Sub-cavity induced passive control of confined supersonic cavity flows across varying freestream Mach numbers

The self-sustaining cavity oscillations enhance fluid mixing, promoting energy and momentum transport. Despite their utility, the associated oscillation frequencies can amplify acoustic loading, potentially damaging structures that house cavities. Understanding cavity flow dynamics across various geometries and freestream conditions is, therefore, crucial, along with devising strategies to regulate these oscillations without compromising performance. In this study, we investigate the role of sub-cavities placed at the front and aft walls of a cavity confined by a top wall with a deflection angle of 2.29°, under freestream Mach numbers M∞=2 and 3. We perform large-eddy simulations (LES) using OpenFOAM for each configuration and analyze the unsteady pressure signals through spectral methods. The results show that the aft-wall sub-cavity most effectively suppresses the dominant oscillation frequency at the lower Mach number (M∞=2), while the front-wall sub-cavity achieves the greatest suppression at the higher Mach number (M∞=3). We examine the density gradient contours (numerical schlieren), vorticity fields, normalized acoustic impedance, and global wavelet power to identify the flow mechanisms responsible for frequency attenuation. At M∞=2, the aft-wall sub-cavity entrains mass and interrupts the convectively driven feedback loop. At M∞=3, the front-wall sub-cavity weakens the hydrodynamic–acoustic coupling between the cavity pressure waves and the shear layer near the leading edge, thereby disrupting the compressibility-driven feedback loop. We observe that these configurations suppress the major oscillation frequencies by 5.45% and 23.4% for M∞=2 and 3, respectively. We further validate these findings by computing the cross correlation between pressure probes located near the front and aft edges and by analyzing the spatiotemporal coherence in pressure and velocity fields using dynamic mode decomposition, which confirms the mechanisms behind the observed frequency suppression.