  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. Using
experiments and numerical simulations, we study the possibility of modifying
these secondary flows by using 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
secondary structures called Taylor rolls that persist even at significant
levels of turbulence. We generate these structures by rotating only the inner
cylinder of the system, and show that a spanwise superhydrophobic treatment can
weaken the rolls through a mismatching surface heterogeneity, as long as the
roll size can be fixed. The minimum hydrophobicity of the treatment required
for this flow control is rationalized, and its effectiveness beyond the
Reynolds numbers studied here is also discussed.

Fluid Dynamics