Base Flow Measurements of a Slender Cone in Hypersonic Flow

<p dir="ltr">Base and wake flows in the hypersonic regime are critical areas of study, as they involve complex interactions between shock waves, viscous effects, and high-temperature gas dynamics. The base flow field, occurring at the aft end of a body, is influenced by the separation of the boundary-layer and the formation of a low-pressure wake region. This region contributes to significant drag and potential aerodynamic instability. Incoming flow conditions, such as the state of the boundary-layer and freestream conditions, influence both base and wake flows. Understanding these flows is essential for the design and optimization of hypersonic vehicles, as they affect overall aerodynamic performance, thermal management, and structural integrity.</p><p dir="ltr">This project consists of base flow field experiments performed in the Boeing/AFOSR Mach-6 Quiet Tunnel, which is capable of producing a low freestream noise environment comparable to flight. The tunnel can also be run conventionally with increased freestream noise similar to conventional hypersonic wind tunnels. The model used in this project is strut mounted to the tunnel wall, as opposed to the commonly used sting mount to minimize interference. Sensors are routed internally through the strut to ensure that no upstream effects are imparted on the base and wake flow fields by the wires. Using a rotatable base plate and radially similar sensor locations, measurements can be made at the same radial locations with different sensors across multiple tests.</p><h4>Initial experimental investigations were carried out over a range of unit Reynolds numbers from approximately 7X10⁶/m to 15X10⁶/m. These experiments focused on measuring mean static base pressure and heat transfer, as well as base pressure and heat transfer fluctuations. An assessment of the boundary layer was first conducted to determine incoming flow conditions. In quiet flow, a fully laminar boundary layer was measured, except at the highest Reynolds numbers tested. At the highest Reynolds numbers tested, the boundary layer shows signs indicating transition. These indicators include high band-integrated root-mean-square values, schlieren imaging showing the presence of the second-mode instability, and a significant drop in base pressure. When run conventionally, the tunnel produces higher freestream noise, which produces a fully turbulent boundary layer. </h4><h4>Under laminar boundary-layer conditions, the base pressure coefficient was shown to decrease with increasing Reynolds number. Under turbulent boundary-layer conditions, the base pressure coefficient decreased slightly with increasing Reynolds number. Normalized and scaled static base pressures were compared with static base pressure correlations developed by Lamb and Oberkampf and correlations developed by DeChant and Wagnild. Comparisons show good agreement with Lamb and Oberkampf and poor agreement with DeChant and Wagnild. Static heat transfer measurements show extremely low levels of heat transfer. The highest heat transfer measured was approximately 1000 W/m². Under laminar boundary-layer conditions, radial gradients in heat transfer were measured, with cooling measured in the heat transfer sensor closest to the base center. Under turbulent boundary-layer conditions, similar radial gradients were present, but no cooling was measured. Normalized and scaled heat transfer measurements show poor agreement with heat transfer correlations developed by Lamb and Oberkampf.</h4><h4>Pressure fluctuation measurements on the base exhibit a novel correlation with Mack's second-mode instability measured on the surface near the aft end of the model. Double-pass schlieren imaging was used to capture flow features in the near-base flow field. Spectral proper orthogonal decomposition of schlieren images showed the second-mode instability in the boundary-layer and its effects on the shear layer and base flow field. Heat transfer fluctuation measurements showed no dominant frequencies. Higher broadband noise was seen towards the center of the base in heat transfer fluctuation measurements. </h4><h4>Subsequent experimental investigations examined geometric effects, focusing on the effects of the strut mounting system and trailing edge radius. Due to a drop in the maximum quiet pressure of the Boeing/AFOSR Mach-6 Quiet Tunnel, experiments were conducted over a unit Reynolds number range of approximately 7X10⁶/m to 9X10⁶/m. Strut thickness was varied from its original size up to 200% of the baseline value. Fluorescent pink dye suspended in polydimethylsiloxane oil was used to visualize the footprints of horseshoe vortices generated by the strut. Vortices were observed originating from the mounting strut. Increasing the thickness of the strut resulted in vortex positions moving farther from the centerline. Static base pressures and pressure fluctuations were also measured for each strut configuration.</h4><h4>Under laminar boundary-layer conditions, static base pressures were largely unaffected by strut thickness. Under turbulent boundary-layer conditions, static base pressures increased with increasing strut thickness. The presence of the strut introduced broadband noise on the strut side of the base and tripped the flow at higher Reynolds numbers.</h4><h4>Trailing-edge configurations included a nominally sharp trailing edge, a radius of 0.25 cm, and a radius of 0.64 cm. The base pressure coefficient increased with increasing trailing-edge radius under both laminar and turbulent boundary-layer conditions. Under laminar conditions, the amplitudes of high-frequency signals near 270 kHz and broadband noise on the base decreased with increasing trailing-edge radius. This dampening effect was also observed on the cone surface immediately upstream of the trailing edge. Under turbulent boundary-layer conditions, no such dampening effect was detected.</h4><p></p>