Effects of the intrinsic three-dimensionalization of laminar separation bubbles on the amplification of external disturbances

Abstract

The size and behavior of laminar separation bubbles is strongly related to the laminar-to-turbulent transition process that takes place downstream of the separation of the laminar boundary layer. Wind-tunnel experiments and numerical simulations in which external fluctuations are imposed show a clear dominance of instability waves of convective nature in the transition process. On the other hand, stability analysis demonstrate the existence of a self-excited mechanism that introduces a three-dimensionalization of the recirculation region. This paper studies the co-existence of both mechanisms, i.e. how the spanwise modulation of the bubbles can alter the properties of instability waves. It is found that a large convective amplification is induced by the spanwise velocity gradients.