Spectral Analysis of Wall-Pressure Measurements in the BOLT-1B Flight Experiment

The Boundary Layer Transition (BOLT) flight experiments were developed to study hypersonic boundary-layer transition and turbulence on an experimental configuration featuring low-curvature concave surfaces and highly swept leading edges. Building upon BOLT-1A, the BOLT-1B flight experiment was launched on September 2, 2024, from Andøya Space, Norway. This paper presents results from time- and frequency-domain analysis of wall-pressure measurements acquired on the primary surface of the BOLT-1B vehicle during ascent and descent experiment windows. Time-series data from Kulite pressure transducers revealed spatial variations in the onset and delay of boundary-layer transition and relaminarization, depending on each sensor’s location relative to the leading edge. The corresponding energy spectra exhibited a characteristic [Formula: see text] slope within the inertial frequency range; however, differences in energy distribution were observed between ascent and descent phases, believed to be an effect of local Reynolds number at the Kulite sensor locations. Time–frequency analysis of pressure fluctuations from PCB sensors near the swept edges showed broadband spectra, indicating turbulent flow, whereas sensors closer to the centerline exhibited disturbances occurring within a frequency range commonly associated with high-frequency boundary-layer (vortical type) instabilities. Furthermore, a redistribution of spectral energy toward lower frequencies occurred during the relaminarization of the turbulent boundary layer near the swept edges, which is compared to the characteristic feature of the transition process in shear-layer and mixing-layer flows.