Focused laser differential interferometry measurement of instability wave in a hypersonic boundary-layer

The wind tunnel experiment is one of the most important methods to conduct the hypersonic boundary-layer transition research. However, the experimental technology that can be used for three-dimensional hypersonic boundary-layer measurement is still extremely lacking, and the dynamic response of the existing measurement technology is quite restricted. In order to solve the above problems, a non-instrusive Focused Laser Differential Interferometry (FLDI) measurement system is set up based on the light refraction and interference principle and it can effectively measure the density disturbance of the flow field at spatial points. A hypersonic laminar/turbulent boundary-layer transition experiment was carried out on a 7° half angle sharp cone model in a conventional Mach 8 hypersonic wind tunnel with FLDI being the main diagnostic. The results show that FLDI successfully captures the second mode instability wave at 327 kHz and its 645 kHz harmonics. In comparison with PCB test results, FLDI has the advantages of high Signal to Noise Ratio, high dynamic response and high spatial resolution (less than 1 mm along the flow direction). Considering its excellent characteristics such as high spatial and temporal resolution, FLDI can be used as a promising diagnostic for the hypersonic boundary-layer transition and receptivity study.