Forced Control of SWBLI in a 24deg Compression Ramp Flow with Air-jet Vortex-generator

Abstract

Beneficial effects of air-jet vortex generators (AJVGs) in controlling shock-induced flow separation motivated us to examine the influence of sinusoidally-forced injection of an array of spanwise-inclined AJVGs to control a shock-wave/boundary-layer interaction (SWBLI). We performed implicit large-eddy simulations of a 24deg-compression-ramp interaction for an incoming flow at Mach 2.5 and with a momentum-thickness Reynolds number of 7000. Two forcing frequencies with St^pl_d = 0.0015 and 0.0062 were considered. Spanwise-inclined jet injection embeds an asymmetric counter-rotating vortex pair (CVP) in the boundary layer. The jet-induced major CVPs energize the near-wall region of the boundary layer, resulting in a reduced separation length. The fundamental nature of the jet/jet interactions are similar for both unforced and forced AJVG cases at a fixed jet spacing of D = 8d. Therefore, the overall mean-flow topology of the forced AJVG-control cases are very similar to the unforced case. The total mean separation length is not affected by forcing, which indicates that unforced AJVGs are equally effective at reducing flow-separation, and these steady devices are much easier to implement. Triple decomposition of the flow field revealed that injection of forced jets affect the jet/jet interactions: low frequency forcing results in coherent momentum distribution in the region between the adjacent major vortices, whereas, high frequency forcing, results in turbulent mixing, similar to the unforced AJVG case. The SWBLI cases with both unforced and forced AJVG-control are dominated by the low-frequency unsteadiness of the shock and recirculation bubble, with constant St_Lsep in the range 0.03 – 0.05. A 3D DMD analysis revealed that dominant modes of jet/jet interactions excite and influence the SWBLI dynamics. Moreover, the results reveal that SWBLI has similar dynamic behavior in all AJVG-control cases, however, their characteristic frequencies change, indicating that SWBLI is a dynamically balanced mechanism. Due to the additional installation complexities of forced control setup, we find unforced AJVGs to be practically more effective compared to the forced AJVGs.