Nicholas Ouellette

All Publications

• Dispersion of inertial particles in turbulent canopy flows with buoyant and nonbuoyant plumes PHYSICAL REVIEW FLUIDS Chung, H., Sunberg, L. C., MacDonald, E., Ouellette, N. T., Koseff, J. R. 2024; 9 (9)

  View details for DOI 10.1103/PhysRevFluids.9.093801

  View details for Web of Science ID 001327714700001

• Spectral energy transfer on complex networks: a filtering approach. Scientific reports MacMillan, T., Ouellette, N. T. 2024; 14 (1): 20691

  Abstract

  The spectral analysis of dynamical systems is a staple technique for analyzing a vast range of systems. But beyond its analytical utility, it is also the primary lens through which many physical phenomena are defined and interpreted. The turbulent energy cascade in fluid mechanics, a dynamical consequence of the three-dimensional Navier-Stokes equations in which energy "cascades" from large injection scales to smaller dissipation scales, is a well-known example that is precisely defined only in reciprocal space. Related techniques in the context of networked dynamical systems have been employed with great success in deriving reduced order models. But what such techniques gain in analytical tractability, they often lose in interpretability and locality, as the lower degree of freedom system frequently contains information from all nodes of the network. Here, we demonstrate that a network of nonlinear oscillators exhibits spectral energy transfer facilitated by an effective force akin to the Reynolds stress in turbulence, an example of an emergent higher order interaction. Then, introducing a filter-based decomposition motivated by large eddy simulation, we show that such higher order interactions can be localized to individual nodes and study the effects of local topology on such interactions.

  View details for DOI 10.1038/s41598-024-71756-x

  View details for PubMedID 39237704

  View details for PubMedCentralID 3497713

• Parametric Study of the dispersion of inertial ellipsoidal particles in a wave-current flow Physical Review Fluids Sunberg, L. K., DiBenedetto, M. H., Ouellette, N. T., Koseff, J. R. 2024; 9

  View details for DOI 10.1103/PhysRevFluids.9.034302

• Quantifying the pedestrian access potential of suburban street network retrofits ENVIRONMENT AND PLANNING B-URBAN ANALYTICS AND CITY SCIENCE Aras, R. L., Ouellette, N. T., Jain, R. K. 2023

  View details for DOI 10.1177/23998083231190974

  View details for Web of Science ID 001037149700001

• Swarm formation as backward diffusion. Physical biology Reynolds, A. M., Ouellette, N. 2023

  Abstract

  Considerable progress has been made in understanding insect swarms - forms of collective animal behaviour that unlike bird flocks, fish schools and animal herds do not possess global order. Nonetheless, little is known about swarm formation. Here we posit a mechanism for the formation of insect swarms that is consistent with recent empirical observations reported by [Patel and Ouellette 2022]. It correctly predicts new features of swarm formation that have not been reported on previously. Our simple analytically tractable model shows how harmonic potential wells, a characteristic feature of swarming, and so swarm cohesion, arise from diffusion and local fission-fusion dynamics and how, in accord with observations, these wells deepen over time. The overall form of these potential wells is predicted to depend on the number and spatial distribution of all individuals, making them manifestly a collective phenomenon. Finally, swarms are predicted to 'cool' (that is, condense) as they form.

  View details for DOI 10.1088/1478-3975/acb986

  View details for PubMedID 36745925

• Non-uniform spatial sampling by individuals in midge swarms. Journal of the Royal Society, Interface Feng, Y., Ouellette, N. T. 2023; 20 (199): 20220521

  Abstract

  Individual animals engaged in collective behaviour can interchange their relative positions on a wide range of time scales. In situations where some regions of the group are more desirable, it is thought that more fit individuals will preferentially occupy the more favourable locations. However, this notion is difficult to test for animal groups like insect swarms that fluctuate rapidly and display little apparent structure. Here, we study the way that individuals in mating swarms of the non-biting midge Chironomus riparius sample the space available to them. We use Voronoi tessellation to define different regions of the swarm in a dynamic way, and show that midges indeed sample the swarm non-uniformly. However, individuals that preferentially reside in the interior or exterior of the swarm do not display statistically distinct flight behaviour, suggesting that differences in fitness must be assessed in a different way. Nevertheless, our results indicate that midge swarms are not random configurations of individuals but rather possess non-trivial internal structure.

  View details for DOI 10.1098/rsif.2022.0521

  View details for PubMedID 36722071

• A barrier too far: Understanding the role of intersection crossing distance on bicycle rider behavior in Chicago ENVIRONMENT AND PLANNING B-URBAN ANALYTICS AND CITY SCIENCE Aras, R. L., Ouellette, N. T., Jain, R. K. 2023

  View details for DOI 10.1177/23998083221147922

  View details for Web of Science ID 000909520900001

• Dispersion of finite-size, non-spherical particles by waves and currents JOURNAL OF FLUID MECHANICS Clark, L. K., DiBenedetto, M. H., Ouellette, N. T., Koseff, J. R. 2022; 954

  View details for DOI 10.1017/jfm.2022.968

  View details for Web of Science ID 000901456800001

• Velocity correlations in jackdaw flocks in different ecological contexts. Physical biology O'Coin, D., Mclvor, G. E., Thornton, A., Ouellette, N. T., Ling, H. 2022; 20 (1)

  Abstract

  Velocity correlation is an important feature for animal groups performing collective motions. Previous studies have mostly focused on the velocity correlation in a single ecological context. It is unclear whether correlation characteristics vary in a single species in different contexts. Here, we studied the velocity correlations in jackdaw flocks in two different contexts: transit flocks where birds travel from one location to another, and mobbing flocks where birds respond to an external stimulus. We found that in both contexts, although the interaction rules are different, the velocity correlations remain scale-free, i.e. the correlation length (the distance over which the velocity of two individuals is similar) increases linearly with the group size. Furthermore, we found that the correlation length is independent of the group density for transit flocks, but increases with increasing group density in mobbing flocks. This result confirms a previous observation that birds obey topological interactions in transit flocks, but switch to metric interactions in mobbing flocks. Finally, in both contexts, the impact of group polarization on correlation length is not significant. Our results suggest that wild animals are always able to respond coherently to perturbations regardless of context.

  View details for DOI 10.1088/1478-3975/aca862

  View details for PubMedID 36541516

• Linking multiple stressor science to policy opportunities through network modeling MARINE POLICY Wedding, L. M., Green, S. J., Reiter, S., Arrigo, K. R., Hazen, L., Ruckelshaus, M., van der Grient, J. A., Bailey, R. M., Cameron, M. A., Leape, J., Levi, M., Merkl, A., Mills, M. M., Monismith, S., Ouellette, N. T., van Dijken, G., Micheli, F. 2022; 146

  View details for DOI 10.1016/j.marpol.2022.105307

  View details for Web of Science ID 000868989000003

• Extracting free-surface expressions of underwater features EXPERIMENTS IN FLUIDS Gakhar, S., Koseff, J. R., Ouellette, N. T. 2022; 63 (9)

  View details for DOI 10.1007/s00348-022-03491-w

  View details for Web of Science ID 000842157700001

• Stochastic modelling of bird flocks: accounting for the cohesiveness of collective motion. Journal of the Royal Society, Interface Reynolds, A. M., McIvor, G. E., Thornton, A., Yang, P., Ouellette, N. T. 2022; 19 (189): 20210745

  Abstract

  Collective behaviour can be difficult to discern because it is not limited to animal aggregations such as flocks of birds and schools of fish wherein individuals spontaneously move in the same way despite the absence of leadership. Insect swarms are, for example, a form of collective behaviour, albeit one lacking the global order seen in bird flocks and fish schools. Their collective behaviour is evident in their emergent macroscopic properties. These properties are predicted by close relatives of Okubo's 1986 [Adv. Biophys. 22, 1-94. (doi:10.1016/0065-227X(86)90003-1)] stochastic model. Here, we argue that Okubo's stochastic model also encapsulates the cohesiveness mechanism at play in bird flocks, namely the fact that birds within a flock behave on average as if they are trapped in an elastic potential well. That is, each bird effectively behaves as if it is bound to the flock by a force that on average increases linearly as the distance from the flock centre increases. We uncover this key, but until now overlooked, feature of flocking in empirical data. This gives us a means of identifying what makes a given system collective. We show how the model can be extended to account for intrinsic velocity correlations and differentiated social relationships.

  View details for DOI 10.1098/rsif.2021.0745

  View details for PubMedID 35440203

• Formation and dissolution of midge swarms. Physical review. E Patel, M. L., Ouellette, N. T. 2022; 105 (3-1): 034601

  Abstract

  Using external illumination cues, we induce the formation and dissolution of laboratory swarms of the nonbiting midge Chironomus riparius and study their behavior during these transient processes. In general, swarm formation is slower than swarm dissolution. We find that the swarm property that appears most rapidly during formation and disappears most rapidly during dissolution is an emergent mean radial acceleration pointing toward the center of the swarm. Our results strengthen the conjecture that this central effective force may be used as an indicator to distinguish when the midges are swarming from when they are not.

  View details for DOI 10.1103/PhysRevE.105.034601

  View details for PubMedID 35428071

• Directional strengthening and weakening in hydrodynamically sheared granular beds PHYSICAL REVIEW FLUIDS Galanis, M., Shattuck, M. D., O'Hern, C. S., Ouellette, N. T. 2022; 7 (1)

  View details for DOI 10.1103/PhysRevFluids.7.013802

  View details for Web of Science ID 000747749300003

• A physics perspective on collective animal behavior. Physical biology Ouellette, N. 1800

  Abstract

  The beautiful dynamic patterns and coordinated motion displayed by groups of social animals are a beautiful example of self-organization in natural farfrom-equilibrium systems. Recent advances in active-matter physics have enticed physicists to begin to consider how their results can be extended from microscale physical or biological systems to groups of real, macroscopic animals. At the same time, advances in measurement technology have led to the increasing availability of high-quality empirical data for the behavior of animal groups both in the laboratory and in the wild. In this review, I survey this available data and the ways that it has been analyzed. I then describe how physicists have approached synthesizing, modeling, and interpreting this information, both at the level of individual animals and at the group scale. In particular, I focus on the kinds of analogies that physicists have made between animal groups and more traditional areas of physics.

