We pursue research across a range of topics under the overall theme of fluid mechanics in environmental processes:

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Particle motion in environmental and industrial systems: sediment, microswimmers, microplastics, etc.

Under this topic, we investigate the dynamics of particles in fluid motion in various contexts.

Marine debris and microplastics are found everywhere in the aquatic environment. On a global scale, plastics in the environment are increasing due to continued inputs from anthropogenic activities and from the breakdown of larger plastic pieces due to fragmentation and degradation. On more local scales, the complexities of debris and microplastic distributions and their transport mechanisms are only starting to be understood. We are interested in improving predictions of how these emerging contaminants are transported and transformed by flow in various settings.

When small organisms travel through a turbulent flow, they are subject to powerful fluid vortices and currents that they must navigate. We are interested in understanding how they do this and still manage to find food, find mating partners, and avoid predators. By looking at simplified models of such swimmers, we investigate how their shape and swimming ability couples them to the flow.

From the standpoint of more fundamental physics, if small, neutrally-buoyant spherical particles are inserted into a flow, they faithfully follow the same trajectories as small fluid parcels and rotate with the local rate of fluid rotation. If, however, the particles are non-spherical and/or large enough (as is often the case in environmental and industrial processes), the particle motion will differ from the fluid. We examine this situation using numerical simulations and experiments for the fundamental cases.

Pujara, N., Koehl, M. A. R., and Variano, E. A., 2018. Rotations and accumulation of ellipsoidal microswimmers in isotropic turbulence. Journal of Fluid Mechanics, 838, pp. 356–368.

Pujara, N., and Variano, E. A., 2017. Rotations of small, inertialess triaxial ellipsoids in isotropic turbulence. Journal of Fluid Mechanics, 821, pp. 517–538

Pujara, N., Oehmke, T. B., Bordoloi, A. D., and Variano, E. A., 2018. Rotations of large, inertial cubes, cuboids, cones, and cylinders in turbulence. Physical Review Fluids, 3, 054605.

Pujara, N., Voth, G. A., and Variano, E. A., 2019. Scale-dependent alignment, tumbling and stretching of slender rods in isotropic turbulence. Journal of Fluid Mechanics, 860, pp. 465–486.

Wave-driven flow, transport, and coastal processes

Waves on the surface of water are fascinating phenomena. At coastlines, they create a continuous but never repeating mesmerising flow by breaking on the shore. This flow is also very important for whether the coast is undergoing erosion (a global problem), whether the coast will be flooded, and whether the various biochemical processes that make for healthy and sustainable coastlines are in balance. This informs our interest in the fluid mechanics of wave-driven flows.

Pujara, N., Liu, P. L.-F. and Yeh, H., 2015. The swash of solitary waves on a plane beach: flow evolution, bed shear stress and run-up. Journal of Fluid Mechanics, 779, pp. 556–597.

Pujara, N., Liu, P. L.-F., and Yeh, H., 2015. An experimental study of the interaction of two successive solitary waves in the swash: A strongly interacting case and a weakly interacting case. Coastal Engineering, 105, pp. 66–74.

Pujara, N., Liu, P. L.-F., and Yeh, H., 2016. An integral treatment of friction during a swash uprush. Coastal Engineering, 114, pp. 295–300.

Flow measurement and diagnostics

A lot of our work is done in the laboratory where we use a wave flume, water channels, and turbulence tanks to do controlled experiments. Along the way, we also develop new methods for making measurements. One such example is a sensor to measure the shear stress that a flowing fluid exerts on a solid boundary. This is a fundamentally important quantity in many areas of fluid mechanics since it determines the drag on a streamlined body or the capacity for the flow to pick up loose material like sediment.

Pujara, N. and Liu, P. L.-F., 2014. Direct measurements of local bed shear stress in the presence of pressure gradients. Experiments in fluids, 55, 1767.

Pujara N, Du Clos K T, Ayres S, Variano E A, and Karp-Boss, L, 2021. Measurements of trajectories and spatial distributions of diatoms (Coscinodiscus spp.) at dissipation scales of turbulence. Experiments in Fluids 62, 149.