Spectral Content in a Supersonic Backward-Facing Step Flow
Logan P. Riley, Rajesh Ranjan, Datta V. Gaitonde
Abstract
Backward-facing step (BFS) configurations are relevant to many aerospace applications. An understanding of the underlying dynamics of such flows can aid in developing suitable control strategies. To this end, an unsteady analysis of a Mach 2 BFS flow is conducted. Large-eddy simulations (LESs), informed by linear perturbation analysis, are employed to characterize the range of frequencies encountered due to shear-layer separation from the step and its subsequent reattachment. Of particular interest are low frequencies in the separated region behind the step and in the region of the reattachment shock, which are not captured in simpler linear analysis. Near the step, detailed frequency analysis of the shear layer identifies higher-frequency content (Strouhal number , based on boundary-layer thickness ) associated with the incoming turbulent boundary layer. The signature of Kelvin/Helmholtz-like shedding associated with a convective shear-layer instability manifests at frequency , in agreement with linear analysis. Below the shear layer, the LES provides insight into a low-frequency signature of the recirculation zone at the reattachment location. Additionally, both linear and nonlinear analyses indicate an increase in the length scale of convecting vortex structures at reattachment, a region characterized by enhanced wall-pressure fluctuations.