  View details for DOI 10.1088/1478-3975/ac4bef

  View details for PubMedID 35038691

• Lagrangian scale decomposition via the graph Fourier transform PHYSICAL REVIEW FLUIDS MacMillan, T., Ouellette, N. T. 2022; 7 (12)

  View details for DOI 10.1103/PhysRevFluids.7.124401

• On the interaction between oncoming internal waves and a dense gravity current in a two-layer stratification JOURNAL OF FLUID MECHANICS Tanimoto, Y., Ouellette, N. T., Koseff, J. R. 2021; 932

  View details for DOI 10.1017/jfm.2021.1006

  View details for Web of Science ID 000726848800001

• Spectral condensation in laboratory two-dimensional turbulence PHYSICAL REVIEW FLUIDS Fang, L., Ouellette, N. T. 2021; 6 (10)

  View details for DOI 10.1103/PhysRevFluids.6.104605

  View details for Web of Science ID 000708674900002

• Onset of grain motion in eroding subaqueous bimodal granular beds PHYSICAL REVIEW FLUIDS Galanis, M., Wang, P., Shattuck, M. D., O'Hern, C. S., Ouellette, N. T. 2021; 6 (9)

  View details for DOI 10.1103/PhysRevFluids.6.094301

  View details for Web of Science ID 000693640700005

• Extending the reach of Lagrangian analysis in turbulence JOURNAL OF FLUID MECHANICS Ouellette, N. T. 2021; 924

  View details for DOI 10.1017/jfm.2021.493

  View details for Web of Science ID 000683416900001

• Goals and Limitations of Modeling Collective Behavior in Biological Systems FRONTIERS IN PHYSICS Ouellette, N. T., Gordon, D. M. 2021; 9

  View details for DOI 10.3389/fphy.2021.687823

  View details for Web of Science ID 000667045200001

• Secondary generation of breaking internal waves in confined basins by gravity currents JOURNAL OF FLUID MECHANICS Tanimoto, Y., Ouellette, N. T., Koseff, J. R. 2021; 917

  View details for DOI 10.1017/jfm.2021.309

  View details for Web of Science ID 000646052900001

• Shear response of granular packings compressed above jamming onset PHYSICAL REVIEW E Wang, P., Zhang, S., Tuckman, P., Ouellette, N. T., Shattuck, M. D., O'Hern, C. S. 2021; 103 (2): 022902

  Abstract

  We investigate the mechanical response of jammed packings of repulsive, frictionless spherical particles undergoing isotropic compression. Prior simulations of the soft-particle model, where the repulsive interactions scale as a power law in the interparticle overlap with exponent α, have found that the ensemble-averaged shear modulus 〈G(P)〉 increases with pressure P as ∼P^{(α-3/2)/(α-1)} at large pressures. 〈G〉 has two key contributions: (1) continuous variations as a function of pressure along geometrical families, for which the interparticle contact network does not change, and (2) discontinuous jumps during compression that arise from changes in the contact network. Using numerical simulations, we show that the form of the shear modulus G^{f} for jammed packings within near-isostatic geometrical families is largely determined by the affine response G^{f}∼G_{a}^{f}, where G_{a}^{f}/G_{a0}=(P/P_{0})^{(α-2)/(α-1)}-P/P_{0}, P_{0}∼N^{-2(α-1)} is the characteristic pressure at which G_{a}^{f}=0, G_{a0} is a constant that sets the scale of the shear modulus, and N is the number of particles. For near-isostatic geometrical families that persist to large pressures, deviations from this form are caused by significant nonaffine particle motion. We further show that the ensemble-averaged shear modulus 〈G(P)〉 is not simply a sum of two power laws, but 〈G(P)〉∼(P/P_{c})^{a}, where a≈(α-2)/(α-1) in the P→0 limit and 〈G(P)〉∼(P/P_{c})^{b}, where b≳(α-3/2)/(α-1), above a characteristic pressure that scales as P_{c}∼N^{-2(α-1)}.

  View details for DOI 10.1103/PhysRevE.103.022902

  View details for Web of Science ID 000619139100004

  View details for PubMedID 33736049

• An equation of state for insect swarms. Scientific reports Sinhuber, M., van der Vaart, K., Feng, Y., Reynolds, A. M., Ouellette, N. T. 2021; 11 (1): 3773

  Abstract

  Collective behaviour in flocks, crowds, and swarms occurs throughout the biological world. Animal groups are generally assumed to be evolutionarily adapted to robustly achieve particular functions, so there is widespread interest in exploiting collective behaviour for bio-inspired engineering. However, this requires understanding the precise properties and function of groups, which remains a challenge. Here, we demonstrate that collective groups can be described in a thermodynamic framework. We define an appropriate set of state variables and extract an equation of state for laboratory midge swarms. We then drive swarms through "thermodynamic" cycles via external stimuli, and show that our equation of state holds throughout. Our findings demonstrate a new way of precisely quantifying the nature of collective groups and provide a cornerstone for potential future engineering design.

  View details for DOI 10.1038/s41598-021-83303-z

  View details for PubMedID 33580191

• Assessing the information content of complex flows PHYSICAL REVIEW E Fang, L., Ouellette, N. T. 2021; 103 (2)

  View details for DOI 10.1103/PhysRevE.103.023301

  View details for Web of Science ID 000615718100013

• Automated identification of urban substructure for comparative analysis. PloS one Aras, R. L., Ouellette, N. T., Jain, R. K. 2021; 16 (1): e0245067

  Abstract

  Neighborhoods are the building blocks of cities, and thus significantly impact urban planning from infrastructure deployment to service provisioning. However, existing definitions of neighborhoods are often ill suited for planning in both scale and pattern of aggregation. Here, we propose a generalized, scalable approach using topological data analysis to identify barrier-enclosed neighborhoods on multiple scales with implications for understanding social mixing within cities and the design of urban infrastructure. Our method requires no prior domain knowledge and uses only readily available building parcel information. Results from three American cities (Houston, New York, San Francisco) indicate that our method identifies neighborhoods consistent with historical approaches. Additionally, we uncover a consistent scale in all three cities at which physical isolation drives neighborhood emergence. However, our methods also reveal differences between these cities: Houston, although more disconnected on larger spatial scales than New York and San Francisco, is less disconnected at smaller scales.

  View details for DOI 10.1371/journal.pone.0245067

  View details for PubMedID 33444347

• Detection of evolving Lagrangian coherent structures: A multiple object tracking approach PHYSICAL REVIEW FLUIDS MacMillan, T., Ouellette, N. T., Richter, D. H. 2020; 5 (12)

  View details for DOI 10.1103/PhysRevFluids.5.124401

  View details for Web of Science ID 000600850200004

• Settling of inertial nonspherical particles in wavy flow PHYSICAL REVIEW FLUIDS Clark, L. K., DiBenedetto, M. H., Ouellette, N. T., Koseff, J. R. 2020; 5 (12)

  View details for DOI 10.1103/PhysRevFluids.5.124301

  View details for Web of Science ID 000595856700004

• Temporal dynamics of the alignment of the turbulent stress and strain rate PHYSICAL REVIEW FLUIDS Ballouz, J. G., Johnson, P. L., Ouellette, N. T. 2020; 5 (11)

  View details for DOI 10.1103/PhysRevFluids.5.114606

  View details for Web of Science ID 000589182500004

• On the surface expression of bottom features in free-surface flow JOURNAL OF FLUID MECHANICS Gakhar, S., Koseff, J. R., Ouellette, N. T. 2020; 900

  View details for DOI 10.1017/jfm.2020.548

  View details for Web of Science ID 000559967400001

• Disentangling resolution, precision, and inherent stochasticity in nonlinear systems PHYSICAL REVIEW RESEARCH Fang, L., Balasuriya, S., Ouellette, N. T. 2020; 2 (2)

  View details for DOI 10.1103/PhysRevResearch.2.023343

  View details for Web of Science ID 000603628900002

• Vorticity gradient stretching in the direct enstrophy transfer process of two-dimensional turbulence PHYSICAL REVIEW FLUIDS Zhou, Z., Fang, L., Ouellette, N. T., Xu, H. 2020; 5 (5)

  View details for DOI 10.1103/PhysRevFluids.5.054602

  View details for Web of Science ID 000531459800001

• Interaction between an inclined gravity current and a pycnocline in a two-layer stratification JOURNAL OF FLUID MECHANICS Tanimoto, Y., Ouellette, N. T., Koseff, J. R. 2020; 887

  View details for DOI 10.1017/jfm.2020.9

  View details for Web of Science ID 000508241400001

• Geometric constraints on energy transfer in the turbulent cascade PHYSICAL REVIEW FLUIDS Ballouz, J. G., Ouellette, N. T. 2020; 5 (3)

  View details for DOI 10.1103/PhysRevFluids.5.034603

  View details for Web of Science ID 000518991600003

• Similarities between insect swarms and isothermal globular clusters PHYSICAL REVIEW RESEARCH Gorbonos, D., van der Vaart, K., Sinhuber, M., Puckett, J. G., Reynolds, A. M., Ouellette, N. T., Gov, N. S. 2020; 2 (1)

  View details for DOI 10.1103/PhysRevResearch.2.013271

  View details for Web of Science ID 000602695400004

• Environmental perturbations induce correlations in midge swarms. Journal of the Royal Society, Interface van der Vaart, K., Sinhuber, M., Reynolds, A. M., Ouellette, N. T. 2020; 17 (164): 20200018

  Abstract

  Although collectively behaving animal groups often show large-scale order (such as in bird flocks), they need not always (such as in insect swarms). It has been suggested that the signature of collective behaviour in disordered groups is a residual long-range correlation. However, results in the literature have reported contradictory results as to the presence of long-range correlation in insect swarms, with swarms in the wild displaying correlation but those in a controlled laboratory environment not. We resolve these apparently incompatible results by showing that the external perturbations generically induce the emergence of correlations. We apply a range of different external stimuli to laboratory swarms of the non-biting midge Chironomus riparius, and show that in all cases correlations appear when perturbations are introduced. We confirm the generic nature of these results by showing that they can be reproduced in a stochastic model of swarms. Given that swarms in the wild will always have to contend with environmental stimuli, our results thus harmonize previous findings. These findings emphasize that collective behaviour cannot be understood in isolation without considering its environmental context, and that new research is needed to disentangle the distinct roles of intrinsic dynamics and external stimuli.

  View details for DOI 10.1098/rsif.2020.0018

  View details for PubMedID 32208820

• Synergistic interactions among growing stressors increase risk to an Arctic ecosystem. Nature communications Arrigo, K. R., van Dijken, G. L., Cameron, M. A., van der Grient, J., Wedding, L. M., Hazen, L., Leape, J., Leonard, G., Merkl, A., Micheli, F., Mills, M. M., Monismith, S., Ouellette, N. T., Zivian, A., Levi, M., Bailey, R. M. 2020; 11 (1): 6255

  Abstract

  Oceans provide critical ecosystem services, but are subject to a growing number of external pressures, including overfishing, pollution, habitat destruction, and climate change. Current models typically treat stressors on species and ecosystems independently, though in reality, stressors often interact in ways that are not well understood. Here, we use a network interaction model (OSIRIS) to explicitly study stressor interactions in the Chukchi Sea (Arctic Ocean) due to its extensive climate-driven loss of sea ice and accelerated growth of other stressors, including shipping and oil exploration. The model includes numerous trophic levels ranging from phytoplankton to polar bears. We find that climate-related stressors have a larger impact on animal populations than do acute stressors like increased shipping and subsistence harvesting. In particular, organisms with a strong temperature-growth rate relationship show the greatest changes in biomass as interaction strength increased, but also exhibit the greatest variability. Neglecting interactions between stressors vastly underestimates the risk of population crashes. Our results indicate that models must account for stressor interactions to enable responsible management and decision-making.

