ENHANCING THE EFFECTIVE TEMPORAL RESOLUTION OF SCHLIEREN IMAGING FOR UNSTEADY COMPRESSIBLE FLOWS

The unsteady flow mechanisms that can be extracted from high-speed shadowgraph and schlieren methods in compressible flows are often limited by the maximum frequencies recorded, which are defined by the frame rate of the system camera. This paper proposes a data processing method to enhance the effective temporal resolution of flow visualisation data by taking advantage of
camera capabilities to record a sub-region of the domain at frame rates higher than the nominal value. By using proper orthogonal decomposition to integrate some higher-frequency information into the original data set, a time series of schlieren images is reconstructed with increased effective frame rate. The approach is applied to shadowgraph data of a Mach 1.3 underexpanded jet flow
recorded at 20,000 frames per second over the entire field of view, with a smaller subdomain recorded at 40,000 frames per second. The reconstruction produces a series of full-domain shadowgraph images at 40,000 frames per second. The cross-correlation coefficient compared to known true values is 0.949, which corresponds to 13% enhancement from typical inter-frame correlations towards perfect reconstruction.