Experimental Investigation of Boundary-Layer Transition on a Slender Cone at Mach 4

Abstract

Boundary-layer transition over a 5 degree half-angle straight cone model was examined in University of Maryland's Multiphase flow Investigations Tunnel (MIST), a Mach 4 Ludweig tube. In order to prepare for future studies with particle-laden flow, this study was conducted to characterize the boundary-layer of the cone under dry conditions. Furthermore, both first-mode (Tollmien-Schlichting) and second-mode (Mack) boundary-layer instabilities are expected at Mach 4 (Mack), with the former being not widely studied. The boundary-layer on the top surface of the cone was visualized using high-speed Schlieren and analyzed using image and signal processing techniques. Experimentation was conducted over a range of unit Reynolds numbers from 31.3-40.3 10^6 m^-1 in order to vary the transition location. Power Spectral Density (PSD) and Spectral Proper Orthogonal Decomposition (SPOD) revealed the most coherent wave packets propagating within the boundary layer at frequencies ranging from 10 to 40 kHz, frequencies that are consistent with the first mode. Frequencies between 110-140 kHz contained additional content consistent with the first mode but less coherent. The presence of these structures was verified with a bandpass filter allowing 10-40 kHz and a highpass filter allowing >100 kHz each separately applied to the original footage. Future work is planned to compare the results of this paper with multi-phase flow experiments conducted with the same model at the same freestream conditions.