MEAM Seminar: “Computational Study on the Influence of Roughness at Low and Very-High Reynolds Numbers”

Many fluid problems of interest, such as turbulent flow over an airplane or transport processes in geophysical flows, contain wall-bounded regions that form boundary layers. Oftentimes, both numerical and experimental studies are simplified by using smooth surfaces. This simplification has allowed us to gain a greater understanding of near-wall processes for flows of engineering interest, yet most surfaces are inherently rough. In many cases, especially at higher Reynolds numbers, the roughness protrudes far enough into the boundary layer to disrupt the flow. This roughness can induce form drag, reducing efficiency for ships, planes, and turbines, or alter transport of particles in atmospheric flows. For lower Reynolds number flows, roughness may produce the opposite effect, reducing drag or enhancing lift capabilities. This is with the addition of dimples on a golf ball or the alula on a bird wing.

In this talk, I present numerical results on the effects of roughness at two opposite ends of the spectrum. At low Reynolds number (O(10³)), roughness elements displayed an ability to decrease drag, reducing power required, and augment lift capabilities for a propeller designed for use with a micro-unmanned aerial vehicle. I discuss the physical mechanisms at play which influence near-wall vortical structures, enhancing the aerodynamics of the propeller in a particularly viscous regime. The results of the simulations are corroborated with an in-house experimental study. Next, I present preliminary simulation results to a very-high Reynolds number flow (O(10⁶)) over a real topography featuring a step-change in roughness. An atmospheric boundary layer flow over the dunes at White Sands National Park in New Mexico is studied to discern the mechanism behind reduced sediment transport downstream of the step-change. I then discuss current efforts to explore how the internal boundary layer formed by the new wall condition may influence small near-wall scale interaction with large-scales in the outer portion of the flow.