Axisymmetric cavities in hypersonic flow

TL;DR

This study investigates the shear layer dynamics of axisymmetric cavities in hypersonic flow at Mach 6, revealing how geometry and Reynolds number influence vortex formation, mode switching, and transition to turbulence.

Contribution

It provides detailed experimental insights into shear layer behavior, mode transitions, and frequency characteristics for various cavity geometries at hypersonic speeds.

Findings

Shear layer remains laminar at low Reynolds numbers and develops vortices as Re increases.

Transition to turbulence occurs at higher Re for longer cavities.

Mode switching depends on rear-face height difference, with pressure build-up influencing dominant modes.

Abstract

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.