Sustainable Drag Reduction in Turbulent Taylor-Couette Flows by Depositing Sprayable Superhydrophobic Surfaces

We demonstrate a reduction in the measured inner wall shear stress inmoderately turbulent Taylor-Couette flows by depositing sprayable superhydrophobic microstructures on the inner rotor surface. The magnitude of reduction becomes progressively larger as the Reynolds number increases up to a value of 22% at Re = 8.0 x 10(4). We show that the mean skin friction coefficient C-f in the presence of the superhydrophobic coating can be fitted to a modified Prandtl-von Karman-type relationship of the form (C-f/2)(-1/2) = M ln (Re(C-f/2)(1/2)) + N + (b/Delta r)Re(C-f/2)(1/2) from which we extract an effective slip length of b approximate to 19 mu m. The dimensionless effective slip length b(+) = b/delta(nu), where delta(nu) is the viscous length scale, is the key parameter that governs the drag reduction and is shown to scale as b(+) similar to Re-1/2 in the limit of high Re.