First and Mack-mode instabilities in a flat-plate boundary layer at Mach 4

Boundary layer stability experiments are performed in a Mach 4 quiet wind tunnel on a flat plate. Clusters of surface-mounted, high-frequency pressure transducers are installed at two locations on the model. Transducer clusters allow the computation of cross-spectra and the determination of the phase speeds and wave angles of passing boundary-layer waves. At an edge Mach number of 4, the coexistence of Mack's first and second modes is of particular interest. Peaks in the autospectral densities provide weak and inconsistent evidence of unstable waves within frequency bands relevant to either mode of interest. Cross-spectral densities show high coherence at frequencies associated with the first mode. Wave angles appear to match well with those predicted for the first mode, while other propagation information matches poorly, likely due to shortcomings in the analysis tools in dealing with low signal to noise ratio situations.