Turbulent flow control through superhydrophobic surfaces

Turbulent shear flows are abundant in geo 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 and mass and heat transfer. We study the possibility of modifying these secondary flows by using superhydrophobic surface treatments which 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. We show that a spanwise superhydrophobic treatment can destructively interfere with Taylor rolls by inducing additional secondary flows through surface heterogeneity, as long as the structure size can be fixed. We compare experiments and Direct Numerical Simulations and quantify the effects of heterogeneity caused by superhydrophobic surface on Taylor rolls at Re_s=O(10⁴) . We rationalize the minimum treatment hydrophobicity required for this flow control, and show that it can be effective beyond the Reynolds numbers here studied.