Experiments on Separation Shear Layer Instabilities in Hypervelocity Flows

Citation

Yu, Wesley Minlai (2024) Experiments on Separation Shear Layer Instabilities in Hypervelocity Flows. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/1m4t-5n55. https://resolver.caltech.edu/CaltechTHESIS:05202024-181545768

Abstract

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.

Extensive freestream characterization of the T5 Free-Piston Reflected Shock Tunnel was performed over the course of numerous experimental campaigns using high-speed shadowgraph/schlieren imaging, static and pitot pressure probes, tunable diode laser absorption spectroscopy (TDLAS), and focused laser differential interferometry (FLDI). Accurate time-resolved static pressure measurements are key to characterizing the operation and freestream thermodynamic state in hypervelocity reflected shock tunnels, through both direct measurement and for interpretation of TDLAS signals. A series of three static pressure probes were built for use in T5 at a range of conditions from 8-16 MJ/kg stagnation enthalpies, and measurements agreed well with TDLAS-inferred pressure and numerical simulations of the static probe response. At higher enthalpy conditions, TDLAS measurements showed a substantial decrease in freestream temperature (~1000 K) while velocity was constant. This finding motivated the need for a method to characterize the arrival time and degree of driver gas contamination in T5. An opposing-wedge detector was designed to leverage the sensitivity of the canonical Mach stem flow to the freestream γ, such that the flow would choke at a prescribed increase in γ corresponding to the arrival of a specific mole fraction of monatomic driver gas. With high-speed schlieren/shadowgraph imaging, driver gas arrival times and mole fractions were obtained for the 8 MJ/kg test condition.

Informed by these freestream characterization experiments, near-surface FLDI measurements of instabilities in a separation shear layer on a 25°-55° double-cone model were performed with simultaneous static pressure and freestream tunnel noise measurements in hypervelocity conditions. Three main frequency regimes were considered: i) low-frequency content associated with Kelvin-Helmholtz instabilities and streamwise acoustic disturbances along the shear layer, ii) a strong medium-frequency (peak ~370-450 kHz) signal associated with shear layer instabilities communicating with the model surface, and iii) high-frequency features associated with Mack (second-mode) disturbances. Length scaling arguments are discussed for each case, informed by axisymmetric simulations of the mean flow over the double-cone. A ray-tracing model was used to simulate the FLDI response to certain disturbances. The low-frequency Kelvin-Helmholtz and streamwise acoustic disturbance frequencies did not vary beyond uncertainty bounds along the shear layer. The medium-frequency content had a clear dependence on the local separation height, with the mean frequency decreasing with streamwise position. The high-frequency Mack mode disturbances were only observed in some experiments, suggesting the disturbance is limited only to within the shear layer, making detection difficult if any bulk shear layer motion occurs relative to the FLDI beam positions. This study provides the first known FLDI data on shear layers in hypervelocity flows, together with simultaneous freestream characterization, with the aim to inform future experiments in hypervelocity ground testing facilities and high-resolution numerical simulations.

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