Controlling Secondary Flows in Turbulent Taylor-Couette Flow Using Surface Heterogeneity

Turbulent shear flows are abundant in geophysical and astrophysical systems and in engineering-technology applications. They are often riddled with large-scale secondary flows that drastically modify the characteristics of the primary stream, preventing or enhancing mixing, mass, and heat transfer. In this thesis, we study the possibility of modifying these secondary flows by using stress reducing surfaces including free-slip and superhydrophobic surface treatments that reduce the local shear. We focus on the canonical problem of Taylor-Couette flow, the flow between two coaxial and independently-rotating cylinders, which has robust pinned secondary structures called Taylor rolls that persist even at significant levels of turbulence. It is shown through experiments and simulations that stress reducing surfaces arranged in a spanwise manner destructively interfere with Taylor rolls by inducing additional secondary flows through surface heterogeneity, as long as the structure size can be fixed. Simulations also find that slanted free-slip surfaces, when applied at certain angles and wavelengths, induce a velocity that causes the large-scale structures to move when the domain is periodic. The minimum slip-lengths of the treatment required for this flow control to work are determined and rationalized, and their effectiveness beyond the Reynolds numbers studied here is also discussed. We also explore a novel framework for understanding the origins of Taylor rolls through a two-way coupling between velocities caused by Coriolis forces, and apply it to other flows finding that the framework is not valid.