On Instabilities and Transition in Laminar Separation Bubbles

During the past several years we have performed a considerable amount of investigations of basic instability mechanisms in various laminar separation bubbles. Because of the high sensitivity of the flow with respect to intrusive measurement techniques and the difficulties of computing the flow field via boundary-layer equations or RANS our research relies primarily on direct numerical simulations (DNS) based on the complete Navier-Stokes equations, backed by linear stability theory, wind tunnel and (non-intrusive) water channel experiments. Using LDA and PIV it is now possible to measure unsteady flow quantities without influencing the flow. The investigations concentrate on different instability mechanisms and their possible contributions to laminar-turbulent transition in laminar separation bubbles. In the present contribution we shall present an overview on this research.