Localization, Characterization, and Modeling of Freestream Disturbances in Hypersonic Wind Tunnels

Hypersonic ground-test facilities are critical for evaluating aerodynamic forces, aerothermodynamic heating, and boundary-layer transition (BLT) behavior on high-speed flight vehicles. However, conventional hypersonic wind tunnels are plagued by high-intensity freestream noise present in the test section. Because these disturbances build, focus, and propagate downstream, the fundamental understanding of their evolution may require a potentially deep analysis within the upstream components of the facility. Historically, Laufer [1] established that nozzle-wall turbulent boundary layers radiate downstream-directed Mach waves, making them a principal source of freestream noise. However, especially in high-enthalpy and hypervelocity impulse facilities, the noise field may be further enhanced by complex upstream dynamics that are not optically accessible. Examples of focus areas include, but are not limited to: • Nozzle Throats and Walls (all tunnels): The extreme thermal and velocity gradients in the nozzle throat pose an opportunity for disturbance generation and initiation of turbulence over the nozzle walls. However, severe restricted throat geometry and high heat flux make optical diagnostic access virtually impossible. Diagnostics on tunnel walls are also very challenging although there are examples of previous successful attempts [12]. • Reflected-Shock Tunnels: Reservoir entropy, pressure fluctuations, and driver-gas contamination originate from the complex interaction between the reflected shock wave and the shock tube wall boundary layer [5, 6]. Furthermore, upstream diaphragm particulate-laden flow [7] acts as a continuous source of premature model transition downstream. • Expansion Tubes and Tunnels: Subject to driver-gas acoustic focusing [9, 10], where upstream driver unsteadiness is focused directly into the test gas when sound speed ratios fall into unfavorable ranges. This is exacerbated by secondary diaphragm rupture wave systems [3, 11] propagating from upstream. Characterizing these regions is exceptionally difficult because standard optical diagnostic techniques (such as Femtosecond Laser Electronic Excitation Tagging (FLEET), Focused Laser Differential Interferometry (FLDI), or Rayleigh scattering) require optical-grade line-of-sight window access, which may not withstand the extreme pressures, temperatures, or geometric constraints of regions such as the driver, reservoir, or nozzle throat. To bridge this gap, this STTR topic focuses on the development of specialized diagnostics, informed by multi-fidelity computational flow models ranging in scope from system-scale to component-specific. Collaborative proposals from Small Business Concerns (SBCs) must address the following technical pillars: • Upstream Noise Characterization & Modeling: Modeling facility-specific noise generation phenomena such as transient or statistically steady boundary-layer effects, driver-gas acoustic focusing, diaphragm bursting, starting from the most upstream location ultimately responsible for noise contamination of the test section. • Diagnostics for Non-Optical Zones: Developing and demonstrating novel non-intrusive or minimally intrusive diagnostic systems for flow and acoustic characterization, capable of operating without traditional optical windows. Promising approaches include but are not limited to flush-mounted high-frequency pressure/thermal sensor arrays, micro-bore fiber-optic probes, laser-based acoustic sensing, and/or hybrid data-assimilation techniques that computationally reconstruct upstream flow states from sparse wall measurements.