Research
Particle Transport in Fluid Flows
In a vast array of situations in nature and industry, fluid flows transport particulate matter. In some situations, these particles are purely carried by the flow: they move just as fluid elements do. But in the more generic case, the particles do not follow the flow and have their own, complex dynamics. This situation may arise due to density differences between the particles and the carrier fluid, to particle size or shape, or to intrinsic particle activity. The modification of particle transport by such effects can have surprising consequences, ranging from the acceleration of rain formation in clouds to the formation of phytoplankton layers in the ocean. We are interested in understanding how the underlying particle and flow properties lead to the macroscopic behavior. We aim both to gain insight into geophysical processes and to harness such effects for engineering applications.
Aspherical Particles
Spherical particles with finite size may not follow the flow, since they feel a velocity field that is averaged over their surface. But when the particles are not round, even more complex dynamics may result. Now the particles couple not only to the flow advection and rotation but also to the strain rate. In addition, if the particles do not have fore/aft symmetry, their transport dynamics may be coupled to the body shape (especially if the particle Reynolds number is appreciable).
We are studying the behavior of aspherical particles in both 2D and 3D turbulence by extending our particle-tracking tools. In 2D, we can simultaneoulsy measure the flow field and the particle dynamics, allowing us to pinpoint the detailed deviations between the particles and the flow. In 3D, where the flow lengthscales are smaller, we measure only the particle dynamics and infer the dynamics from the modification of the statistical properties.
Active Particles
Some particles have their own intrinsic motility; that is, they can swim. Although most typically this particle "activity" is biological (for living particles like plankton), it can come from mechanical or chemical effects. In all cases, self-propelled particles clearly do not follow the flow!
We have been studying the behavior of active particles using simple computational models. Our primary interest has been to understand how particle activity and complex (chaotic or turbulent) flow fields give rise to unexpected behavior. We have found, for example, that small levels of particle motility (only a few percent of the overall flow speed) can surprisingly lead to slower overall particle transport, as the particles often become stuck in non-mixing regions of the flow. These effects are enhanced when the swimmers are elongated. But when the particles swim just a bit faster, their transport can be significanly enhanced relative to a fluid element. We are currently interested in understanding how particle motility, particularly when it is variable, can modify encounter rates for small organisms.
Representative Publications
N. T. Ouellette, P. J. J. O'Malley, and J. P. Gollub, "Transport of finite-sized particles in chaotic flow," Phys. Rev. Lett. 101, 174504 (2008).
S. Parsa, J. S. Guasto, M. Kishore, N. T. Ouellette, J. P. Gollub, and G. A. Voth, "Rotation and alignment of rods in two-dimensional chaotic flow," Phys. Fluids 23, 043302 (2011).
T. P. Sapsis, N. T. Ouellette, J. P. Gollub, and G. Haller, "Neutrally buoyant particle dynamics in fluid flows: Comparison of experiments with Lagrangian stochastic models," Phys. Fluids 23, 093304 (2011).
N. Khurana, J. Blawzdziewicz, and N. T. Ouellette, "Reduced transport of swimming particles in chaotic flow due to hydrodynamic trapping," Phys. Rev. Lett. 106, 198104 (2011).
N. Khurana and N. T. Ouellette, "Interactions between active particles and dynamical structures in chaotic flow," Phys. Fluids 24, 091902 (2012).
R. Ni, N. T. Ouellette, and G. A. Voth, "Alignment of vorticity and rods with Lagrangian fluid stretching in turbulence," J. Fluid Mech. 743, R3 (2014).
R. Ni, S. Kramel, N. T. Ouellette, and G. A. Voth, "Measurements of the coupling between the tumbling of rods and the velocity gradient tensor in turbulence," J. Fluid Mech. 766, 202-225 (2015).