Near-Wake Structure and Dynamics of a Cylinder in Hypervelocity Flows

The near-wake of a circular cylinder at hypersonic conditions was investigated experimentally to assess the influence of high stagnation enthalpy thermochemical effects on mean wake structure and near-wake dynamics. Experiments were conducted in the T5 Free-Piston Reflected Shock Tunnel and the Caltech Ludwieg Tube using high-speed schlieren and simultaneous focused laser differential interferometry and high-speed schlieren. Mean flow quantities, including the separation point, separation region size, shock angles, and neck width, were extracted from schlieren data. Relative to perfect-gas conditions, high stagnation enthalpy flow exhibited a downstream shift of the separation point and a reduction in the separation region size. These features retained a strong dependence on Reynolds number at high stagnation enthalpy. The recompression shock angle was found to be largely insensitive to key flow parameters, remaining approximately constant within experimental uncertainty, while the neck width decreased with increasing stagnation enthalpy and followed a Re-1/2 scaling consistent with laminar perfect gas behavior. Near-wake dynamics were examined using power spectral density (PSD) analysis and Spectral Proper Orthogonal Decomposition (SPOD). A dominant shear layer frequency and a secondary frequency were identified in both facilities, with both remaining approximately constant along the shear layer. Although this frequency in high stagnation enthalpy flow was higher than that observed in perfect-gas experiments at similar Reynolds numbers, scaling by the shear layer length collapsed the data onto a Strouhal number consistent with previous studies. PSD contours and SPOD modes at the dominant frequency revealed standing wave structures in the separation region and banded structures between the shear layer and the separation shock. SPOD, correlation analysis, and direct schlieren visualization revealed downstream-propagating disturbances consistent with the Kelvin Helmholtz instability, accompanied by waves indicative of Mach wave radiation; upstream-propagating waves between the shear layer and the separation shock, originating near the neck and hypothesized to be associated with upstream-traveling acoustic disturbances within the recirculation region; and stationary standing wave structures. These observations are consistent with a resonant aeroacoustic feedback mechanism in which downstream-propagating instabilities generate acoustic waves that travel upstream through the recirculation region and perturb the shear layer at separation. The physical origin of the secondary frequency remains uncertain; the data are analyzed in the context of existing theories to evaluate their consistency with current observations, although further data are required for conclusive identification.