Numerical simulation of the bursting of a laminar separation bubble

Abstract

Numerical simulations of laminar separation bubbles are carried out to investigate the so-called bubble bursting, i.e. the changeover from a short to a long bubble by means of very small variation of one governing parameter. A laminar separation bubble is formed if a laminar boundary layer separates in a region of adverse pressure gradient on a flat plate and undergoes transition, leading to a reat-tached turbulent boundary layer. Bubble bursting denotes a phenomenon, in which a local, in average closed region of reverse flow (the short separation bubble) suddenly becomes considerably longer as a result of only small changes in the conditions of the surrounding flow. Here, this condition is the disturbance input upstream of separation. Both, long laminar separation bubbles and bubble bursting, are not yet well understood on a fundamental level, but it is commonly accepted that the transition process plays an important role. Simulations in which transition is or is not explicitly triggered are carried out. Depending on this triggering, either a short laminar separation bubble develops or the bursting process is initiated and the flow develops towards a long-bubble state. If the flow is tripped to turbulence prior to the adverse pressure gradient, the boundary layer remains attached. Performance data on a NEC SX-8 super computer are given for two different resolutions.