About

Nick Ouellette is a Professor of Civil and Environmental Engineering at Stanford University and leads the Environmental Complexity Lab. He is broadly interested in the behavior of complex systems far from equilibrium, with a particular focus on dynamical self-organization. His research seeks to understand the physical principles governing the spontaneous emergence of low-dimensional structure in high-dimensional systems and to harness this self-organization for engineering applications. His research interests include turbulent flows in two and three dimensions, the transport of inertial, anisotropic, and active particles in turbulence, fluid-driven erosion of granular beds and sediment transport, quantitative measurements of collective behavior in insect swarms and bird flocks, and emergent, self-organized structure and dynamics in cities.

Research topics

• Biology
• Computer Science
• Artificial Intelligence
• Ecology
• Evolutionary biology
• Statistical physics
• Epistemology
• Psychology
• Data science
• Sociology
• Physics
• Cognitive science
• Theoretical physics
• Mathematics education
• Biological system
• Classical mechanics
• Mathematics
• Environmental science
• Environmental health
• Philosophy
• Geography
• Quantum mechanics
• Management science
• Engineering

Selected publications

• Effect of sub-critical fluid shear flow on granular bed strength

  arXiv (Cornell University) · 2026-04-16

  preprintOpen access

  Interactions between fluids and granular materials are prevalent on the Earth's surface. In the case of fluid flow over a sediment bed, the fluid imparts a shear stress to the granular materials. When the applied shear stress is above a critical value, the grains become entrained in the fluid flow. Prior experimental studies have shown that granular beds subjected to a sub-critical fluid flow can strengthen in the same direction as the sub-critical flow. In contrast, granular beds can become weaker in the direction opposite to the sub-critical fluid flow. To investigate the grain-scale mechanisms that control directional strengthening and weakening, we perform discrete element method (DEM) simulations of granular beds subjected to model fluid flows in two (2D) and three (3D) dimensions with varied inter-particle static friction coefficients and conditioning flow speeds. In these studies, the sub-critical grain motion does not cause significant bed compaction. Instead, we find that the strength of a granular bed in a particular direction is highly correlated with the fraction of {\it surface} grains that can be dislodged by a fluid force applied in that direction. Further, the anisotropic bed strength only persists over a finite time scale that is set by the Shields number. We also show that inter-particle static friction is not required for bed strength anisotropy, but varying the friction affects the magnitude of the anisotropy. This research enhances the grain-scale understanding of erosion of granular beds caused by fluid flows and underscores the importance of tracking the history of the fabric of the bed surface since it couples strongly to bed strength.

  PublisherDOI

• Effect of sub-critical fluid shear flow on granular bed strength

  ArXiv.org · 2026-04-16

  articleOpen access

  Interactions between fluids and granular materials are prevalent on the Earth's surface. In the case of fluid flow over a sediment bed, the fluid imparts a shear stress to the granular materials. When the applied shear stress is above a critical value, the grains become entrained in the fluid flow. Prior experimental studies have shown that granular beds subjected to a sub-critical fluid flow can strengthen in the same direction as the sub-critical flow. In contrast, granular beds can become weaker in the direction opposite to the sub-critical fluid flow. To investigate the grain-scale mechanisms that control directional strengthening and weakening, we perform discrete element method (DEM) simulations of granular beds subjected to model fluid flows in two (2D) and three (3D) dimensions with varied inter-particle static friction coefficients and conditioning flow speeds. In these studies, the sub-critical grain motion does not cause significant bed compaction. Instead, we find that the strength of a granular bed in a particular direction is highly correlated with the fraction of {\it surface} grains that can be dislodged by a fluid force applied in that direction. Further, the anisotropic bed strength only persists over a finite time scale that is set by the Shields number. We also show that inter-particle static friction is not required for bed strength anisotropy, but varying the friction affects the magnitude of the anisotropy. This research enhances the grain-scale understanding of erosion of granular beds caused by fluid flows and underscores the importance of tracking the history of the fabric of the bed surface since it couples strongly to bed strength.

