Coherent structure in the turbulent planar jet. Part 1. Extraction of proper orthogonal decomposition eigenmodes and their self-similarity

In this paper the coherent structure in the similarity region of the turbulent planar jet is examined experimentally by application of the proper orthogonal decomposition (POD). In particular, twin cross-stream rakes of X-wire probes are used to take cross-spectral measurements with different spanwise separations between the rakes and at several locations throughout the similarity region. The resulting POD spatial eigenfunctions for each of the three velocity components depend on cross-stream spatial coordinate, Strouhal number, and spanwise wavenumber. Corresponding eigenvalue distributions are obtained in Strouhal number–spanwise wavenumber space. Eigenvalue convergence is demonstrated to be rapid. When properly scaled the eigenfunctions and eigenvalues are shown to exhibit self-similarity though the streamwise location at which this commences depends on the particular velocity component. The results suggest that the flow supports a planar structure aligned in the spanwise direction as well as an essentially three-dimensional structure with asymmetrical shape in the cross-stream direction and pseudo-periodically distributed in the spanwise direction. Comparison of the single- and dual-rake implementations of the POD presented in this paper demonstrate that measurements confined to a single plane are incapable of properly extracting the planar modes. Rather, the single-rake implementation results in modes that appear to be a weighted sum of modes corresponding to different spanwise wavenumbers.