Hypersonic Boundary-Layer Instability on a Highly Cooled Cone. Part II: Schlieren Analysis of Boundary-Layer Disturbances

Abstract

A campaign of hypervelocity experiments was conducted with a blunted 5d-half-angle cone in the T5 shock tunnel. A modified Z-type schlieren setup was utilized to image the boundary-layer with total enthalpies around 10 MJ/kg and freestream-to-wall temperature ratios of approximately 5. Second-mode instabilities and transitional behavior in the boundary layer were identified imaged within the field of view between x/L = 0.57 and 0.73. Second-mode wavepackets manifested as alternating dark/light waves beginning right at the wall and permeating up to 30% of the boundary-layer height, d. Spectral analysis of the two lower-Reynolds-number shots demonstrated that these packets could be characterized by peak wavenumbers 315 - 365 /m at 10% d. In the highest-Reynolds-number shot, structures passing out of the boundary layer indicated the beginning of turbulent breakdown. For discrete examples where the onset of these structures was imaged, breakdown appeared to begin at x/L = 0.66. A Gabor-filter-based image analysis indicated that spectral content was transferred out of the boundary layer at disturbance Mach numbers 2.75-3.75. A larger range of wavenumbers, 305 - 415 /m, populated the near-wall content in this transitional shot, and high-wavenumber content up to 450 /m was identified outside the boundary layer. It is to be noted that this paper serves as a companion study to that of Ref. 1, which interrogated the same boundary layer with focused-laser differential interferometry (FLDI).