Numerical investigation of turbulent-drag reduction induced by active control of streamwise travelling waves of wall-normal velocity

Abstract

Direct Numerical Simulations studies in a fully-developed turbulent channel flow were performed to analyse the efficacy of the active control of streamwise travelling waves of wall-normal velocity on turbulent drag-reduction. A key result of application of control was reduction in cross-flow velocity fluctuations in the buffer layer zone (10<z+≤40). This leads to the generation of more stable near-wall streaks of weaker magnitude which results in suppression of bursting events in the buffer layer. This eventually caused significant reduction in turbulent kinetic energy which further lead to reduction of turbulent skin-friction drag at the controlled wall (generally around 10%–11%). Upstream travelling waves of wall-normal velocity were most effective in bringing about considerable drag-reduction. For the case of downstream travelling waves, low phase-speed waves produced drag-reduction while waves with phase-speeds approaching c+≃10 showed a significant increase in turbulent drag. The considerations of power-budget show the practical efficacy of upstream travelling waves and low phase-speed downstream travelling waves in producing net-power savings.