  View details for DOI 10.1038/s41467-020-19899-z

  View details for PubMedID 33288746

• Pair formation in insect swarms driven by adaptive long-range interactions. Journal of the Royal Society, Interface Gorbonos, D. n., Puckett, J. G., van der Vaart, K. n., Sinhuber, M. n., Ouellette, N. T., Gov, N. S. 2020; 17 (171): 20200367

  Abstract

  In swarms of flying insects, the motions of individuals are largely uncoordinated with those of their neighbours, unlike the highly ordered motion of bird flocks. However, it has been observed that insects may transiently form pairs with synchronized relative motion while moving through the swarm. The origin of this phenomenon remains an open question. In particular, it is not known if pairing is a new behavioural process or whether it is a natural by-product of typical swarming behaviour. Here, using an 'adaptive-gravity' model that proposes that insects interact via long-range gravity-like acoustic attractions that are modulated by the total background sound (via 'adaptivity' or fold-change detection) and that reproduces measured features of real swarms, we show that pair formation can indeed occur without the introduction of additional behavioural rules. In the model, pairs form robustly whenever two insects happen to move together from the centre of the swarm (where the background sound is high) towards the swarm periphery (where the background sound is low). Due to adaptivity, the attraction between the pair increases as the background sound decreases, thereby forming a bound state since their relative kinetic energy is smaller than their pair-potential energy. When the pair moves into regions of high background sound, however, the process is reversed and the pair may break up. Our results suggest that pairing should appear generally in biological systems with long-range attraction and adaptive sensing, such as during chemotaxis-driven cellular swarming.

  View details for DOI 10.1098/rsif.2020.0367

  View details for PubMedID 33023396

• Comparison of shear and compression jammed packings of frictional disks GRANULAR MATTER Xiong, F., Wang, P., Clark, A. H., Bertrand, T., Ouellette, N. T., Shattuck, M. D., O'Hern, C. S. 2019; 21 (4)

  View details for DOI 10.1007/s10035-019-0964-9

  View details for Web of Science ID 000500812200003

• Collective turns in jackdaw flocks: kinematics and information transfer. Journal of the Royal Society, Interface Ling, H., Mclvor, G. E., Westley, J., van der Vaart, K., Yin, J., Vaughan, R. T., Thornton, A., Ouellette, N. T. 2019; 16 (159): 20190450

  Abstract

  The rapid, cohesive turns of bird flocks are one of the most vivid examples of collective behaviour in nature, and have attracted much research. Three-dimensional imaging techniques now allow us to characterize the kinematics of turning and their group-level consequences in precise detail. We measured the kinematics of flocks of wild jackdaws executing collective turns in two contexts: during transit to roosts and anti-predator mobbing. All flocks reduced their speed during turns, probably because of constraints on individual flight capability. Turn rates increased with the angle of the turn so that the time to complete turns remained constant. We also find that context may alter where turns are initiated in the flocks: for transit flocks in the absence of predators, initiators were located throughout the flocks, but for mobbing flocks with a fixed ground-based predator, they were always located at the front. Moreover, in some transit flocks, initiators were far apart from each other, potentially because of the existence of subgroups and variation in individual interaction ranges. Finally, we find that as the group size increased the information transfer speed initially increased, but rapidly saturated to a constant value. Our results highlight previously unrecognized complexity in turning kinematics and information transfer in social animals.

  View details for DOI 10.1098/rsif.2019.0450

  View details for PubMedID 31640502

• Nonlinear dynamics captures brain states at different levels of consciousness in patients anesthetized with propofol. PloS one Eagleman, S. L., Chander, D., Reynolds, C., Ouellette, N. T., MacIver, M. B. 2019; 14 (10): e0223921

  Abstract

  The information processing capability of the brain decreases during unconscious states. Capturing this decrease during anesthesia-induced unconsciousness has been attempted using standard spectral analyses as these correlate relatively well with breakdowns in corticothalamic networks. Much of this work has involved the use of propofol to perturb brain activity, as it is one of the most widely used anesthetics for routine surgical anesthesia. Propofol administration alone produces EEG spectral characteristics similar to most hypnotics; however, inter-individual and drug variation render spectral measures inconsistent. Complexity measures of EEG signals could offer better measures to distinguish brain states, because brain activity exhibits nonlinear behavior at several scales during transitions of consciousness. We tested the potential of complexity analyses from nonlinear dynamics to identify loss and recovery of consciousness at clinically relevant timepoints. Patients undergoing propofol general anesthesia for various surgical procedures were identified as having changes in states of consciousness by the loss and recovery of response to verbal stimuli after induction and upon cessation of anesthesia, respectively. We demonstrate that nonlinear dynamics analyses showed more significant differences between consciousness states than spectral measures. Notably, attractors in conscious and anesthesia-induced unconscious states exhibited significantly different shapes. These shapes have implications for network connectivity, information processing, and the total number of states available to the brain at these different levels. They also reflect some of our general understanding of the network effects of consciousness in a way that spectral measures cannot. Thus, complexity measures could provide a universal means for reliably capturing depth of consciousness based on EEG changes at the beginning and end of anesthesia administration.

  View details for DOI 10.1371/journal.pone.0223921

  View details for PubMedID 31665174

• Interaction of a downslope gravity current with an internal wave JOURNAL OF FLUID MECHANICS Ouillon, R., Meiburg, E., Ouellette, N. T., Koseff, J. R. 2019; 873: 889–913

  View details for DOI 10.1017/jfm.2019.414

  View details for Web of Science ID 000473183100001

• Modeling Environmental DNA Transport in the Coastal Ocean Using Lagrangian Particle Tracking FRONTIERS IN MARINE SCIENCE Andruszkiewicz, E. A., Koseff, J. R., Fringer, O. B., Ouellette, N. T., Lowe, A. B., Edwards, C. A., Boehm, A. B. 2019; 6

  View details for DOI 10.3389/fmars.2019.00477

  View details for Web of Science ID 000479073400001

• The Most Active Matter of All MATTER Ouellette, N. T. 2019; 1 (2): 297–99

  View details for DOI 10.1016/j.matt.2019.07.012

  View details for Web of Science ID 000519688200002

• Local interactions and their group-level consequences in flocking jackdaws. Proceedings. Biological sciences Ling, H., Mclvor, G. E., van der Vaart, K., Vaughan, R. T., Thornton, A., Ouellette, N. T. 2019; 286 (1906): 20190865

  Abstract

  As one of nature's most striking examples of collective behaviour, bird flocks have attracted extensive research. However, we still lack an understanding of the attractive and repulsive forces that govern interactions between individuals within flocks and how these forces influence neighbours' relative positions and ultimately determine the shape of flocks. We address these issues by analysing the three-dimensional movements of wild jackdaws ( Corvus monedula) in flocks containing 2-338 individuals. We quantify the social interaction forces in large, airborne flocks and find that these forces are highly anisotropic. The long-range attraction in the direction perpendicular to the movement direction is stronger than that along it, and the short-range repulsion is generated mainly by turning rather than changing speed. We explain this phenomenon by considering wingbeat frequency and the change in kinetic and gravitational potential energy during flight, and find that changing the direction of movement is less energetically costly than adjusting speed for birds. Furthermore, our data show that collision avoidance by turning can alter local neighbour distributions and ultimately change the group shape. Our results illustrate the macroscopic consequences of anisotropic interaction forces in bird flocks, and help to draw links between group structure, local interactions and the biophysics of animal locomotion.

  View details for DOI 10.1098/rspb.2019.0865

  View details for PubMedID 31266425

• Mechanical spectroscopy of insect swarms. Science advances van der Vaart, K., Sinhuber, M., Reynolds, A. M., Ouellette, N. T. 2019; 5 (7): eaaw9305

  Abstract

  Social animals routinely form groups, which are thought to display emergent, collective behavior. This hypothesis suggests that animal groups should have properties at the group scale that are not directly linked to the individuals, much as bulk materials have properties distinct from those of their constituent atoms. Materials are often probed by measuring their response to controlled perturbations, but these experiments are difficult to conduct on animal groups, particularly in the wild. Here, we show that laboratory midge swarms have emergent continuum mechanical properties, displaying a collective viscoelastic response to applied oscillatory visual stimuli that allows us to extract storage and loss moduli for the swarm. We find that the swarms strongly damp perturbations, both viscously and inertially. Thus, unlike bird flocks, which appear to use collective behavior to promote lossless information flow through the group, our results suggest that midge swarms use it to stabilize themselves against environmental perturbations.

  View details for DOI 10.1126/sciadv.aaw9305

  View details for PubMedID 31501772

  View details for PubMedCentralID PMC6719412

• Transport across a bathymetric interface in quasi-two-dimensional flow PHYSICAL REVIEW FLUIDS Fang, L., Ouellette, N. T. 2019; 4 (6)

  View details for DOI 10.1103/PhysRevFluids.4.064501

  View details for Web of Science ID 000470896500002

• Costs and benefits of social relationships in the collective motion of bird flocks NATURE ECOLOGY & EVOLUTION Ling, H., McIvor, G. E., van der Vaart, K., Vaughan, R. T., Thornton, A., Ouellette, N. T. 2019; 3 (6): 943–48

  View details for DOI 10.1038/s41559-019-0891-5

  View details for Web of Science ID 000470917200018

• Orientation dynamics of nonspherical particles under surface gravity waves PHYSICAL REVIEW FLUIDS DiBenedetto, M. H., Koseff, J. R., Ouellette, N. T. 2019; 4 (3)

  View details for DOI 10.1103/PhysRevFluids.4.034301

  View details for Web of Science ID 000460680000002

• Three-dimensional time-resolved trajectories from laboratory insect swarms SCIENTIFIC DATA Sinhuber, M., van der Vaart, K., Ni, R., Puckett, J. G., Kelley, D. H., Ouellette, N. T. 2019; 6

  View details for DOI 10.1038/sdata.2019.36

  View details for Web of Science ID 000460588400001

• Response of insect swarms to dynamic illumination perturbations. Journal of the Royal Society, Interface Sinhuber, M., van der Vaart, K., Ouellette, N. T. 2019; 16 (150): 20180739

  Abstract

  Many animal species across taxa spontaneously form aggregations that exhibit collective behaviour. In the wild, these collective systems are unavoidably influenced by ubiquitous environmental perturbations such as wind gusts, acoustic and visual stimuli, or the presence of predators or other animals. The way these environmental perturbations influence the animals' collective behaviour, however, is poorly understood, in part because conducting controlled quantitative perturbation experiments in natural settings is challenging. To circumvent the need for controlling environmental conditions in the field, we study swarming midges in a laboratory experiment where we have full control over external perturbations. Here, we consider the effect of controlled variable light exposure on the swarming behaviour. We find that not only do individuals in the swarm respond to light changes by speeding up during brighter conditions but also the swarm as a whole responds to these perturbations by compressing and simultaneously increasing the attraction of individual midges to its centre of mass. The swarm-level response can be described by making an analogy to classical thermodynamics, with the state of the swarm moving along an isotherm in a thermodynamic phase plane.

