Proper Orthogonal Decomposition of High-Speed Particle Image Velocimetry in an Open Cavity

Two-dimensional velocity data of flow over an open cavity acquired by pulse-burst particle image velocimetry at 37.5 kHz for freestream Mach numbers of 0.6, 0.8, and 0.94 have been analyzed using proper orthogonal decomposition (POD). The snapshot application of the POD resulted in modes consistent with previous studies on cavity flows under similar conditions. The time-resolved nature of velocity datasets also allowed for the application of the classical POD in both space and time which, for a stationary flow, is solved in the frequency domain. Eigenvalue spectra and convergence plots revealed that these POD modes are organized by capturing the maximum energy associated with dominant frequencies. The extracted eigenvectors revealed a strong dependency on frequency, with the most dominant modes corresponding to frequencies associated with Rossiter modes known to dominate open cavities. Mach number comparisons from the classical POD application reveal that their modes efficiently capture energy of dominant frequencies associated with the flow dynamics, regardless of the freestream conditions. Using a similarity parameter between spatial unitary basis functions, it was shown that the modes at different Mach numbers were quantitatively similar at the same Strouhal and POD mode numbers.