Flow structure measurement of high-enthalpy plasma based on laser-speckle background oriented schlieren

The inductively coupled plasma wind tunnel is an important facility for simulating high-enthalpy fluid. Due to its extreme conditions, such as high enthalpy, very low density, and strong self luminosity, traditional flow visualization and measurement methods cannot effectively capture flow structures. Focusing on the flow field characteristics of the inductively coupled plasma wind tunnel, this study investigates the laser speckle background oriented schlieren technique. Compared to conventional printed or sprayed backgrounds, the quality of laser speckle imaging is independent of the camera’s focus length, and it offers high brightness, monochromaticity, and short pulse time, which significantly improves the system’s sensitivity and anti-interference capabilities. The optical flow algorithm is applied to calculate the displacement of the laser speckles, enhancing the accuracy of small displacement calculations in low-density flow fields. The structure of the detached shock wave over a spherical cylinder was obtained in the inductively coupled plasma wind tunnel. The shape and position of the shock wave were in good agreement with the numerical results, verifying the effectiveness of the background oriented schlieren technique under extreme flow conditions.