Opposition control in compressible turbulent flows

Opposition control using out-of-phase, combined spanwise and wall-normal velocities at the walls has been studied. The control aims to establish a virtual wall away from the boundary wall, restricting and hindering momentum transport in respective directions. The study is carried out for flow regimes at bulk Mach numbers Mab = 0.8, 1.5, and 2.5, for bulk Reynolds number Reb = 3000. The combined velocity control achieves higher skin friction drag than the solitary wall-normal velocity control. The control application achieves a high drag reduction of over 30% for Mab = 0.8 and 1.5. However, for Mab = 2.5, the drag reduction tapers, signifying the loss of control effectiveness for higher Mab flows. At higher Mab number flows, the turbulent contribution to skin friction decreases significantly while the fixed laminar component increases. The quadrant analysis reveals an overall increase in the sweep events for Mab = 2.5 cases—the increase in sweep results from the channel's mean temperature profile. The high-temperature fluid in the channel core leads to the higher viscosity of the fluid as compared to the lower-viscosity near-wall fluid and assists sweeps toward the walls.