Schlieren visualization of hypersonic boundary layer transition on a circular cone

To develop the experimental technique for investigating the hypersonic boundary layer transition, schlieren visualization is used to study the characteristics of the boundary layer transition in the FD-14 shock tunnel at the Hypervelocity Aerodynamics Institute of China Aerodynamics Research and Development Center(CARDC). The test model is a 7° half-angle blunt cone with two different nosetip radiuses, 2.0 and 0.5mm, respectively. The nominal Mach numbers are 8 and 10 and the unit Renolds number varies from 1.6×10⁷ to 4.4×10⁷ m^-1. The high-speed camera's frame frequency is 20kHz. Structures of the second mode instability waves and turbulent spots are obtained by schlieren visualization and the power spectral density is analyzed based on the gray scale distribution of schlieren images. Results indicate that the wavelength of the second mode wave is nearly twice as large as the boundary layer thickness. Analysis results of the schlieren image sequence show that the propagation speed of the turbulent spot front is higher than that of the turbulent spot tail, and is slightly higher than the velocity of the boundary layer outer edge.