Experimental Investigation of Mach 5 Laminar and Turbulent Fin-Induced Junction Flow

Fins provide stability and control to high-speed flight vehicles. Fin-junction flows are known to be prone to low-frequency unsteadiness with possibly negative implications for the operation of flight vehicles. The origin of the unsteadiness and the conditions for which it occurs remain poorly understood. This paper reports on laminar and turbulent fin junction flow experiments in the Mach 5 shock tunnel at New Mexico State University for leading edge fin sweep anglesof 0°, 15°, and 30°. Kulite measurements ahead of the junction indicate a 30ms time interval with sufficiently constant freestream conditions. The turbulent approach flow boundary layer profile is measured with a femtosecond laser electronic excitation tagging system. A folded Z-type high-speed Schlieren system and a self aligned focused Schlieren system are employed to visualize the junction flow. Schlieren visualizations for the turbulent interactions reveal a stochastic motion of the separation shock. With increasing fin sweep angle, the size of the interaction region and the unsteadiness subside. Using inverse theory, the unsteady heat flux was estimated from thin film temperature gauge measurements. For the unswept turbulent interaction, the heat flux has a peak at 35kHz, corresponding to a Strouhal number based on boundary layer thickness of 0.5 in agreement with the literature. Wavelet analyses of PCB wall pressure data reveal broad frequency content for the approach boundary layer and a strong increase in root mean square pressure fluctuations for the junction region with a broad peak near 35kHz. Proper orthogonal decomposition analysis was applied to the Schlieren images to provide insight into the dominant unsteady flow structures. Modes that capture the separation shock are weakly correlated with the heat flux and wall pressure data at 35kHz, providing further evidence that the frequency is associated with low-frequency unsteadiness of the junction flow. A preliminary self-aligned focused Schlieren image of the laminar junction flow reveals a large separated flow region with a shallow separation shock.