Quantification of convection velocity and dominant scale of large-scale structures by high-speed schlieren imaging

We proposed a new method to evaluate simultaneously both convective velocity and scale of dominant turbulent structures captured in a high speed framing schlieren movie. We took 90 time-series schlieren images of a transverse jet into a Mach 2 supersonic flow with 1 MHz sampling. We focused on periodical image gradations due to the turbulent structures within the jet and boundary layer. We remapped the image intensities on the structures along their trajectory as a time-space map. The time-space map showed swept stripe patterns. The presence of the stripe patterns indicates stable structures were periodically generated and convected downstream. Slope of the stripes corresponds to the convective velocities of the structures. Cycles in the space- and time-directions of the stripes correspond to the scale and formation cycle of the dominant structure. Two-dimensional Fourier transform efficiently extracted the dominant ones from the time-space map. We demonstrated this newlyproposed method using two-dimensional Fourier transform accurately evaluated the convective velocity and scale of the dominant turbulent structures within the jet and boundary layer.