  View details for DOI 10.1098/rsif.2018.0739

  View details for PubMedID 30958145

  View details for PubMedCentralID PMC6364661

• Flowing crowds SCIENCE Ouellette, N. T. 2019; 363 (6422): 27–28

  View details for DOI 10.1126/science.aav9869

  View details for Web of Science ID 000455315800043

• Local linearity, coherent structures, and scale-to-scale coupling in turbulent flow PHYSICAL REVIEW FLUIDS Fang, L., Balasuriya, S., Ouellette, N. T. 2019; 4 (1)

  View details for DOI 10.1103/PhysRevFluids.4.014501

  View details for Web of Science ID 000455067200002

• Behavioural plasticity and the transition to order in jackdaw flocks. Nature communications Ling, H. n., Mclvor, G. E., Westley, J. n., van der Vaart, K. n., Vaughan, R. T., Thornton, A. n., Ouellette, N. T. 2019; 10 (1): 5174

  Abstract

  Collective behaviour is typically thought to arise from individuals following fixed interaction rules. The possibility that interaction rules may change under different circumstances has thus only rarely been investigated. Here we show that local interactions in flocks of wild jackdaws (Corvus monedula) vary drastically in different contexts, leading to distinct group-level properties. Jackdaws interact with a fixed number of neighbours (topological interactions) when traveling to roosts, but coordinate with neighbours based on spatial distance (metric interactions) during collective anti-predator mobbing events. Consequently, mobbing flocks exhibit a dramatic transition from disordered aggregations to ordered motion as group density increases, unlike transit flocks where order is independent of density. The relationship between group density and group order during this transition agrees well with a generic self-propelled particle model. Our results demonstrate plasticity in local interaction rules and have implications for both natural and artificial collective systems.

  View details for DOI 10.1038/s41467-019-13281-4

  View details for PubMedID 31729384

• Computational and Structural Advantages of Pairwise Flocking Nagy, G., Thornton, A., Ling, H., McIvor, G., Ouellette, N. T., Vaugha, R., Sabattini, L. IEEE. 2019: 133–35

  View details for Web of Science ID 000614713600028

• Preferential orientation of spheroidal particles in wavy flow JOURNAL OF FLUID MECHANICS DiBenedetto, M. H., Ouellette, N. T. 2018; 856: 850–69

  View details for DOI 10.1017/jfm.2018.738

  View details for Web of Science ID 000447209900004

• Simultaneous measurements of three-dimensional trajectories and wingbeat frequencies of birds in the field JOURNAL OF THE ROYAL SOCIETY INTERFACE Ling, H., Mclvor, G. E., Nagy, G., MohaimenianPour, S., Vaughan, R. T., Thornton, A., Ouellette, N. T. 2018; 15 (147)

  View details for DOI 10.1098/rsif.2018.0653

  View details for Web of Science ID 000448813900030

• Do Complexity Measures of Frontal EEG Distinguish Loss of Consciousness in Geriatric Patients Under Anesthesia? Frontiers in neuroscience Eagleman, S. L., Vaughn, D. A., Drover, D. R., Drover, C. M., Cohen, M. S., Ouellette, N. T., MacIver, M. B. 2018; 12: 645

  Abstract

  While geriatric patients have a high likelihood of requiring anesthesia, they carry an increased risk for adverse cognitive outcomes from its use. Previous work suggests this could be mitigated by better intraoperative monitoring using indexes defined by several processed electroencephalogram (EEG) measures. Unfortunately, inconsistencies between patients and anesthetic agents in current analysis techniques have limited the adoption of EEG as standard of care. In attempts to identify new analyses that discriminate clinically-relevant anesthesia timepoints, we tested 1/f frequency scaling as well as measures of complexity from nonlinear dynamics. Specifically, we tested whether analyses that characterize time-delayed embeddings, correlation dimension (CD), phase-space geometric analysis, and multiscale entropy (MSE) capture loss-of-consciousness changes in EEG activity. We performed these analyses on EEG activity collected from a traditionally hard-to-monitor patient population: geriatric patients on beta-adrenergic blockade who were anesthetized using a combination of fentanyl and propofol. We compared these analyses to traditional frequency-derived measures to test how well they discriminated EEG states before and after loss of response to verbal stimuli. We found spectral changes similar to those reported previously during loss of response. We also found significant changes in 1/f frequency scaling. Additionally, we found that our phase-space geometric characterization of time-delayed embeddings showed significant differences before and after loss of response, as did measures of MSE. Our results suggest that our new spectral and complexity measures are capable of capturing subtle differences in EEG activity with anesthesia administration-differences which future work may reveal to improve geriatric patient monitoring.

  View details for DOI 10.3389/fnins.2018.00645

  View details for PubMedID 30294254

  View details for PubMedCentralID PMC6158339

• Probing the strain-rotation balance in non-Newtonian turbulence with inertial particles PHYSICAL REVIEW FLUIDS Sinhuber, M., Ballouz, J. G., Ouellette, N. T. 2018; 3 (8)

  View details for DOI 10.1103/PhysRevFluids.3.082602

  View details for Web of Science ID 000441239400001

• Critical scaling near the yielding transition in granular media PHYSICAL REVIEW E Clark, A. H., Thompson, J. D., Shattuck, M. D., Ouellette, N. T., O'Hern, C. S. 2018; 97 (6)

  View details for DOI 10.1103/PhysRevE.97.062901

  View details for Web of Science ID 000434259700010

• Generalized Lagrangian coherent structures PHYSICA D-NONLINEAR PHENOMENA Balasuriya, S., Ouellette, N. T., Rypina, I. I. 2018; 372: 31–51

  View details for DOI 10.1016/j.physd.2018.01.011

  View details for Web of Science ID 000430892700003

• Critical scaling near the yielding transition in granular media. Physical review. E Clark, A. H., Thompson, J. D., Shattuck, M. D., Ouellette, N. T., O'Hern, C. S. 2018; 97 (6-1): 062901

  Abstract

  We show that the yielding transition in granular media displays second-order critical-point scaling behavior. We carry out discrete element simulations in the low-inertial-number limit for frictionless, purely repulsive spherical grains undergoing simple shear at fixed nondimensional shear stress Σ in two and three spatial dimensions. To find a mechanically stable (MS) packing that can support the applied Σ, isotropically prepared states with size L must undergo a total strain γ_{ms}(Σ,L). The number density of MS packings (∝γ_{ms}^{-1}) vanishes for Σ>Σ_{c}≈0.11 according to a critical scaling form with a length scale ξ∝|Σ-Σ_{c}|^{-ν}, where ν≈1.7-1.8. Above the yield stress (Σ>Σ_{c}), no MS packings that can support Σ exist in the large-system limit L/ξ≫1. MS packings generated via shear possess anisotropic force and contact networks, suggesting that Σ_{c} is associated with an upper limit in the degree to which these networks can be deformed away from those for isotropic packings.

  View details for DOI 10.1103/PhysRevE.97.062901

  View details for PubMedID 30011584

• Remifentanil and Nitrous Oxide Anesthesia Produces a Unique Pattern of EEG Activity During Loss and Recovery of Response FRONTIERS IN HUMAN NEUROSCIENCE Eagleman, S. L., Drover, C. M., Drover, D. R., Ouellette, N. T., MacIver, M. 2018; 12: 173

  Abstract

  Nitrous oxide (N2O) and remifentanil (remi) are used along with other anesthetic and adjuvant agents for routine surgical anesthesia, yet the electroencephalogram (EEG) changes produced by this combination are poorly described. N2O administered alone produces EEG spectral characteristics that are distinct from most hypnotics. Furthermore, EEG frequency-derived trends before and after clinically relevant time points vary depending on N2O concentration. Remifentanil typically increases low frequency and decreases high frequency activity in the EEG, but how it influences N2O's EEG effect is not known. Previous attempts to characterize EEG signals of patients anesthetized with N2O using frequency-derived measures have shown conflicts and inconsistencies. Thus, in addition to determining the spectral characteristics of this unique combination, we also test whether a newly proposed characterization of time-delayed embeddings of the EEG signal tracks loss and recovery of consciousness significantly at clinically relevant time points. We retrospectively investigated the effects of remi and N2O on EEG signals recorded from 32 surgical patients receiving anesthesia for elective abdominal surgeries. Remifentanil and N2O (66%) were co-administered during the procedures. Patients were tested for loss and recovery of response (ROR) to verbal stimuli after induction and upon cessation of anesthesia, respectively. We found that the addition of remifentanil to N2O anesthesia improves the ability of traditional frequency-derived measures, including the Bispectral Index (BIS), to discriminate between loss and ROR. Finally, we found that a novel analysis of EEG using nonlinear dynamics showed more significant differences between states than most spectral measures.

  View details for PubMedID 29867405

• Shoaling internal waves may reduce gravity current transport ENVIRONMENTAL FLUID MECHANICS Hogg, C. R., Egan, G. C., Ouellette, N. T., Koseff, J. R. 2018; 18 (2): 383–94

  View details for DOI 10.1007/s10652-017-9554-8

  View details for Web of Science ID 000429374800003

• Transport of anisotropic particles under waves JOURNAL OF FLUID MECHANICS DiBenedetto, M. H., Ouellette, N. T., Koseff, J. R. 2018; 837: 320–40

  View details for DOI 10.1017/jfm.2017.853

  View details for Web of Science ID 000418739300002

• Influence of lateral boundaries on transport in quasi-two-dimensional flow CHAOS Fang, L., Ouellette, N. T. 2018; 28 (2): 023113

  Abstract

  We assess the impact of lateral coastline-like boundaries on mixing and transport in a laboratory quasi-two-dimensional turbulent flow using a transfer-operator approach. We examine the most coherent sets in the flow, as defined by the singular vectors of the transfer operator, as a way to characterize its mixing properties. We study three model coastline shapes: a uniform boundary, a sharp embayment, and a sharp headland. Of these three, we show that the headland affects the mixing deep into the flow domain because it has a tendency to pin transport barriers to its tip. Our results may have implications for the siting of coastal facilities that discharge into the ocean.

