Numerical investigation of the evolution and control of two-dimensional unsteady separated flow over a Stratford ramp

The applicability of flow control by periodic blowing and suction through a slot in a spatially evolving Stratford ramp flow is investigated by numerical simulations based on the two-dimensional incompressible Navier-Stokes equations. The simulations have shown that beyond certain Reynolds numbers, instability waves are developing even when no “controlled” disturbances are introduced. These instability waves increase the momentum exchange between the outer and inner part of the boundary layer and thus can suppress the massive separation which would have occurred for the steady laminar flow under the same adverse pressure gradient. This natural instability can be exploited by introducing controlled disturbances (in the present case, by periodic blowing and suction through a slot). For example, a small separation bubble, which occurred at the upstream end of the Stratford ramp when no controlled disturbances were introduced, could be completely suppressed by periodically forcing the flow with the frequency that was observed for the “natural” (uncontrolled) case. However, for this “controlled” case another separation bubble may be generated further downstream. By carefully adjusting the forcing frequency (to about half of the natural frequency), the size and intensity of the newly generated separation bubble can be greatly reduced. When the Reynolds number was increased, extensive separation occurred without forcing, although instability waves were developing as for the low Reynold number case. However, the separation could again be suppressed by periodic forcing with large amplitudes. Furthermore, the required forcing amplitude could be minimized by using “optimal” frequencies.