Effect of micro-grooves on drag reduction in Taylor–Couette flow

Abstract

Taylor–Couette flow with micro-grooves on the rotating inner cylinder is investigated to reveal the effect of surface structures on drag reduction. The Reynolds number (Re) ranges from 160 to 18 700. On the one hand, in the regimes of wave vortex flow (WVF, 160 < Re < 1010) and modulated wavy vortices (MWV, 1010 < Re < 1380) flow, the micro-grooves always reduce the torque, indicating drag reduction. Increasing either the size of micro-groove or Re, drag reduction will be enhanced. On the other hand, when the flow regime enters turbulent Taylor vortices (TTV, Re > 1380), drag reduction will be suppressed as Re increases and eventually turns to drag increase. The bigger the groove size, the smaller the critical Re where it turns from drag reduction to drag increase. To reveal the underlying mechanism of the effect of micro-grooves on drag reduction, particle image velocimetry measurements are conducted to observe the vortex flow structures, which demonstrates two aspects affecting the drag of Taylor–Couette flow over micro-grooved wall. First, the weakening of the large-scale Taylor vortex will lead to drag reduction. Second, the roughness effect will result in drag increase. In WVF/MWV, the former plays a dominant role, while in TTV, the latter dominates. In addition, a relationship between the micro-groove size and the predictive critical Reynolds number (Rec) is developed, providing a method for controlling the wall drag.