A detailed experimental campaign is conducted to assess the shear layer characteristics of an axisymmetric open cavity configuration exposed to a Mach 6 freestream flow. Experiments are carried out in a Ludwieg tunnel across varying Reynolds number cases $(23000\le {\text{Re}}_D\le 74000)$ based on the cavity depth $\left(D\right)$. The influence of associated geometrical parameters in the form of length-to-depth ratio $\left(\right[L/D]=[2,4,6\left]\right)$ and nondimensional excess height of the rear face with respect to the front face $\left(\right[\mathrm{\Delta }h/D]=[-0.5,-0.25,0,0.25,0.5\left]\right)$ is assessed. Thereby, a detailed interpretation of the shear layer evolution is made based on qualitative flow structures captured using schlieren imaging and planar laser Rayleigh scattering (PLRS), along with quantitative measures obtained through unsteady pressure probes. Regardless of the $[L/D]$, the state of the shear layer is observed to be laminar at lower ${\text{Re}}_D\mathrm{s}$, and when increased, reveals the presence of Kelvin-Helmholtz (K-H) vortices. Nevertheless, for the longest aspect-ratio case $\left(\right[L/D]=6.0)$, the shear layer transitions to turbulence in the extreme ${\text{Re}}_D$ cases owing to the availability of a longer length scale for K-H propagation. The spectral content obtained from the pressure response and the light intensity from the PLRS snapshots reveals an increase in the dominant frequency from the first Rossiter mode to higher-order modes for $[L/D]=6.0$. Apart from $[L/D]=6,[\mathrm{\Delta }h/D]=0$, the dominant frequency for all the cases considered herein matches the prediction of Rossiter modes and is found to be invariant of Reynolds number. While varying the $[\mathrm{\Delta }h/D]$, shifting of modes occurs as obtained using proper orthogonal decomposition analysis of PLRS snapshots. For the negative $[\mathrm{\Delta }h/D]$, the K-H mode is dominant (fifth and sixth Rossiter modes), whereas for the positive, the strong flapping mode (first Rossiter mode) prevails owing to significant pressure build-up inside the cavity recirculation region. For $[\mathrm{\Delta }h/D]=0$, both modes may coexist depending on ${\text{Re}}_D$. The azimuthal uniformity of the flow is also probed, indicating the dominance of the axisymmetric mode in the flapping scenario and much less correlated behavior in the case of K-H vortices in the shear layer.

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• High-speed flow
• Kelvin-Helmholtz instability
• Shear flows
• Shock waves