Measurement of the second-mode's propagation velocity in the hypersonic boundary layers of a flared cone with dual-frame focusing schlieren and optical flow velocimetry

Understanding the transition mechanism in the hypersonic boundary layer is essential for hypersonic vehicles , where the second-mode wave plays a key role in the process. An experiment utilizing dual-frame focusing schlieren and optical flow velocimetry is performed in the Mach 6 wind tunnel at Peking University. Detailed two-dimensional velocity field , frequency, and density fluctuation intensity of the second-mode waves in the near-wall region are analyzed and discussed. The experiment observes the density gradient's velocity fluctuating in the horizontal direction within the sonic line , while its mean velocity remains uniform. The measurement of the second-mode frequency meets the results by PCB® and FLDI but with a much lower sampling frequency. • Measure the second-mode's velocity via Focusing Schlieren and Optical Flow. • A Detailed two-dimensional velocity field of the second-mode in the near wall region is obtained and analyzed. • Measure the second-mode's frequency with a significantly lower data acquisition rate.