Drag reduction for swept flat plate flow

Drag reduction analyses for turbulent flow over a swept flat plate are conducted. To be more precise, large-eddy simulations of oblique traveling transversal surface waves in turbulent flat plate boundary layer flow at a momentum thickness based Reynolds number of Re-theta = 1000 are performed. For two parameter sets of the actuation parameters wavelength, period, and amplitude the angle between the wave propagation direction and the streamwise direction, i.e., the sweep angle q', is varied. The goal is to investigate the susceptibility of drag reduction, net power saving, and turbulence to different sweep angles of the traveling waves. The extension of the technique of spanwise traveling transversal surface waves allows partially up- or downstream traveling waves, i.e., opposite to or in the main flow direction. For downstream traveling waves, the periodic spanwise shear is decreased and the reduction of the wall-shear stress is diminished. Nevertheless, strong decreases of the turbulent stress tensor components persist even for the largest sweep angle. For upstream traveling waves, there is an increase of the lowered friction drag reduction. However, the turbulence level in the outer flow is also higher since the pressure drag is increased such that the total drag reduction is massively lowered. Compared to a purely spanwise traveling wave, i.e., phi = 0 degrees, the sweep angle phi not equal 0 degrees leads to a much stronger phase dependence of the skin friction since the partially up- and downstream traveling waves induce also periodic fluctuations in the streamwise direction. A shift of energy towards larger scale motions is observed which is also reflected by an increase of the integral length scale close to the wall.