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 ($[Δh/D]=[-0.5,-0.25,0,0.25,0.5]$). 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 $[L/D]=6, [Δh/D]=0$, dominant frequencies agree with Rossiter predictions and remain largely Reynolds-number independent. Variation of $[Δh/D]$ leads to mode switching identified using POD of PLRS snapshots. Negative $[Δh/D]$ 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 $[Δh/D]=0$, 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.
Cavities in high-speed flows have crucial aspects owing to the complex interaction of associated shock waves, separated shear layers, and the recirculation region 1 . Cavities find applications in various areas, including landing gear housings 2 , weapon bays 3,4 , scramjet components 5 , flow control 6 , and flame stabilization 7,8 , among others. The cavity dynamics, specifically shock-shear layer interaction and recirculation bubble characteristics, vary significantly due to compressibility effects as the Mach number is changed, resulting in an unsteady flowfield in most cases, accompanied by spanwise coherent vortices 9,10 . The current investigation focuses on hypersonic flow past an axisymmetric cavity, which often finds application in both small-and large-scale contexts, such as sensor housing provisions 11 and trapped a) Electronic mail: soumyananda224@gmail.com b) Electronic mail: sri.vamsi1432@gmail.com c) Electronic mail: skkarthick@mae.iith.ac.in (corresponding author) d) Electronic mail: aerycyc@technion.ac.il combustor 12 . Nevertheless, the applicability point of view differs in both cases. Small-scale cavities are used as a thermal shielding mechanism to isolate the sensors from the elevated heating zone, providing a laminar separated shear layer that results in reduced heat transfer 13 . Large-scale cavities, on the other hand, are typically used to aid rapid flow mixing. Moreover, upon reattachment of the separated flowfield, a hotspot forms 14 in the cavity's trailing edge. The transition from a laminar to a turbulent state in the incoming shear layer results in a further increment in the trailing edge heat load 15 . Therefore, it is essential to comprehend the state of the shear layer, its characteristics, and potential interaction mechanisms with the cavity's trailing edge, which may trigger unsteadiness.
The overall cavity flow is typically governed by the feedback loop of shear layer impingement on the aft wall, along with the generation and upstream propagation of acoustic waves, which then perturb the shear layer, thereby completing the feedback cycle 16 . The perturbations introduce vortical structures in the shear layer through K-H instability, which, when in a proper phase with the acoustic waves, results in a strong resonance leading to self-sustained oscillations, referred to as Rossiter modes [17][18][19] . The incoming flow properties and the geometrical parameters concerning the cavity are among the key attributes that drive the overall flow oscillation. Rossiter 17 , followed by Heller et al. 20 , proposed a semi-empirical relation to predict the associated resonance frequency, which is presented in terms of the Strouhal number (St D ) as,
where M represents the Mach number of the flow upstream of the cavity, and the integer number n = [1, 2, 3, 4, . . .] indicates the mode number. The empirical constants α = 0.25 and k = 0.57 10 correspond to the phase lag of the feedback loop, and the velocity ratio of the K-H convection to the freestream, respectively. Amplification or stabilization of the Rossiter modes is typically reported for varying geometric situations (cavities with negative and positive asymmetry 21 ) and incoming flow compressibility 22 . In contrast, cavity flows in relatively higher supersonic flow regimes are observed to be driven by the contribution of additional waves in excess of the impingement of the trailing-edge vortices 23 . The additional wave structures arise from the differences in the relative speed of the convecting large-scale structures with respect to the freestream flow and the internal cavity flow 24 .
An increase in the Mach number also led to the dominance of three-dimensional (3D) modes over two-dimensional (2D) modes [25][26][27] .
Two-dimensional cavities are relevant to real-world applications and can also serve as a canonical model to capture complex fields. Several researchers have investigated such cavities across various Mach numbers, Reynolds numbers, and cavity geometries. Recently, modal and resolvent analysis have helped establish the K-H growth rates 28 and crossfrequency interactions involving energy transfer from the fundamental Rossiter mode to harmonic modes 29 , thereby identifying the flow behavior as convectively amplified or highly oscillatory. The stability analysis 19,30 is reported to demonstrate the significance of 2D Rossiter modes, along with 3D eigenmodes, including centrifugal instabilities and spanwise structures, in longer cavities. The evolution of tornado-like structures 31 and suppression of turbulence intensity with increase in Mach number 1 also happens to be altering the dominant spectral content through disruption of coherence in Rossiter modes. Most of the available literature emphasizes the interplay between compressibility effects, modal instabilities, and three-dimensional structures in the evolution of flow in two-dimensional cavities 32 .
Although 2D cavity configurations have been extensive
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