Hypersonic boundary-layer transition on the BOLT forebody in the R2Ch facility

Abstract

Laminar-to-turbulent boundary-layer transition is investigated on a 1:3-subscale BOLT model within the ONERA R2Ch hypersonic wind tunnel. The study is conducted at Mach numbers of 6 and 7 over a wide range of Reynolds numbers, enabling observations of the entire transition process over a large portion of the geometry. Infrared thermography measurements are used to track the evolution of transition onset. High-frequency wall pressure transducers (PCB) provide insights into transitional instabilities on the central and outboard regions. Pressure spectra analysis reveals amplified spectral energy content, associated with different instabilities, when the Reynolds number is increased. At Mach 7, a high-frequency instability (\(f\approx 140\)–250 kHz), believed to be a second Mack mode, is found to be amplified on the outboard region. However, given its absence from Mach 6 results which display very similar transition fronts and the small spatial amplification, the role it plays in the transition process remains unclear. A different instability mechanism is found in the central region, with a lower-frequency range. The computation of the instabilities’ streamwise propagation velocities also highlights significant disparities between the central and outboard regions. Comparing these values with laminar CFD results provides further insights into characterizing these instabilities. Additionally, the study investigates signal intermittency by examining the high-frequency content of spectrograms to point out the emergence of turbulent spots.