Drag reduction in turbulent channel flows by a spanwise traveling wave of wall blowing and suction

A spanwise traveling wave, generated by blowing and suction at walls, is applied in direct numerical simulations to reduce the drag in a fully developed turbulent channel flow with Reτ≈180. The wave traveling direction reverses periodically to induce a spanwise motion of the flow near the wall, similar to that from spanwise wall oscillations. Such a motion can be approximated by an infinite series of the Stokes layers in an asymptotic expansion. A relation linking the blowing and suction velocity and frequency for the traveling wave is derived to achieve a drag reduction similar to the case with spanwise wall oscillation. A drag reduction of 24% is achieved in the present study. From the analyses of the energy budget of Reynolds stresses, the drag reduction mechanism is found strongly related to the pressure–strain correlations. The reduced wall-normal strain rate and pressure–strain correlation are the main causes of the decrease in energy components of turbulence. The induced spanwise motion also weakens the streamwise strain rate through inclining the vortex structures periodically and modulates the spanwise strain rate through its production term.