Application of proper orthogonal decomposition to a supersonic axisymmetric jet

Results are presented from the application of the snapshot proper orthogonal decomposition (POD) method to a spatiotemporal flowfield generated from large eddy simulations (LES) of a Mach 1.4 ideally expanded jet. This is part of ongoing research in the development and use of the POD method in conjunction with advanced laser-based optical measurements in high-speed flows. The POD application goal is twofold: to extract dynamically significant information on the large-scale coherent structures in a high-speed jet and to facilitate low-dimensional modeling of the jet. It was found that the spatial eigenmodes obtained using weakly correlated snapshots, but spanning tens of convective timescale and uncorrelated snapshots, are similar. It was also found that a short-duration temporally resolved LES data (simulating data obtainable from pulse burst laser-based measurements) could be used to calculate the time evolution coefficients of the eigenmodes. The use of a few modes (namely, 12) was sufficient for a reasonable reconstruction of the spatiotemporal flowfield. The use of POD with a vector norm instead of a scalar norm did reduce the energy captured in the first few modes and also changed their rank order, but did not substantially alter the reconstructed flow. In the early jet development region, the first and dominant mode was found to be axisymmetric, followed by either another axisymmetric or asymmetric (probably helical) mode, whereas higher modes in this region and all of the modes farther downstream were more complex and three-dimensional. The POD modes and their temporal coefficients obtained at various streamwise locations suggest that the large-scale jet structures undergo a process of disorganization near the end of potential core, followed by reorganization farther downstream.