  View details for PubMedID 29495670

• Tensor geometry in the turbulent cascade JOURNAL OF FLUID MECHANICS Ballouz, J. G., Ouellette, N. T. 2018; 835: 1048–64

  View details for DOI 10.1017/jfm.2017.802

  View details for Web of Science ID 000416946600003

• Determining the onset of hydrodynamic erosion in turbulent flow PHYSICAL REVIEW FLUIDS Salevan, J. C., Clark, A. H., Shattuck, M. D., O'Hern, C. S., Ouellette, N. T. 2017; 2 (11)

  View details for DOI 10.1103/PhysRevFluids.2.114302

  View details for Web of Science ID 000415690300002

• Characterizing free-surface expressions of flow instabilities by tracking submerged features EXPERIMENTS IN FLUIDS Mandel, T. L., Rosenzweig, I., Chung, H., Ouellette, N. T., Koseff, J. R. 2017; 58 (11)

  View details for DOI 10.1007/s00348-017-2435-6

  View details for Web of Science ID 000413694700002

• Multiple stages of decay in two-dimensional turbulence PHYSICS OF FLUIDS Fang, L., Ouellette, N. T. 2017; 29 (11)

  View details for DOI 10.1063/1.4996776

  View details for Web of Science ID 000416067400005

• Phase Coexistence in Insect Swarms PHYSICAL REVIEW LETTERS Sinhuber, M., Ouellette, N. T. 2017; 119 (17): 178003

  Abstract

  Animal aggregations are visually striking, and as such are popular examples of collective behavior in the natural world. Quantitatively demonstrating the collective nature of such groups, however, remains surprisingly difficult. Inspired by thermodynamics, we applied topological data analysis to laboratory insect swarms and found evidence for emergent, material-like states. We show that the swarms consist of a core "condensed" phase surrounded by a dilute "vapor" phase. These two phases coexist in equilibrium, and maintain their distinct macroscopic properties even though individual insects pass freely between them. We further define a pressure and chemical potential to describe these phases, extending theories of active matter to aggregations of macroscopic animals and laying the groundwork for a thermodynamic description of collective animal groups.

  View details for PubMedID 29219441

• Are midge swarms bound together by an effective velocity-dependent gravity? EUROPEAN PHYSICAL JOURNAL E Reynolds, A. M., Sinhuber, M., Ouellette, N. T. 2017; 40 (4)

  Abstract

  Midge swarms are a canonical example of collective animal behaviour where local interactions do not clearly play a major role and yet the animals display group-level cohesion. The midges appear somewhat paradoxically to be tightly bound to the swarm whilst at the same time weakly coupled inside it. The microscopic origins of this behaviour have remained elusive. Models based on Newtonian gravity do, however, agree well with experimental observations of laboratory swarms. They are biologically plausible since gravitational interactions have similitude with long-range acoustic and visual interactions, and they correctly predict that individual attraction to the swarm centre increases linearly with distance from the swarm centre. Here we show that the observed kinematics implies that this attraction also increases with an individual's flight speed. We find clear evidence for such an attractive force in experimental data.

  View details for DOI 10.1140/epje/i2017-11531-7

  View details for Web of Science ID 000399715200001

  View details for PubMedID 28417322

• Role of grain dynamics in determining the onset of sediment transport PHYSICAL REVIEW FLUIDS Clark, A. H., Shattuck, M. D., Ouellette, N. T., O'Hern, C. S. 2017; 2 (3)

  View details for DOI 10.1103/PhysRevFluids.2.034305

  View details for Web of Science ID 000399155900003

• Hyperbolic neighbourhoods as organizers of finite-time exponential stretching JOURNAL OF FLUID MECHANICS Balasuriya, S., Kalampattel, R., Ouellette, N. T. 2016; 807: 509-545

  View details for DOI 10.1017/jfm.2016.633

  View details for Web of Science ID 000386452000022

• Advection and the Efficiency of Spectral Energy Transfer in Two-Dimensional Turbulence. Physical review letters Fang, L., Ouellette, N. T. 2016; 117 (10): 104501-?

  Abstract

  We report measurements of the geometric alignment of the small-scale turbulent stress and the large-scale rate of strain that together lead to the net flux of energy from small scales to large scales in two-dimensional turbulence. We find that the instantaneous alignment between these two tensors is weak and, thus, that the spectral transport of energy is inefficient. We show, however, that the strain rate is much better aligned with the stress at times in the past, suggesting that the differential advection of the two is responsible for the inefficient spectral transfer. We provide evidence for this conjecture by measuring the alignment statistics conditioned on weakly changing stress history. Our results give new insight into the relationship between scale-to-scale energy transfer, geometric alignment, and advection in turbulent flows.

  View details for DOI 10.1103/PhysRevLett.117.104501

  View details for PubMedID 27636478

• Long-range acoustic interactions in insect swarms: an adaptive gravity model NEW JOURNAL OF PHYSICS Gorbonos, D., Ianconescu, R., Puckett, J. G., Ni, R., Ouellette, N. T., Gov, N. S. 2016; 18

  View details for DOI 10.1088/1367-2630/18/7/073042

  View details for Web of Science ID 000381880000001

• Concentration effects on turbulence in dilute polymer solutions far from walls. Physical review. E de Chaumont Quitry, A., Ouellette, N. T. 2016; 93 (6): 063116-?

  Abstract

  We report measurements of the modification of turbulence far from any walls by small concentrations of long-chain polymers. We consider a range of statistical properties of the flow, including Eulerian and Lagrangian velocity structure functions, Eulerian acceleration correlation functions, and the relative dispersion of particle pairs. In all cases, we find that the polymer concentration has a strong effect on the extent to which the statistical properties are changed compared to their values in pure water. These effects can be captured by the recently proposed energy flux-balance model (when suitably extended into the time domain for Lagrangian statistics). However, unlike previous measurements, which found that the concentration effect could be completely scaled out, we consistently find that our data collapse onto two different master curves, one for small concentration and one for larger concentration. We suggest that the difference between the two may be related to the onset of interactions among polymer chains, which is likely to be more easily observed at the small Weissenberg numbers we consider here.

  View details for DOI 10.1103/PhysRevE.93.063116

  View details for PubMedID 27415367

• Stretching and folding in finite time. Chaos Ma, T., Ouellette, N. T., Bollt, E. M. 2016; 26 (2): 023112-?

  Abstract

  Complex flows mix efficiently, and this process can be understood by considering the stretching and folding of material volumes. Although many metrics have been devised to characterize stretching, fewer are able to capture folding in a quantitative way in spatiotemporally variable flows. Here, we extend our previous methods based on the finite-time curving of fluid-element trajectories to nonzero scales and show that this finite-scale finite-time curvature contains information about both stretching and folding. We compare this metric to the more commonly used finite-time Lyapunov exponent and illustrate our methods using experimental flow-field data from a quasi-two-dimensional laboratory flow. Our new analysis tools add to the growing set of Lagrangian methods for characterizing mixing in complex, aperiodic fluid flows.

  View details for DOI 10.1063/1.4941256

  View details for PubMedID 26931593

• Correlating Lagrangian structures with forcing in two-dimensional flow PHYSICS OF FLUIDS Ouellette, N. T., Hogg, C. A., Liao, Y. 2016; 28 (1)

  View details for DOI 10.1063/1.4938495

  View details for Web of Science ID 000374595500043

• On the tensile strength of insect swarms. Physical biology Ni, R., Ouellette, N. T. 2016; 13 (4): 045002-?

  Abstract

  Collective animal groups are often described by the macroscopic patterns they form. Such global patterns, however, convey limited information about the nature of the aggregation as a whole. Here, we take a different approach, drawing on ideas from materials testing to probe the macroscopic mechanical properties of mating swarms of the non-biting midge Chironomus riparius. By manipulating ground-based visual features that tend to position the swarms in space, we apply an effective tensile load to the swarms, and show that we can quasi-statically pull single swarms apart into multiple daughter swarms. Our results suggest that swarms surprisingly have macroscopic mechanical properties similar to solids, including a finite Young's modulus and yield strength, and that they do not flow like viscous fluids.

  View details for DOI 10.1088/1478-3975/13/4/045002

  View details for PubMedID 27559838

• Swarm dynamics may give rise to Lévy flights. Scientific reports Reynolds, A. M., Ouellette, N. T. 2016; 6: 30515-?

  Abstract

  "Continuous-time correlated random walks" are now gaining traction as models of scale-finite animal movement patterns because they overcome inherent shortcomings with the prevailing paradigm - discrete random walk models. Continuous-time correlated random walk models are founded on the classic Langevin equation that is driven by purely additive noise. The Langevin equation is, however, changed fundamentally by the smallest of multiplicative noises. The inclusion of such noises gives rise to Lévy flights, a popular but controversial model of scale-free movement patterns. Multiplicative noises have not featured prominently in the literature on biological Lévy flights, being seen, perhaps, as no more than a mathematical contrivance. Here we show how Langevin equations driven by multiplicative noises and incumbent Lévy flights arise naturally in the modelling of swarms. Model predictions find some support in three-dimensional, time-resolved measurements of the positions of individual insects in laboratory swarms of the midge Chironomus riparius. We hereby provide a new window on Lévy flights as models of movement pattern data, linking patterns to generative processes.

  View details for DOI 10.1038/srep30515

  View details for PubMedID 27465971

  View details for PubMedCentralID PMC4964348

• Mixing and sink effects of air purifiers on indoor PM2.5 concentrations: A pilot study of eight residential homes in Fresno, California AEROSOL SCIENCE AND TECHNOLOGY Cheng, K., Park, H., Tetteh, A. O., Zheng, D., Ouellette, N. T., Nadeau, K. C., Hildemann, L. M. 2016; 50 (8): 835-845

  View details for DOI 10.1080/02786826.2016.1197375

  View details for Web of Science ID 000379625400008

• Velocity correlations in laboratory insect swarms EUROPEAN PHYSICAL JOURNAL-SPECIAL TOPICS Ni, R., Ouellette, N. T. 2015; 224 (17-18): 3271-3277

  View details for DOI 10.1140/epjst/e2015-50077-5

  View details for Web of Science ID 000367309500011

• Optimal directional volatile transport in retronasal olfaction PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA Ni, R., Michalski, M. H., Brown, E., Ngoc Doan, N., Zinter, J., Ouellette, N. T., Shepherd, G. M. 2015; 112 (47): 14700-14704

  Abstract

  The ability of humans to distinguish the delicate differences in food flavors depends mostly on retronasal smell, in which food volatiles entrained into the airway at the back of the oral cavity are transported by exhaled air through the nasal cavity to stimulate the olfactory receptor neurons. Little is known whether food volatiles are preferentially carried by retronasal flow toward the nasal cavity rather than by orthonasal flow into the lung. To study the differences between retronasal and orthonasal flow, we obtained computed tomography (CT) images of the orthonasal airway from a healthy human subject, printed an experimental model using a 3D printer, and analyzed the flow field inside the airway. The results show that, during inhalation, the anatomical structure of the oropharynx creates an air curtain outside a virtual cavity connecting the oropharynx and the back of the mouth, which prevents food volatiles from being transported into the main stream toward the lung. In contrast, during exhalation, the flow preferentially sweeps through this virtual cavity and effectively enhances the entrainment of food volatiles into the main retronasal flow. This asymmetrical transport efficiency is also found to have a nonmonotonic Reynolds number dependence: The asymmetry peaks at a range of an intermediate Reynolds number close to 800, because the air curtain effect during inhalation becomes strongest in this range. This study provides the first experimental evidence, to our knowledge, for adaptations of the geometry of the human oropharynx for efficient transport of food volatiles toward the olfactory receptors in the nasal cavity.

