Drag reduction of blowing-based active control in a turbulent boundary layer

Abstract

Direct numerical simulations 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.