Large-scale Unsteadiness in a Two-dimensional Diffuser: Numerical Study toward Active Separation Control

We develop a reduced order model for large-scale unsteadiness (vortex shedding) in a two-dimensional diffuser and study the mechanisms of active flow separation control. This model can estimate the vortex shedding frequency for inviscid flows by accounting for the accumulated vorticity flux in the diffuser. The model can also predict the stagnation pressure loss, which consists of two parts: A steady part corresponds to static pressure loss on the detached area, and an unsteady part is associated with vortex shedding. To validate this model, we perform direct numerical simulation (DNS) of compressible, laminar diffuser flows. The comparison between the model and DNS shows good agreement at various Mach numbers and area ratios of the diffuser in terms of vortex shedding time scale and stagnation pressure loss. To investigate the effects of periodic mass injection near the separation point, we also perform DNS over a wide range of the forcing frequency. The DNS results show that periodic mass injection can pinch off vortices with a smaller size; accordingly, their convective velocity is increased, absorption of circulation from the wall is enhanced, and the extent of the separated region is reduced. As a result, the stagnation pressure recovery, particularly the unsteady part, is substantially improved as predicted by the model.