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 the BOLT-1A mission, the follow-on BOLT-1B flight experiment was successfully 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 both the 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 re-laminarization, depending on each sensor’s distance from the leading edge. The corresponding energy spectra exhibited a characteristic $-7/3$ slope within the inertial frequency range; however, finite differences in energy distribution were observed between the ascent and descent phases, which are believed to be an effect of local Reynolds number at the Kulite sensor locations. Time–frequency analysis of pressure fluctuations from PCB sensors located near the swept edges show broadband energy spectra indicating turbulent flow at those locations, while the sensors closer to the centreline show second mode instabilities associated with transition/re-laminarisation. Further, there is a redistribution of spectral energy to lower frequencies during the re-laminarization process of turbulent flow near the swept edges, which requires further investigation.