Bio


Nick Ouellette is broadly interested the behavior of complex systems far from equilibrium. In particular, a running theme in his research is dynamical self-organization. He seeks both to understand the physical principles that govern the spontaneous emergence of low-dimensional structure in high-dimensional systems and to harness this self-organization for engineering applications. His current research includes studies of turbulent flows in two and three dimensions, in both simple and complex fluids; the transport of inertial, anisotropic, and active particles in turbulence; the erosion of granular beds by fluid flows and subsequent sediment transport; and quantitative measurements of collective behavior in insect swarms and other animal groups.

Before coming to Stanford in 2015, Ouellette spent seven years on the faculty in Mechanical Engineering and Materials Science at Yale University, where he won the Yale Provost's Teaching Prize in 2014. Before beginning his faculty career, he held postdoctoral positions at the Max Planck Institute for Dynamics and Self-Organization and in the Physics Department at Haverford College.

Academic Appointments


Professional Education


  • Ph.D., Cornell University, Physics (2006)
  • M.S., Cornell University, Physics (2005)
  • B.A., Swarthmore College, Physics and Computer Science (2002)

Current Research and Scholarly Interests


The Environmental Complexity Lab studies self-organization in a variety of complex systems, ranging from turbulent fluid flows to granular materials to collective motion in animal groups. In all cases, we aim to characterize the macroscopic behavior, understand its origin in the microscopic dynamics, and ultimately harness it for engineering applications. Most of our projects are experimental, though we also use numerical simulation and mathematical modeling when appropriate. We specialize in high-speed, detailed imaging and statistical analysis.

Our current research includes studies of turbulence in two and three dimensions, with a focus on coherent structures and the geometry of turbulence; the transport of inertial, anisotropic, and active particles in turbulence; the erosion of granular beds by fluid flows and subsequent sediment transport; quantitative measurements of collective behavior in insect swarms and bird flocks; the stability of ocean ecosystems; neural signal processing; and uncovering the natural, self-organized spatiotemporal scales in urban systems.

Stanford Advisees


All Publications


  • 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)
  • Generalized Lagrangian coherent structures PHYSICA D-NONLINEAR PHENOMENA Balasuriya, S., Ouellette, N. T., Rypina, I. I. 2018; 372: 31–51
  • 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 DOI 10.3389/fnhum.2018.00173

    View details for Web of Science ID 000431524500001

    View details for PubMedID 29867405

    View details for PubMedCentralID PMC5950731

  • 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
  • Transport of anisotropic particles under waves JOURNAL OF FLUID MECHANICS DiBenedetto, M. H., Ouellette, N. T., Koseff, J. R. 2018; 837: 320–40
  • 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 DOI 10.1063/1.5003893

    View details for Web of Science ID 000427002200014

    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
  • 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)
  • 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 DOI 10.1103/PhysRevLett.119.178003

    View details for Web of Science ID 000413516800013

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

  • 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

  • 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

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

  • 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)
  • Correlations between the instantaneous velocity gradient and the evolution of scale-to-scale fluxes in two-dimensional flow PHYSICAL REVIEW E Liao, Y., Ouellette, N. T. 2015; 92 (3)
  • 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

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

  • 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

  • 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

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

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

  • 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
  • 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)
  • 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
  • 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
  • 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
  • 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
  • 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
  • An experimental study of turbulent relative dispersion models NEW JOURNAL OF PHYSICS Ouellette, N. T., Xu, H., Bourgoin, M., Bodenschatz, E. 2006; 8
  • Small-scale anisotropy in Lagrangian turbulence NEW JOURNAL OF PHYSICS Ouellette, N. T., Xu, H., Bourgoin, M., Bodenschatz, E. 2006; 8
  • 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
  • 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