Hypersonic Boundary-Layer Instability on a Highly Cooled Cone. Part I: Q-FLDI Measurement and Instability Calculations

Abstract

In this paper, a Q-FLDI system was constructed to correlate disturbances inside and outside of the boundary layer over a blunted cone in high-enthalpy, hypersonic flow at T5, the free- piston-driven reflected-shock tunnel at California Institute of Technology. We present results for three shots representing a fully-turbulent case with a 5 mm nose-tip radius, an unstable case with a 5 mm nose-tip radius at moderate enthalpy (6 MJ/kg), and an unstable case with a 2 mm nose-tip radius at higher enthalpy (10.5 MJ/kg). In the fully-turbulent case, the spectra indicate the possibility of a region where there is a power-law variation in the spectrum. For the unstable case with a 2 mm nose-tip radius at higher enthalpy (10.5 MJ/kg), the FLDI detected the second mode instability at approximately 1 MHz. This agrees well with results obtained using the schlieren technique for experiments performed in this campaign at similar conditions. The mismatch when compared to stability calculations for this case could be attributed to run-condition calculation error. An unstable case with a 5 mm nose-tip radius at moderate enthalpy (6 MJ/kg) is also presented. In this case, there is more content measured outside of the boundary layer than inside. For this case, computations of the mean flow show that the entropy-layer had not been swallowed at the point of measurement suggesting the elevated energy content outside the boundary layer could be due to entropy layer instabilities or oblique waves not considered in the stability analysis.