Modal analysis of a mach 1.5 underexpanded jet using time-resolved optical diagnostics

An experimental study concerning an underexpanded, screeching, Mach 1.5 jet at NPR = 4.4 is presented. Experimental data were acquired from high-speed schlieren imaging (100,000 fps) as well as particle image velocimetry (PIV) at rates of either 10 kHz (full frame) or 50 kHz (time-resolved). The modal analysis techniques of POD and SPOD were then applied to these datasets. The ensuing comparisons revealed both schlieren images and time-resolved PIV vector fields could be decomposed to capture the same 17 kHz oscillation (screech) that emerged in the jet. Both techniques captured strong oscillatory behavior in the shear layer of the jet, where the screech phenomenon is theoretically the strongest. Excellent agreement was found between SPOD spectra generated from both sets of experimental data, and a spectrum generated by a near-field microphone. Projecting the dominant 17 kHz mode from both SPOD datasets onto the respective mean data revealed a strong oscillation in the jet, and the reality of this oscillation was confirmed by phase-averaging the schlieren data at the appropriate frequency. In addition to further demonstrating the applicability of SPOD to unsteady, high-speed flows, the results gathered from this experiment demonstrated similarity between two different experimental techniques in isolating the most dominant fluid characteristics. Applying various reduced-order modeling techniques such as POD and SPOD may lead to better understanding of the critical flow phenomena associated with high-speed flows, and carefully-crafted experiments and simulations may be able to leverage such techniques to design more efficient and effective flow control systems.