Loïc SOMBAERT, Mathieu LUGRIN, Sébastien ESQUIEU, Reynald BUR
DOI Number: N/A
Conference number: HiSST-2025-256
This support paper for the oral presentation at the HiSST conference investigates the laminar-to-turbulent transition over the BOLT geometry, based on experimental data acquired in the ONERA R2Ch hypersonic blowdown wind tunnel using a 1/3-subscale model at Mach 7. Infrared thermography and high-speed wall-pressure measurements (PCB sensors) were obtained across a broad range of freestream Reynolds numbers, from fully laminar to well-developed turbulent regimes. Transition typically appears firstly in the outboard region of the geometry, characterized by the appearance of two symmetric heating lobes away from the centerline. Particular attention is given to this region, where a high-frequency instability near 150 kHz is consistently detected in the Power Spectral Densities at Reynolds numbers around 5×106 m-1. A lower-frequency bump near 20 kHz is also observed, though less distinctly. To investigate the physical origin and nature of these instabilities, a Linear Stability Theory (LST) analysis is conducted based on numerically computed laminar baseflows. Results from this analysis suggest that the observed spectral bumps are associated with the amplification of the second Mack mode and traveling crossflow instabilities. This is consistent with existing literature, as these two instability mechanisms are commonly identified as dominant in hypersonic boundary-layer transition over complex three-dimensional geometries. However, the application of classical LST to such a complex three-dimensional geometry leads to relatively low N-factors, below 1.5, despite the experimental data clearly indicating the occurrence of transition. This discrepancy highlights the limitations of LST in predicting transition onset in realistic configurations,
where non-parallel effects and mode interactions may play a significant role and are not captured by local, linear analyses.