  PublisherOA PDF

• Anisotropic Stress History Effects in Erodible Sediment Beds

  Journal of Geophysical Research Earth Surface · 2026-01-01 · 1 citations

  articleSenior author

  Abstract Bedload transport occurs when the shear stress, or non‐dimensional Shields stress, imparted by a fluid onto a sediment bed exceeds a critical value for sediment entrainment. The history of fluid stress imparted onto a sediment bed influences this critical Shields stress, with bed strengthening occurring under unidirectional flows and bed weakening occurring when the flow direction is reversed. In this study, we examine directional strengthening and weakening in a sediment bed for multiple fluid stress orientations using a rotating bed of sand in a laboratory flume. This sediment bed is exposed to an initial subcritical conditioning flow followed by a subsequent erosive flow at an offset angle of , , , , or . We identify the particle trajectories of a population of sediment grains to measure their velocity, activity, and associated bulk statistics. We confirm bed strengthening (i.e., lower grain velocity and activity) in the unidirectional case, especially for flows at or below the nominal critical Shields stress. As the angular offset increases between the conditioning and erosive flows, both grain velocity and activity increase, with the greatest bed weakening at offsets of and . Our results confirm that stress history is stored anisotropically in the sediment bed, supporting mechanisms such as shear jamming where an anisotropic granular fabric develops in response to shear. These results inform our understanding of how subcritical and critical fluid‐imposed stresses can modify the grain contact and force networks in geophysical contexts.

  PublisherDOI

• How will AI affect patent disclosures?

  Nature Biotechnology · 2025-01-01 · 3 citations

  articleSenior author
  PublisherDOI

• Transport of rod-shaped particles in a canopy flow with a buoyant plume

  International Journal of Multiphase Flow · 2025-06-24

  article
  PublisherDOI

• Expected correlation in time-series analysis

  Physical review. E · 2025-02-14

  articleSenior author

  Time-series analysis often involves the characterization of order or predictability, qualities that are related to internal structure and autocorrelation. Investigating a recently proposed algorithm for solving a density prediction task, we demonstrate that if the same system can be viewed on multiple time scales, there is an inevitable degree of expected order and predictability that increases as the system size grows. In particular, we introduce bounds on the expected second-order structure function and autocorrelation function of a time series where multiple observation scales are available, and conclude with a lower bound on the expected correlation time. Such a lower bound shows that there is an inevitable degree of correlation induced when time-series data is aggregated, quantifying a previously overlooked source of bias towards high correlations.

  PublisherDOI

• Settling of actively buoyant particles

  Journal of Fluid Mechanics · 2025-12-01

  articleSenior authorCorresponding

  Not all particulate matter carried by fluid flows has constant buoyancy. In some cases, the buoyancy of a particle can change dynamically based on the local flow. We refer to this phenomenon as ‘active buoyancy.’ Although actively buoyant particles are found throughout nature, their dynamics is not well understood, particularly when they are also highly inertial. Motivated by the problem of the transport of firebrands in wildfires, whose effective buoyancy is modulated by conductive and convective heat transfer to the surrounding fluid, we conducted a series of experiments to investigate the effects of active buoyancy on particle settling in quiescent fluid. We find that, depending on the control parameters, active buoyancy can either hinder or enhance settling, in some cases to a large extent. The details of this settling modulation, however, cannot be simply captured by any single control parameter. Analysis of the trajectories of the falling particles showed that they fall along nearly sinusoidal paths even though the particle Reynolds number is higher than expected for this regime, suggesting that active buoyancy may act to stabilise their wakes. Our results suggest both that models of actively buoyant particles such as firebrands must account for the effects of active buoyancy and that there is still much to be understood about the behaviour of these complex particles.

  PublisherDOI

• Towards mechanistic understanding in a data-driven weather model: internal activations reveal interpretable physical features

