Numerical study of separation bubbles with turbulent reattachment followed by a boundary layer relaxation^*

Publisher Summary

When a laminar boundary layer encounters a strong enough adverse pressure gradient, it separates to form a laminar free shear layer. The separated shear layer undergoes transition to turbulent flow and the turbulent shear layer may reattach to form a closed laminar separation bubble. A second type of separation bubble is one that occurs after a backward-facing step, where the separation point is fixed. A family of suction-induced laminar separation bubbles on a smooth surface has been investigated by a spectral numerical method which solves the full 3D time-dependent incompressible Navier-Stokes equations. A comparison study with experimental results shows that the present simulations capture the main features associated with turbulent reattachment and the relaxing boundary layer that follows. The main observations downstream of reattachment are that the relaxation to an equilibrium boundary layer is very slow; Reynolds stresses are initially very high and change rapidly, eventually falling below the equilibrium boundary layer values. These observations suggest that there is a region of rapid distortion in the immediate vicinity of reattachment while further downstream there is a slowly decaying shear layer above the relaxing boundary layer.