Active Control for Turbulent Drag Reduction by Periodic Blowing and Suction

This study examines the effects of spatially and temporally periodic blowing and suction on turbulent drag and coherent structures in channel flow. We first determine the minimal domain necessary for consistent analysis of the impact of harmonic blowing and suction. We then conduct a systematic exploration of wavenumber and frequency domains for spanwise-constant standing and traveling waves under small wall-transpiration velocity amplitudes to identify parameters that lead to drag increase or decrease. Our findings indicate that optimal parameters result in a 2.93% drag reduction for standing waves and a 3.62% drag reduction for traveling waves. Turbulent kinetic energy (TKE) analysis shows that drag reduction is closely linked to TKE reduction in the near-wall region above the blowing section, whereas drag increase occurs when TKE reduction in the blowing section is offset by TKE increase in the suction section. Furthermore, two-point velocity correlation analysis reveals a consistent enhancement of streamwise rolls in drag-reducing scenarios, with streamwise streaks inclined toward the wall. In contrast, drag-increasing scenarios exhibit attenuation of these coherent structures, with streamwise streaks inclined away from the wall.