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 18700. 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 into 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 mechanisam of the effect of micro-grooves on drag reduction, particle image velocimetry (PIV) 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.
Direct numerical simulations (DNS) are conducted to gain insight into the blowing-based active control in a spatially developing turbulent boundary layer at a low Reynolds number. The drag reduction properties and mechanisms of different blowing velocity distribution forms under constant wall-normal mass flux are comparatively studied, including uniform blowing and blowing-only opposition control (BOOC). After the application of blowing control, the self-similarity of the Reynolds shear stress is influenced. The property of drag reduction and control gain of the blowing-based active control schemes in the turbulent boundary layer is similar to that in turbulent channel flow, i.e., the BOOC scheme can achieve higher drag reduction than uniform blowing, but the control gain reduces. Due to the coexistence of the opposition effect and the induction effect, the negative wall-normal velocity fluctuations accompanied by the sweep motion are induced to form small-scale flow structures in the near-wall region. The decomposition of the skin-friction drag coefficient shows that the changes of each contribution term are basically the same for different blowing schemes, except that the BOOC scheme has a more substantial influence on mean convection and spatial development. According to the property that the drag reduction of the BOOC scheme with additional threshold limitation is equivalent to that without the restriction, it can be determined that the effect of blowing-based active control is mainly based on the temporal and spatial averaging effects of blowing, including the opposition effect and the induction effect.
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