Drag reduction by spanwise wall oscillation in wall-bounded turbulent flows

Drag reduction in turbulent channel and pipe flows by spanwise (circumferential) wall oscillations is studied numerically. The influence of the wall oscillation on near-wall streamwise vortices is examined. By the use of the Stokes second problem, a wall-normal distance parameter and an acceleration parameter are obtained to estimate the drag reduction rate. A simple equation is derived for expressing the drag reduction rate by spanwise wall oscillations. The relation between near-wall streamwise vortices and low- and high-speed fluids is scrutinized to extract the key parameters. The drag reduction mechanism is analyzed in terms of the attenuation of Reynolds shear stress.