Axisymmetric cavities in hypersonic flow

A detailed experimental campaign is conducted to investigate the shear layer characteristics of an axisymmetric open cavity exposed to a Mach $6$ freestream. Experiments are performed in a Ludwieg tunnel for varying Reynolds numbers ($23000\leq Re_D \leq 74000$) based on cavity depth ($D$). The effects of geometry are examined through length-to-depth ratios ($[L/D]=[2,4,6]$) and non-dimensional rear-face height differences ($[\Delta h/D]=[-0.5,-0.25,0,0.25,0.5]$). Shear layer evolution is interpreted using qualitative schlieren and Planar Laser Rayleigh Scattering (PLRS) along with quantitative unsteady pressure measurements. For all $[L/D]$, the shear layer remains laminar at low $Re_D$ and develops Kelvin-Helmholtz (K-H) vortices as $Re_D$ increases. For the longest cavity ($[L/D]=6$), transition to turbulence occurs at the highest $Re_D$ due to a longer K-H growth length. Spectral analysis of pressure signals and PLRS intensity shows a shift in dominant frequency from the first Rossiter mode to higher modes for $[L/D]=6$. Except for $[L/D]=6, [\Delta h/D]=0$, dominant frequencies agree with Rossiter predictions and remain largely Reynolds-number independent. Variation of $[\Delta h/D]$ leads to mode switching identified using POD of PLRS snapshots. Negative $[\Delta h/D]$ favors K-H modes (5th-6th Rossiter), whereas positive values promote a strong flapping mode (1st Rossiter) due to pressure build-up inside the cavity. At $[\Delta h/D]=0$, both modes may coexist depending on $Re_D$. Azimuthal measurements indicate dominant axisymmetric behavior in flapping cases and weaker correlation for K-H dominated shear layers.