  View details for DOI 10.1073/pnas.1511495112

  View details for Web of Science ID 000365173100080

  View details for PubMedID 26553982

  View details for PubMedCentralID PMC4664350

• Onset and cessation of motion in hydrodynamically sheared granular beds PHYSICAL REVIEW E Clark, A. H., Shattuck, M. D., Ouellette, N. T., O'Hern, C. S. 2015; 92 (4)

  Abstract

  We performed molecular dynamics simulations of granular beds driven by a model hydrodynamic shear flow to elucidate general grain-scale mechanisms that determine the onset and cessation of sediment transport. By varying the Shields number (the nondimensional shear stress at the top of the bed) and particle Reynolds number (the ratio of particle inertia to viscous damping), we explore how variations of the fluid flow rate, particle inertia, and fluid viscosity affect the onset and cessation of bed motion. For low to moderate particle Reynolds numbers, a critical boundary separates mobile and static states. Transition times between these states diverge as this boundary is approached both from above and below. At high particle Reynolds number, inertial effects become dominant, and particle motion can be sustained well below flow rates at which mobilization of a static bed occurs. We also find that the onset of bed motion (for both low and high particle Reynolds numbers) is described by Weibullian weakest-link statistics and thus is crucially dependent on the packing structure of the granular bed, even deep beneath the surface.

  View details for DOI 10.1103/PhysRevE.92.042202

  View details for Web of Science ID 000362904100005

• Onset and cessation of motion in hydrodynamically sheared granular beds. Physical review. E, Statistical, nonlinear, and soft matter physics Clark, A. H., Shattuck, M. D., Ouellette, N. T., O'Hern, C. S. 2015; 92 (4): 042202

  Abstract

  We performed molecular dynamics simulations of granular beds driven by a model hydrodynamic shear flow to elucidate general grain-scale mechanisms that determine the onset and cessation of sediment transport. By varying the Shields number (the nondimensional shear stress at the top of the bed) and particle Reynolds number (the ratio of particle inertia to viscous damping), we explore how variations of the fluid flow rate, particle inertia, and fluid viscosity affect the onset and cessation of bed motion. For low to moderate particle Reynolds numbers, a critical boundary separates mobile and static states. Transition times between these states diverge as this boundary is approached both from above and below. At high particle Reynolds number, inertial effects become dominant, and particle motion can be sustained well below flow rates at which mobilization of a static bed occurs. We also find that the onset of bed motion (for both low and high particle Reynolds numbers) is described by Weibullian weakest-link statistics and thus is crucially dependent on the packing structure of the granular bed, even deep beneath the surface.

  View details for DOI 10.1103/PhysRevE.92.042202

  View details for PubMedID 26565230

• Intrinsic Fluctuations and Driven Response of Insect Swarms PHYSICAL REVIEW LETTERS Ni, R., Puckett, J. G., Dufresne, E. R., Ouellette, N. T. 2015; 115 (11)

  Abstract

  Animals of all sizes form groups, as acting together can convey advantages over acting alone; thus, collective animal behavior has been identified as a promising template for designing engineered systems. However, models and observations have focused predominantly on characterizing the overall group morphology, and often focus on highly ordered groups such as bird flocks. We instead study a disorganized aggregation (an insect mating swarm), and compare its natural fluctuations with the group-level response to an external stimulus. We quantify the swarm's frequency-dependent linear response and its spectrum of intrinsic fluctuations, and show that the ratio of these two quantities has a simple scaling with frequency. Our results provide a new way of comparing models of collective behavior with experimental data.

  View details for DOI 10.1103/PhysRevLett.115.118104

  View details for Web of Science ID 000360964600015

  View details for PubMedID 26406859

• Correlations between the instantaneous velocity gradient and the evolution of scale-to-scale fluxes in two-dimensional flow. Physical review. E, Statistical, nonlinear, and soft matter physics Liao, Y., Ouellette, N. T. 2015; 92 (3): 033017

  Abstract

  Using high-resolution particle tracking velocimetry and filter-space techniques, we study the links between the scale-to-scale transfer of energy and enstrophy and instantaneously rotational and straining regions as determined by the classic Okubo-Weiss parameter in a quasi-two-dimensional laboratory flow. Although the Okubo-Weiss parameter has shortcomings, we find that, when suitably conditioned, it is surprisingly a good predictor for the future evolution of the spectral fluxes. By studying Lagrangian correlation functions, we explain our findings by showing that both the spectral fluxes and the Okubo-Weiss parameter are independently correlated for long times along fluid-element trajectories and thus that any predictive capacity of the Okubo-Weiss parameter arises because it is coupled to fluid advection. Our results suggest potential strategies for forecasting in complex flows by looking for quantities with long Lagrangian correlation times.

  View details for DOI 10.1103/PhysRevE.92.033017

  View details for PubMedID 26465566

• Time-Frequency Analysis Reveals Pairwise Interactions in Insect Swarms PHYSICAL REVIEW LETTERS Puckett, J. G., Ni, R., Ouellette, N. T. 2015; 114 (25)

  Abstract

  The macroscopic emergent behavior of social animal groups is a classic example of dynamical self-organization, and is thought to arise from the local interactions between individuals. Determining these interactions from empirical data sets of real animal groups, however, is challenging. Using multicamera imaging and tracking, we studied the motion of individual flying midges in laboratory mating swarms. By performing a time-frequency analysis of the midge trajectories, we show that the midge behavior can be segmented into two distinct modes: one that is independent and composed of low-frequency maneuvers, and one that consists of higher-frequency nearly harmonic oscillations conducted in synchrony with another midge. We characterize these pairwise interactions, and make a hypothesis as to their biological function.

  View details for DOI 10.1103/PhysRevLett.114.258103

  View details for Web of Science ID 000356901300006

  View details for PubMedID 26197145

• Long-range ordering of turbulent stresses in two-dimensional flow PHYSICAL REVIEW E Liao, Y., Ouellette, N. T. 2015; 91 (6)

  Abstract

  Using filter-space techniques, we study the spatial structure of the turbulent stress that couples motion on different length scales in a quasi-two-dimensional laboratory flow. As the length scale increases, we observe the appearance of long-range, system-spanning spatial order of this stress, even though the flow field remains disordered. Suggestively, this ordering occurs only in the range of scales over which we find net inverse energy transfer to larger scales. However, we find that a field built from wave vectors with random phases also displays ordering, suggesting that at least some of the ordering we observe is purely kinematic. Our results help to clarify the role played by geometric alignment in the turbulent energy cascade and highlight the importance of the scale-dependent rate of strain in the energy-transfer process.

  View details for DOI 10.1103/PhysRevE.91.063004

  View details for Web of Science ID 000356136400002

• Long-range ordering of turbulent stresses in two-dimensional flow. Physical review. E, Statistical, nonlinear, and soft matter physics Liao, Y., Ouellette, N. T. 2015; 91 (6): 063004

  Abstract

  Using filter-space techniques, we study the spatial structure of the turbulent stress that couples motion on different length scales in a quasi-two-dimensional laboratory flow. As the length scale increases, we observe the appearance of long-range, system-spanning spatial order of this stress, even though the flow field remains disordered. Suggestively, this ordering occurs only in the range of scales over which we find net inverse energy transfer to larger scales. However, we find that a field built from wave vectors with random phases also displays ordering, suggesting that at least some of the ordering we observe is purely kinematic. Our results help to clarify the role played by geometric alignment in the turbulent energy cascade and highlight the importance of the scale-dependent rate of strain in the energy-transfer process.

  View details for DOI 10.1103/PhysRevE.91.063004

  View details for PubMedID 26172789

• Empirical questions for collective-behaviour modelling PRAMANA-JOURNAL OF PHYSICS Ouellette, N. T. 2015; 84 (3): 353-363

  View details for DOI 10.1007/s12043-015-0936-5

  View details for Web of Science ID 000351177300003

• Measurements of the coupling between the tumbling of rods and the velocity gradient tensor in turbulence JOURNAL OF FLUID MECHANICS Ni, R., Kramel, S., Ouellette, N. T., Voth, G. A. 2015; 766

  View details for DOI 10.1017/jfm.2015.16

  View details for Web of Science ID 000349076900012

• Determining asymptotically large population sizes in insect swarms JOURNAL OF THE ROYAL SOCIETY INTERFACE Puckett, J. G., Ouellette, N. T. 2014; 11 (99)

  Abstract

  Social animals commonly form aggregates that exhibit emergent collective behaviour, with group dynamics that are distinct from the behaviour of individuals. Simple models can qualitatively reproduce such behaviour, but only with large numbers of individuals. But how rapidly do the collective properties of animal aggregations in nature emerge with group size? Here, we study swarms of Chironomus riparius midges and measure how their statistical properties change as a function of the number of participating individuals. Once the swarms contain order 10 individuals, we find that all statistics saturate and the swarms enter an asymptotic regime. The influence of environmental cues on the swarm morphology decays on a similar scale. Our results provide a strong constraint on how rapidly swarm models must produce collective states. But our findings support the feasibility of using swarms as a design template for multi-agent systems, because self-organized states are possible even with few agents.

  View details for DOI 10.1098/rsif.2014.0710

  View details for Web of Science ID 000341100800039

  View details for PubMedID 25121646

  View details for PubMedCentralID PMC4233756

• Extracting turbulent spectral transfer from under-resolved velocity fields PHYSICS OF FLUIDS Ni, R., Voth, G. A., Ouellette, N. T. 2014; 26 (10)

  View details for DOI 10.1063/1.4898866

  View details for Web of Science ID 000344593300040

• Searching for effective forces in laboratory insect swarms SCIENTIFIC REPORTS Puckett, J. G., Kelley, D. H., Ouellette, N. T. 2014; 4

  Abstract

  Collective animal behaviour is often modeled by systems of agents that interact via effective social forces, including short-range repulsion and long-range attraction. We search for evidence of such effective forces by studying laboratory swarms of the flying midge Chironomus riparius. Using multi-camera stereoimaging and particle-tracking techniques, we record three-dimensional trajectories for all the individuals in the swarm. Acceleration measurements show a clear short-range repulsion, which we confirm by considering the spatial statistics of the midges, but no conclusive long-range interactions. Measurements of the mean free path of the insects also suggest that individuals are on average very weakly coupled, but that they are also tightly bound to the swarm itself. Our results therefore suggest that some attractive interaction maintains cohesion of the swarms, but that this interaction is not as simple as an attraction to nearest neighbours.

  View details for DOI 10.1038/srep04766

  View details for Web of Science ID 000334836500013

  View details for PubMedID 24755944

  View details for PubMedCentralID PMC3996478

• Impact fragmentation of model flocks PHYSICAL REVIEW E Miller, P. W., Ouellette, N. T. 2014; 89 (4)

  Abstract

  Predicting the bulk material properties of active matter is challenging since these materials are far from equilibrium and standard statistical-mechanics approaches may fail. We report a computational study of the surface properties of a well known active matter system: aggregations of self-propelled particles that are coupled via an orientational interaction and that resemble bird flocks. By simulating the impact of these models flocks on an impermeable surface, we find that they fragment into subflocks with power-law mass distributions, similar to shattering brittle solids but not to splashing liquid drops. Thus, we find that despite the interparticle interactions, these model flocks do not possess an emergent surface tension.

