We pursue research in a range of topics under the overall theme of fluid mechanics in environmental processes:
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Waves on beaches
Beaches are fascinating places, largely due to the mesmerising flow created by waves that continually break on the shore. This flow is also very important for whether the beach is undergoing erosion (a global problem) and for the various biochemical processes that make for healthy and sustainable coastlines. This informs our interest in the fluid mechanics of wave-driven flow on beaches. We ask questions such as: What is the capacity for a wave to erode or add sediment to the beach? And what is the risk of waves overtopping the beach and flooding the roads/houses behind it?
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.
Particles in turbulent flows
Under this topic, we are interested in the effects of size and shape on the dynamics of particles in turbulence. If small, neutrally-buoyant spherical particles are inserted into a turbulent 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 industrial processes and for plankton in environmental water bodies), the particle motion will differ from the fluid. We examine this situation using numerical simulations and experiments for the fundamental case where the background flow is homogenous isotropic turbulence. Even in the simplest case where the particle density is close the fluid density, there are very interesting dynamics of how the particles extract rotation from fluid vortices.
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.
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.
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.
Microplastics are defined as plastic particles less than 5 mm in size that are found in the environment. On a global scale, microplastics in the environment are increasing due to continued plastic inputs from anthropogenic activities and from the breakdown of larger plastic pieces due to fragmentation and degradation. On more local scales, the complexities of 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 fragmented by flow in various settings.
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.