Turbulent spots in an oscillatory flow over a rough wall

We describe the results of a direct numerical simulation inspired by laboratory experiments (Carstensen et al., 2012), which showed the formation of turbulent spots in an oscillatory boundary layer over a rough wall. Even though, differently from the experiments, the wall we consider is characterized by a regular roughness, turbulent spots with similar characteristics are observed. The numerical results provide information on transition to turbulence and on the early stages of formation of the turbulent spots. In particular, the formation of low-speed streaks and the subsequent generation of turbulent spots are described. The speed of the extreme points of the spots is obtained from the numerical results. Moreover, the effects of the wall roughness on the speed of the turbulent spots are discussed.