Turbulent spots in oscillatory boundary layers

The detailed knowledge of the dynamics of vortex structures in an oscillatory boundary layer is essential for the correct modeling of transport processes in many engineering problems and, in particular, of the pick-up and transport of sediments at the bottom of sea waves. In the present contribution, the formation of turbulent spots in an oscillatory boundary layer is investigated by means of direct numerical simulations. Two of the laboratory experiments of Carstensen et al. (2010) are reproduced and, after a comparison of the numerical results with laboratory measurements, a detailed and quantitative characterization of the turbulent spots is given also on the basis of further simulations. The speeds of the head (u1H) and tail (u1T) of the spots are found to scale with the instantaneous free stream velocity Ue and to be similar to those observed in steady boundary layers. The ratios u1H/Ue and u1T/Ue seem to increase with the Reynolds number (Rdelta) while the streamwise expansion rate of the spots appears to be independent of Rdelta.