Laminar boundary layer stability measurements at Mach 7 including wall temperature effects

An experiment was performed to verify and extend earlier observations of instabilities in the hypersonic laminar boundary layer of cones. The present tests were performed at edge Mach number 7 in a continuous wind tunnel, at edge Reynolds numbers from one to three million and wall-to-stagnation temperature ratios of 0.41 to 0.8. Stability data were obtained for non-dimensional frequencies as high as 5 x 10 to the -4 with high data density provided by computerized data reduction techniques, which were also used to re-examine earlier data. The results are consistent with all earlier data and reveal a complex stability diagram with at least three unstable regions. The lower of these regions appears to be generated by the first and second mode instabilities but no theoretical guidance exists to help identify the contributing modes for the other regions. The second-mode instability has the highest amplification rates and generates the optically detectable 'laminar waves'; a 'second harmonic' to these waves is generated by the modes populating the next most prominent unstable region. The stability diagram extends smoothly into the non-linear and transitional regimes, which are marked by the attainment of a maximum amplitude and then a decay, of the laminar waves. Wall cooling causes a substantial increase of the amplification rates with a concurrent upstream movement of the transition zone.