Experimental investigation on the nonlinear development of the crossflow transition on a 75° swept flat plate at Mach 6

Hypersonic crossflow transition typically occurs on the windward side of gliding aircraft, making the study of crossflow instability both scientifically and engineering-wise valuable. In our previous research [Niu et al., Phys. Fluids 36, 124133 (2024)], the early evolution of traveling crossflow instability on a 75° swept flat plate at Mach 6 was studied. In the current paper, we investigate the nonlinear development of crossflow transition during its later stages. The experimental studies were conducted in a Mach 6 quiet wind tunnel, employing nano-tracer-based planar laser scattering (NPLS) technique to capture boundary layer flow structures in the streamwise direction. These results were subsequently analyzed quantitatively using wavelet tools. The findings indicate that the boundary layer experiences significant nonlinear development during the later stages of crossflow transition. Throughout the transition process, the fundamental wavelength of the peak signal, denoted as λ0, is approximately 20 mm. In the zones at z = 40–60 mm, harmonics with wavelengths of 2λ0 and 3λ0 emerge. As the breakdown of the boundary layer begins (z > 60 mm), these harmonics diminish, and new harmonics with wavelengths of 12λ0 and 13λ0 increase in prominence, corresponding to characteristic frequencies of 2f0 and 3f0. This behavior aligns with observations that Mack modes dominate the transition during the later stages.