Symmetric/asymmetric separation transition in a supersonic combustor with single-side expansion

Abstract

Shock wave induced separation in a canonical supersonic combustor is studied through numerical simulation and experiment. Cold flow analysis is implemented to obtain the dynamic features of the symmetric/asymmetric separation transition process. Experiments have been carried out in a single-expanding duct with backpressure produced by a cylinder at Mach number 3. Detached-eddy simulation represents the whole process of the separated region development. Typical simulated transient flow phases are validated by the nano-based planar laser scattering images. The results of the computational study show reasonable agreement with experiments, although the movement of simulated separation shock is slightly faster. It is found that a complex transitional separation occurs when the backpressure is near the threshold. During the dynamic process, the symmetric/asymmetric separation transition is bidirectional. A mechanism for the separation transition is identified based on boundary layer analysis. Results show that the key factor of the separation transition is the velocity/momentum profile fullness (shape factor) distribution of the boundary layers from both walls. An interlaced shape factor distribution means that the separation tendency of two turbulent boundary layers exchanges, which accounts for the switch of separation modes. A lag exists between the boundary layer transformation and the separation transition. A large amplitude, broadband low-frequency shock oscillation exists in the transitional flowfield, which has a relationship with low-frequency unsteadiness in traditional shockwave boundary-layer interaction problems. Future effort is required in discovering the mechanism of low-frequency unsteadiness in complex separation cases.