Separation Shear Layer Instabilities in Hypervelocity Flows

Shock--boundary layer interactions (SBLI) are complex fluid dynamic phenomena that occur when shocks are generated near corners and irregular geometries on vehicles flying near or above supersonic speeds, causing external flow distortion and possible boundary layer separation. Accurate prediction of the mean and unsteady SBLI surface interaction is imperative to avoid failure from highly localized aerodynamic and heating loads, and loss of authority near a control surface. For hypersonic flight-enthalpy matched conditions, current SBLI simulations tend to under-predict thermal loads and have significant disagreements with ground-based experiments in separation location, and location and magnitude of peak heating. These discrepancies are potentially largely due to uncertainties in the modeling and recreation of the coupled real-gas (thermochemical) molecular and gas dynamic processes. To address this issue, efforts have been placed on developing and validating thermochemical gas models, which presents a need for off-surface experimental data. The disagreement between simulations and high-enthalpy ground test experiments have also highlighted the need to better characterize the freestream thermodynamic, velocity, and noise conditions. Near-surface focused laser differential interferometry (FLDI) measurements of instabilities in a separation shear layer on a 25-55 degree double-cone model were performed with simultaneous static pressure and freestream tunnel noise measurements in hypervelocity conditions. A strong medium-frequency band (~370-450 kHz) was observed in the near-surface FLDI beam-pair, with peak signal strength three orders of magnitude above the freestream measurement. Analysis is ongoing to extract frequency content at various regimes and associate them with physical flow features typically found in separation shear layers.