Friction Drag Reduction by Transversal Spanwise Traveling Waves of Ribbed Surfaces

Abstract

Friction drag reduction in zero-pressure gradient turbulent boundary layer flow is investigated by a combined passive and active method. The passive method is riblet structured surfaces, which have been proven to effectively reduce turbulent friction drag. Additionally, the surface undergoes a spanwise traveling transversal wave motion which constitutes the active method. This hybrid, i.e., passive and active, method is tested in a turbulent boundary layer flow at a Reynolds number of. The results are compared against those of a stationary smooth flat plate, a stationary ribbed flat plate, and a moving smooth flat plate. For the transversal wave motion two amplitudes are considered while the wave length and the frequency are not varied. The results show a slightly higher drag reduction for the hybrid method compared to the single drag reduction technique. Furthermore, the hybrid method is less susceptible to variations of the wave motion. The largest relative drag decrease of the hybrid method is achieved for the smallest amplitude, i.e., the lowest additional energy input. The mean velocity gradient near the wall is reduced for the hybrid method compared to the non-actuated cases at the tips and in the valleys of the riblets. The Reynolds stresses show a significant decrease of the turbulent intensities in the near-wall region especially for the large amplitude case. The location of the peak of the streamwise stresses and the wall-normal vorticity fluctuations is shifted off the wall.