  View details for DOI 10.1103/PhysRevE.89.042806

  View details for Web of Science ID 000339489200008

  View details for PubMedID 24827292

• Geometry of scale-to-scale energy and enstrophy transport in two-dimensional flow PHYSICS OF FLUIDS Liao, Y., Ouellette, N. T. 2014; 26 (4)

  View details for DOI 10.1063/1.4871107

  View details for Web of Science ID 000336152700038

• Direct observation of Kelvin waves excited by quantized vortex reconnection PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA Fonda, E., Meichle, D. P., Ouellette, N. T., Hormoz, S., Lathrop, D. P. 2014; 111: 4707-4710

  Abstract

  Quantized vortices are key features of quantum fluids such as superfluid helium and Bose-Einstein condensates. The reconnection of quantized vortices and subsequent emission of Kelvin waves along the vortices are thought to be central to dissipation in such systems. By visualizing the motion of submicron particles dispersed in superfluid (4)He, we have directly observed the emission of Kelvin waves from quantized vortex reconnection. We characterize one event in detail, using dimensionless similarity coordinates, and compare it with several theories. Finally, we give evidence for other examples of wavelike behavior in our system.

  View details for DOI 10.1073/pnas.1312536110

  View details for Web of Science ID 000342747200009

  View details for PubMedID 24704878

  View details for PubMedCentralID PMC3970858

• Alignment of vorticity and rods with Lagrangian fluid stretching in turbulence JOURNAL OF FLUID MECHANICS Ni, R., Ouellette, N. T., Voth, G. A. 2014; 743

  View details for DOI 10.1017/jfm.2014.32

  View details for Web of Science ID 000332844200003

• Stability of model flocks in turbulent-like flow NEW JOURNAL OF PHYSICS Khurana, N., Ouellette, N. T. 2013; 15

  View details for DOI 10.1088/1367-2630/15/9/095015

  View details for Web of Science ID 000324795400005

• Lagrangian coherent structures separate dynamically distinct regions in fluid flows PHYSICAL REVIEW E Kelley, D. H., Allshouse, M. R., Ouellette, N. T. 2013; 88 (1)

  Abstract

  Using filter-space techniques, we study the scale-to-scale transport of energy in a quasi-two-dimensional, weakly turbulent fluid flow averaged along the trajectories of fluid elements. We find that although the spatial mean of this Lagrangian-averaged flux is nearly unchanged from its Eulerian counterpart, the spatial structure of the scale-to-scale energy flux changes significantly. In particular, its features appear to correlate with the positions of Lagrangian coherent structures (LCS's). We show that the LCS's tend to lie at zeros of the scale-to-scale flux, and therefore that the LCS's separate regions that have qualitatively different dynamics. Since LCS's are also known to be impenetrable barriers to advection and mixing, we therefore find that the fluid on either side of an LCS is both kinematically and dynamically distinct. Our results extend the utility of LCS's by making clear the role they play in the flow dynamics in addition to the kinematics.

  View details for DOI 10.1103/PhysRevE.88.013017

  View details for Web of Science ID 000322533200005

  View details for PubMedID 23944559

• Spatial structure of spectral transport in two-dimensional flow JOURNAL OF FLUID MECHANICS Liao, Y., Ouellette, N. T. 2013; 725: 281-298

  View details for DOI 10.1017/jfm.2013.187

  View details for Web of Science ID 000319511200011

• Generation of Lagrangian intermittency in turbulence by a self-similar mechanism NEW JOURNAL OF PHYSICS Wilczek, M., Xu, H., Ouellette, N. T., Friedrich, R., Bodenschatz, E. 2013; 15

  View details for DOI 10.1088/1367-2630/15/5/055015

  View details for Web of Science ID 000319255300002

• Quantifying stretching and rearrangement in epithelial sheet migration NEW JOURNAL OF PHYSICS Lee, R. M., Kelley, D. H., Nordstrom, K. N., Ouellette, N. T., Losert, W. 2013; 15

  View details for DOI 10.1088/1367-2630/15/2/025036

  View details for Web of Science ID 000315525100005

• Emergent dynamics of laboratory insect swarms SCIENTIFIC REPORTS Kelley, D. H., Ouellette, N. T. 2013; 3

  Abstract

  Collective animal behaviour occurs at nearly every biological size scale, from single-celled organisms to the largest animals on earth. It has long been known that models with simple interaction rules can reproduce qualitative features of this complex behaviour. But determining whether these models accurately capture the biology requires data from real animals, which has historically been difficult to obtain. Here, we report three-dimensional, time-resolved measurements of the positions, velocities, and accelerations of individual insects in laboratory swarms of the midge Chironomus riparius. Even though the swarms do not show an overall polarisation, we find statistical evidence for local clusters of correlated motion. We also show that the swarms display an effective large-scale potential that keeps individuals bound together, and we characterize the shape of this potential. Our results provide quantitative data against which the emergent characteristics of animal aggregation models can be benchmarked.

  View details for DOI 10.1038/srep01073

  View details for Web of Science ID 000313551300001

  View details for PubMedID 23323215

  View details for PubMedCentralID PMC3545223

• On the dynamical role of coherent structures in turbulence COMPTES RENDUS PHYSIQUE Ouellette, N. T. 2012; 13 (9-10): 866-877

  View details for DOI 10.1016/j.crhy.2012.09.006

  View details for Web of Science ID 000312513200002

• Effects of forcing geometry on two-dimensional weak turbulence PHYSICAL REVIEW E Liao, Y., Kelley, D. H., Ouellette, N. T. 2012; 86 (3)

  Abstract

  Using high-resolution particle tracking velocimetry, we study the effects of the forcing geometry on the statistics of an electromagnetically stirred thin-layer flow. We consider two forcing arrangements: one that produces a lattice of vortices as a base flow, and one that produces an array of shear bands. We find that the vortex flow drives stronger fluctuating kinetic energy while the shear-band flow leads to more intense fluctuating velocity gradients. We explain our results by considering the spectral flow of energy in the system. Our results have implications for the design of two-dimensional flow experiments.

  View details for DOI 10.1103/PhysRevE.86.036306

  View details for Web of Science ID 000308530800004

  View details for PubMedID 23031012

• Interactions between active particles and dynamical structures in chaotic flow PHYSICS OF FLUIDS Khurana, N., Ouellette, N. T. 2012; 24 (9)

  View details for DOI 10.1063/1.4754873

  View details for Web of Science ID 000309425800016

• Turbulence in two dimensions PHYSICS TODAY Ouellette, N. T. 2012; 65 (5): 68–69

  View details for DOI 10.1063/PT.3.1570

  View details for Web of Science ID 000303909100027

• Spatiotemporal persistence of spectral fluxes in two-dimensional weak turbulence PHYSICS OF FLUIDS Kelley, D. H., Ouellette, N. T. 2011; 23 (11)

  View details for DOI 10.1063/1.3657086

  View details for Web of Science ID 000297939200028

• Path Lengths in Turbulence JOURNAL OF STATISTICAL PHYSICS Ouellette, N. T., Bodenschatz, E., Xu, H. 2011; 145 (1): 93-101

  View details for DOI 10.1007/s10955-011-0323-7

  View details for Web of Science ID 000297134800004

• Neutrally buoyant particle dynamics in fluid flows: Comparison of experiments with Lagrangian stochastic models PHYSICS OF FLUIDS Sapsis, T. P., Ouellette, N. T., Gollub, J. P., Haller, G. 2011; 23 (9)

  View details for DOI 10.1063/1.3632100

  View details for Web of Science ID 000295621800034

• Mechanisms driving shape distortion in two-dimensional flow EPL Quitry, A. d., Kelley, D. H., Ouellette, N. T. 2011; 94 (6)

  View details for DOI 10.1209/0295-5075/94/64006

  View details for Web of Science ID 000291554400016

• Separating stretching from folding in fluid mixing NATURE PHYSICS Kelley, D. H., Ouellette, N. T. 2011; 7 (6): 477-480

  View details for DOI 10.1038/NPHYS1941

  View details for Web of Science ID 000291146500015

• Reduced Transport of Swimming Particles in Chaotic Flow due to Hydrodynamic Trapping PHYSICAL REVIEW LETTERS Khurana, N., Blawzdziewicz, J., Ouellette, N. T. 2011; 106 (19)

  Abstract

  We computationally study the transport of active, self-propelled particles suspended in a two-dimensional chaotic flow. The pointlike, spherical particles have their own intrinsic swimming velocity, which modifies the dynamical system so that the particles can break the transport barriers present in the carrier flow. Surprisingly, we find that swimming does not necessarily lead to enhanced particle transport. Small but finite swimming speed can result in reduced transport, as swimmers get stuck for long times in traps that form near elliptic islands in the background flow. Our results have implications for models of transport and encounter rates for small marine organisms.

  View details for DOI 10.1103/PhysRevLett.106.198104

  View details for Web of Science ID 000290728500020

  View details for PubMedID 21668206

• Rotation and alignment of rods in two-dimensional chaotic flow PHYSICS OF FLUIDS Parsa, S., Guasto, J. S., Kishore, M., Ouellette, N. T., Gollub, J. P., Voth, G. A. 2011; 23 (4)

  View details for DOI 10.1063/1.3570526

  View details for Web of Science ID 000290050000020

• Onset of three-dimensionality in electromagnetically driven thin-layer flows PHYSICS OF FLUIDS Kelley, D. H., Ouellette, N. T. 2011; 23 (4)

  View details for DOI 10.1063/1.3570685

  View details for Web of Science ID 000290050000031

• Using particle tracking to measure flow instabilities in an undergraduate laboratory experiment AMERICAN JOURNAL OF PHYSICS Kelley, D. H., Ouellette, N. T. 2011; 79 (3): 267-273

  View details for DOI 10.1119/1.3536647

  View details for Web of Science ID 000287407900004

• Scale-local velocity fields from particle-tracking data CHAOS Kelley, D. H., Ouellette, N. T. 2010; 20 (4)

  View details for DOI 10.1063/1.3489891

  View details for Web of Science ID 000285768500006

  View details for PubMedID 21198067

• Scale-Dependent Statistical Geometry in Two-Dimensional Flow PHYSICAL REVIEW LETTERS Merrifield, S. T., Kelley, D. H., Ouellette, N. T. 2010; 104 (25)

  Abstract

  By studying the shape dynamics of three-particle clusters, we investigate the statistical geometry of a spatiotemporally chaotic experimental quasi-two-dimensional flow. We show that when shape and size are appropriately decoupled, these Lagrangian triangles assume statistically stationary shape distributions that depend on the flow scale, with smaller scales favoring more distorted triangles. These preferred shapes are not due to trapping by Eulerian flow structures. Since our flow does not have developed turbulent cascades, our results suggest that more careful work is required to understand the specific effects of turbulence on the advection of Lagrangian clusters.

