Drag reduction by spanwise transversal surface waves

A numerical simulation of a spatially evolving turbulent boundary layer impacted by a spanwise traveling transversal sinusoidal wall oscillation is performed. Compared to boundary layer flow over a non-oscillating surface, i.e a standard flat plate boundary layer, a reduction of friction drag of more than 6% is found. A detailed discussion of the near-wall flow reveals similar effects as observed by other excitation methods, such as additionally implemented volume forces or longitudinal sinusoidal wall oscillation. Secondary flow structures induced by the moving wall and a controlled formation of streamwise vorticity close to the wall are identified. The wall-normal vorticity fluctuations are found to be damped such that streak instability and formation of new streaks are reduced yielding an evidently decreased friction drag.