Transition between intermediate states in Taylor vortex flows in the presence of riblet-covered rotors

In flows ranging from boundary layers to internal flows and Taylor-Couette flows, riblet surfaces can alter the frictional drag force on the wall. Mimicking natural patterns such as ribs on denticles of sharks or the ridges on rice leaves, and butterfly wings, 2D textures have been effectively used as a means of passively reducing the drag force. In this talk, the effect of 2D streamwise textures on the onset of instability and transition between different flow states in a Taylor-Couette flow is experimentally examined. Using a wide-gap Taylor-Couette cell, I will present torque measurements as well as velocity profiles in the vicinity of the transitional flow as a function of the geometry of the textures as well as the Reynolds number. Focusing on the Taylor-vortex regime, I will show that unlike the case of smooth rotors, where after the onset of the Taylor vortices the flow settles into a 6-cell Taylor-vortex, periodic and symmetric textures in the circumference of the inner rotors can sustain an intermediate 4-cell Taylor-vortex flow over a range of Reynolds numbers before ultimately transitioning to the 6-cell state. Lastly, I will discuss how the geometric features of the textures affect the critical Reynolds number and the extent of this intermediate state.