  View details for DOI 10.1103/PhysRevLett.104.254501

  View details for Web of Science ID 000279038100001

  View details for PubMedID 20867385

• Bulk turbulence in dilute polymer solutions JOURNAL OF FLUID MECHANICS Ouellette, N. T., Xu, H., Bodenschatz, E. 2009; 629: 375-385

  View details for DOI 10.1017/S0022112009006697

  View details for Web of Science ID 000267770200017

• Detecting topological features of chaotic fluid flow CHAOS Ouellette, N. T., Gollub, J. P. 2008; 18 (4)

  View details for DOI 10.1063/1.2997332

  View details for Web of Science ID 000262224600002

  View details for PubMedID 19123602

• Transport of Finite-Sized Particles in Chaotic Flow PHYSICAL REVIEW LETTERS Ouellette, N. T., O'Malley, P. J., Gollub, J. P. 2008; 101 (17)

  Abstract

  By extending traditional particle tracking techniques, we study the dynamics of neutrally buoyant finite-sized particles in a spatiotemporally chaotic flow. We simultaneously measure the flow field and the trajectories of millimeter-scale particles so that the two can be directly compared. While the single-point statistics of the particles are indistinguishable from the flow statistics, the particles often move in directions that are systematically different from the underlying flow. These differences are especially evident when Lagrangian statistics are considered.

  View details for DOI 10.1103/PhysRevLett.101.174504

  View details for Web of Science ID 000260383600031

  View details for PubMedID 18999753

• Universal intermittent properties of particle trajectories in highly turbulent flows PHYSICAL REVIEW LETTERS Arneodo, A., Benzi, R., Berg, J., Biferale, L., Bodenschatz, E., Busse, A., Calzavarini, E., Castaing, B., Cencini, M., Chevillard, L., Fisher, R. T., Grauer, R., Homann, H., Lamb, D., Lanotte, A. S., Leveque, E., Luethi, B., Mann, J., Mordant, N., Mueller, W., Ott, S., Ouellette, N. T., Pinton, J., Pope, S. B., Roux, S. G., Toschi, F., Xu, H., Yeung, P. K. 2008; 100 (25)

  Abstract

  We present a collection of eight data sets from state-of-the-art experiments and numerical simulations on turbulent velocity statistics along particle trajectories obtained in different flows with Reynolds numbers in the range R{lambda}in[120:740]. Lagrangian structure functions from all data sets are found to collapse onto each other on a wide range of time lags, pointing towards the existence of a universal behavior, within present statistical convergence, and calling for a unified theoretical description. Parisi-Frisch multifractal theory, suitably extended to the dissipative scales and to the Lagrangian domain, is found to capture the intermittency of velocity statistics over the whole three decades of temporal scales investigated here.

  View details for DOI 10.1103/PhysRevLett.100.254504

  View details for Web of Science ID 000257230500028

  View details for PubMedID 18643666

• Lagrangian structure functions in turbulence: A quantitative comparison between experiment and direct numerical simulation PHYSICS OF FLUIDS Biferale, L., Bodenschatz, E., Cencini, M., Lanotte, A. S., Ouellette, N. T., Toschi, F., Xu, H. 2008; 20 (6)

  View details for DOI 10.1063/1.2930672

  View details for Web of Science ID 000257283800027

• Dynamic topology in spatiotemporal chaos PHYSICS OF FLUIDS Ouellette, N. T., Gollub, J. P. 2008; 20 (6)

  View details for DOI 10.1063/1.2948849

  View details for Web of Science ID 000257283800024

• Evolution of geometric structures in intense turbulence NEW JOURNAL OF PHYSICS Xu, H., Ouellette, N. T., Bodenschatz, E. 2008; 10

  View details for DOI 10.1088/1367-2630/10/1/013012

  View details for Web of Science ID 000253084000004

• Acceleration correlations and pressure structure functions in high-reynolds number turbulence PHYSICAL REVIEW LETTERS Xu, H., Ouellette, N. T., Vincenzi, D., Bodenschatz, E. 2007; 99 (20)

  Abstract

  We present measurements of fluid particle accelerations in turbulent water flow between counterrotating disks using three-dimensional Lagrangian particle tracking. By simultaneously following multiple particles with sub-Kolmogorov-time-scale temporal resolution, we measured the spatial correlation of fluid particle acceleration at Taylor microscale Reynolds numbers between 200 and 690. We also obtained indirect, nonintrusive measurements of the Eulerian pressure structure functions by integrating the acceleration correlations. Our measurements are in good agreement with the theoretical predictions of the acceleration correlations and the pressure structure function in isotropic high-Reynolds number turbulence by Obukhov and Yaglom in 1951 [Prikl. Mat. Mekh. 15, 3 (1951)]. The measured pressure structure functions display K41 scaling in the inertial range.

  View details for DOI 10.1103/PhysRevLett.99.204501

  View details for Web of Science ID 000251003600024

  View details for PubMedID 18233145

• Curvature fields, topology, and the dynamics of spatiotemporal chaos PHYSICAL REVIEW LETTERS Ouellette, N. T., Gollub, J. P. 2007; 99 (19)

  Abstract

  The curvature field is measured from tracer-particle trajectories in a two-dimensional fluid flow that exhibits spatiotemporal chaos and is used to extract the hyperbolic and elliptic points of the flow. These special points are pinned to the forcing when the driving is weak, but wander over the domain and interact in pairs at stronger driving, changing the local topology of the flow. Their behavior reveals a two-stage transition to spatiotemporal chaos: a gradual loss of spatial and temporal order followed by an abrupt onset of topological changes.

  View details for DOI 10.1103/PhysRevLett.99.194502

  View details for Web of Science ID 000250810500033

  View details for PubMedID 18233080

• Curvature of Lagrangian trajectories in turbulence PHYSICAL REVIEW LETTERS Xu, H., Ouellette, N. T., Bodenschatz, E. 2007; 98 (5)

  Abstract

  We report measurements of the curvature of Lagrangian trajectories in an intensely turbulent laboratory water flow measured with a high-speed particle-tracking system. The probability density function (PDF) of the instantaneous curvature is shown to have robust power-law tails. We propose a model for the instantaneous curvature PDF, assuming that the acceleration and velocity are uncorrelated Gaussian random variables, and show that our model reproduces the tails of our measured PDFs. We also predict the scaling of the most probable vorticity magnitude in turbulence, assuming Heisenberg-Yaglom scaling. Finally, we average the curvature along trajectories and show that, by removing the effects of large-scale flow reversals, the filtered curvature reveals the turbulent features.

  View details for DOI 10.1103/PhysRevLett.98.050201

  View details for Web of Science ID 000244646100001

  View details for PubMedID 17358827

• Multi-particle statistics - lines, shapes, and volumes in high Reynolds number turbulence PROCEEDINGS OF THE 5TH INTERNATIONAL CONFERENCE ON NONLINEAR MECHANICS Xu, H., Ouellette, N. T., Bodenschatz, E. 2007: 1155-1161

  View details for Web of Science ID 000250953200211

• Experimental measurements of Lagrangian statistics in intense turbulence 11th EUROMECH European Turbulence Conference Xu, H., Ouellette, N. T., Nobach, H., Bodenschatz, E. SPRINGER-VERLAG BERLIN. 2007: 1–10

  View details for Web of Science ID 000251779600001

• Lagrangian particle tracking in high Reynolds number turbulence PARTICLE-LADEN FLOW: FROM GEOPHYSICAL TO KOLMOGOROV SCALES Chang, K., Ouellette, N. T., Xu, H., Bodenschatz, E. 2007; 11: 299-311

  View details for Web of Science ID 000251017300024

• An experimental study of turbulent relative dispersion models NEW JOURNAL OF PHYSICS Ouellette, N. T., Xu, H., Bourgoin, M., Bodenschatz, E. 2006; 8

  View details for DOI 10.1088/1367-2630/8/6/109

  View details for Web of Science ID 000238685800003

• Small-scale anisotropy in Lagrangian turbulence NEW JOURNAL OF PHYSICS Ouellette, N. T., Xu, H., Bourgoin, M., Bodenschatz, E. 2006; 8

  View details for DOI 10.1088/1367-2630/8/6/102

  View details for Web of Science ID 000238292500001

• Multifractal dimension of Lagrangian turbulence PHYSICAL REVIEW LETTERS Xu, H. T., Ouellette, N. T., Bodenschatz, E. 2006; 96 (11)

  Abstract

  We report experimental measurements of the Lagrangian multifractal dimension spectrum in an intensely turbulent laboratory water flow by the optical tracking of tracer particles. The Legendre transform of the measured spectrum is compared with measurements of the scaling exponents of the Lagrangian velocity structure functions, and excellent agreement between the two measurements is found, in support of the multifractal picture of turbulence. These measurements are compared with three model dimension spectra. When the nonexistence of structure functions of order less than -1 is accounted for, the models are shown to agree well with the measured spectrum.

  View details for DOI 10.1103/PhysRevLett.96.114503

  View details for Web of Science ID 000236249900031

  View details for PubMedID 16605829

• The role of pair dispersion in turbulent flow SCIENCE Bourgoin, M., Ouellette, N. T., Xu, H. T., Berg, J., Bodenschatz, E. 2006; 311 (5762): 835-838

  Abstract

  Mixing and transport in turbulent flows-which have strong local concentration fluctuations-are essential in many natural and industrial systems including reactions in chemical mixers, combustion in engines and burners, droplet formation in warm clouds, and biological odor detection and chemotaxis. Local concentration fluctuations, in turn, are intimately tied to the problem of the separation of pairs of fluid elements. We have measured this separation rate in an intensely turbulent laboratory flow and have found, in quantitative agreement with the seminal predictions of Batchelor, that the initial separation of the pair plays an important role in the subsequent spreading of the fluid elements. These results have surprising consequences for the decay of concentration fluctuations and have applications to biological and chemical systems.

  View details for DOI 10.1126/science.1121726

  View details for Web of Science ID 000235374900044

  View details for PubMedID 16469922

• A quantitative study of three-dimensional Lagrangian particle tracking algorithms EXPERIMENTS IN FLUIDS Ouellette, N. T., Xu, H. T., Bodenschatz, E. 2006; 40 (2): 301-313

  View details for DOI 10.1007/s00348-005-0068-7

  View details for Web of Science ID 000234755400015

• High order Lagrangian velocity statistics in turbulence PHYSICAL REVIEW LETTERS Xu, H., Bourgoin, M., Ouellette, N. T., Bodenschatz, E. 2006; 96 (2)

  Abstract

  We report measurements of the Lagrangian velocity structure functions of orders 1 through 10 in a high Reynolds number (Taylor microscale Reynolds numbers of up to R(lambda) = 815 ) turbulence experiment. Passive tracer particles are tracked optically in three dimensions and in time, and velocities are calculated from the particle tracks. The structure function anomalous scaling exponents are measured both directly and using extended self-similarity and are found to be more intermittent than their Eulerian counterparts. Classical Kolmogorov inertial range scaling is also found for all structure function orders at times that trend downward as the order increases. The temporal shift of this classical scaling behavior is observed to saturate as the structure function order increases at times shorter than the Kolmogorov time scale.

  View details for DOI 10.1103/PhysRevLett.96.024503

  View details for Web of Science ID 000234758100044

  View details for PubMedID 16486587