Direct numerical simulation of 'short' laminar separation bubbles with turbulent reattachment

Direct numerical simulation of the incompressible Navier-Stokes equations i s used to study flows where laminar boundary-layer separation is followed b y turbulent reattachment forming a closed region known as a laminar separat ion bubble. In the simulations a laminar boundary layer is forced to separa te by the action of a suction profile applied as the upper boundary conditi on. The separated shear layer undergoes transition via oblique modes and A- vortex-induced breakdown and reattaches as turbulent flow, slowly recoverin g to an equilibrium turbulent boundary layer. Compared with classical exper iments the computed bubbles may be classified as 'short', as the external p otential flow is only affected in the immediate vicinity of the bubble. Nea r reattachment budgets of turbulence kinetic energy are dominated by turbul ence events away from the wall. Characteristics of near-wall turbulence onl y develop several bubble lengths downstream of reattachment. Comparisons ar e made with two-dimensional simulations which fail to capture many of the d etailed features of the full three-dimensional simulations. Stability chara cteristics of mean flow profiles are computed in the separated flow region for a family of velocity profiles generated using simulation data. Absolute instability is shown to require reverse flows of the order of 15-20%. The three-dimensional bubbles with turbulent reattachment have maximum reverse flows of less than 8% and it is concluded that for these bubbles the basic instability is convective in nature.