Compressibility effects on vortex breakdown in subsonic and supersonic regimes

The three-dimensional structure of compressible Vogel–Escudier flow in a closed cylindrical cavity is examined for an aspect ratio Γ=2.5, Reynolds numbers up to Relid=5000, and lid Mach numbers up to Malid=3. The study extends previous investigations on vortex breakdown by analyzing the influence of compressibility on its onset, topology, and stability. For weakly compressible conditions (Malid≤1.5), the breakdown structure remains nearly unchanged, whereas higher Mach numbers produce a downstream displacement of stagnation points, a reduction in bubble length, and a transition from merged to distinct breakdown bubbles. Compressibility, thus, acts to suppress large-scale recirculation and stabilize the flow field. In the unsteady regime (Relid=3400–5000), the azimuthal mode with wavenumber k=4 dominates the flow, with its amplitude and modal interaction decreasing as Malid increases. Velocity spectra and phase-space reconstructions show attenuation of high-frequency oscillations and reduced attractor complexity, indicating diminished dynamical activity at higher Mach numbers. The heat transfer characteristics exhibit strong coupling between Reynolds and Mach number effects. Viscous heating within sidewall shear layers enhances local temperature gradients, while the wall-averaged Stanton and skin-friction coefficients depart from classical similarity relations. These results demonstrate that compressibility modifies both the stability and transport behavior of confined swirling flows.