  ArXiv.org · 2025-12-30

  articleOpen accessSenior author

  Large data-driven physics models like DeepMind's weather model GraphCast have empirically succeeded in parameterizing time operators for complex dynamical systems with an accuracy reaching or in some cases exceeding that of traditional physics-based solvers. Unfortunately, how these data-driven models perform computations is largely unknown and whether their internal representations are interpretable or physically consistent is an open question. Here, we adapt tools from interpretability research in Large Language Models to analyze intermediate computational layers in GraphCast, leveraging sparse autoencoders to discover interpretable features in the neuron space of the model. We uncover distinct features on a wide range of length and time scales that correspond to tropical cyclones, atmospheric rivers, diurnal and seasonal behavior, large-scale precipitation patterns, specific geographical coding, and sea-ice extent, among others. We further demonstrate how the precise abstraction of these features can be probed via interventions on the prediction steps of the model. As a case study, we sparsely modify a feature corresponding to tropical cyclones in GraphCast and observe interpretable and physically consistent modifications to evolving hurricanes. Such methods offer a window into the black-box behavior of data-driven physics models and are a step towards realizing their potential as trustworthy predictors and scientifically valuable tools for discovery.

  PublisherOA PDF

• Anisotropic stress history effects in erodible sediment beds

  2025-12-08

  articleSenior author

  Bedload transport occurs when the shear stress, or non-dimensional Shields stress, imparted by a fluid onto a sediment bed exceeds a critical value for sediment entrainment. The history of fluid stress imparted onto a sediment bed influences this critical Shields stress, with bed strengthening occurring under unidirectional flows and bed weakening occurring when the flow direction is reversed. In this study, we examine directional strengthening and weakening in a sediment bed for multiple fluid stress orientations using a rotating bed of sand in a laboratory flume. This sediment bed is exposed to an initial subcritical conditioning flow followed by a subsequent erosive flow at an offset angle of 0º, 45º, 90º, 135º, or 180º. We identify the particle trajectories of a population of sediment grains to measure their velocity, activity, and associated bulk statistics. We confirm bed strengthening (i.e., lower grain velocity and activity) in the unidirectional case, especially for flows at or below the nominal critical Shields stress. As the angular offset increases between the conditioning and erosive flows, both grain velocity and activity increase, with the greatest bed weakening at offsets of 135° and 180°. Our results confirm that stress history is stored anisotropically in the sediment bed, supporting mechanisms such as shear jamming where an anisotropic granular fabric develops in response to shear. These results inform our understanding of how subcritical and critical fluid-imposed stresses can modify the grain contact and force networks in geophysical contexts.

  PublisherDOI

• Nonballistic transport of particles in a canopy-plume system

  Physical Review Fluids · 2025-05-06 · 1 citations

  article

  We investigated the transport of model firebrands in a canopy-plume system to explore which physical parameters influenced their landing positions. Broad landing distributions motivated a division of particle populations based on landing distance. Particles that travelled further spent more time in the plume and remained higher in the water column. We found that common assumptions in wildfire literature, such as ballistic transport of firebrands, did not capture the dynamics we observed. Stochastic simulations suggested that spatiotemporal coherence in the flow field is an important reason why this ballistic assumption fails.

  PublisherDOI

Recent grants

• Connecting Dynamical Structure and Statistical Analysis in Quasi-2D Fluid Flow

  NSF · $320k · 2009–2012

• Linking Spatial and Spectral Transport in Two-Dimensional Fluid Flow

  NSF · $330k · 2012–2015

• Geometric Structure of the Turbulent Cascade

  NSF · $337k · 2017–2021

• Toward the Design and Control of Dynamical Transport Barriers in Nonlinear Flow

  NSF · $345k · 2016–2020

• Collaborative Research: Theories and Experiments on Scalar Mixing in Chaotic Flows

  NSF · $93k · 2012–2015

Frequent coauthors

• Eberhard Bodenschatz

  111 shared

• Haitao Xu

  Tsinghua University

  96 shared

• Jeffrey R. Koseff

  Mechanics' Institute

  58 shared

• Mickaël Bourgoin

  Centre National de la Recherche Scientifique

  45 shared

• Douglas H. Kelley

  35 shared

• Jacob Berg

  University of Washington

  29 shared

• Kasper van der Vaart

  University of Twente

  28 shared

• Michael Sinhuber

  Carl von Ossietzky Universität Oldenburg

  26 shared

Education

• Ph.D., Civil and Environmental Engineering

  Stanford University

  2009

• M.S., Civil and Environmental Engineering

  Stanford University

  2005

• B.S., Civil Engineering

  University of California, Berkeley

  2003

Awards & honors

• Fellow of the American Physical Society