Abstract
Direct numerical simulations are performed in order to investigate the development of flow field and the friction drag in a subsonic plane boundary layer with a back-step. The boundary layer is forced by normal velocity disturbance of various frequencies at the end of the back-step. The numerical results for no-disturbance case show that the profile of mean streamwise velocity with inflection point is formed behind the back-step and velocity fluctuations are amplified in this region. The rollup of vortexes caused by the growth of the fluctuations are observed around the re-attachment point. In the downstream region of this re-attachment point, irregular vortex street is formed. It is found that the disturbance imposed at the end of the back-step influence the re-attachment point and the vortex street formed downstream of the point. The disturbance with the frequency of about St=0.016 moves the re-attachment point toward the back-step most and is yielded regular distance of vortex street. Although the total drag is larger than laminar boundary layer without back- step, lower total drag compared to turbulent boundary layer is observed in all disturbance cases.
