Snapshot Proper Orthogonal Decomposition (POD) has been applied to reveal the dominant flow structures of an impinging jet flow field. PIV datasets of 200 snapshots were used for the POD analysis. The planar PIV measurements covered the free jet, stagnation and wall jet regions. The jet issued from a long pipe, 112 diameter, D, which ensured fully developed turbulent flow conditions at the outlet. The Reynolds number based on jet mean exit velocity was 16,000 and the pipe-to-plate separation, H, varied between 1D and 8D. The five most energetic modes were identified and found to contain 19–32% of the total turbulent energy. The energetic low modes were most pronounced for H/D = 8. The spatial topology was investigated by using velocity and vorticity fields of the mean flow and first five POD modes. For H/D = 8, the dominant flow structures were vortex tubes along the periphery of the jet. However, for H/D = 1, vortices along the impingement wall, outside of the stagnation region, were most prominent. The formation of vortices and their interaction with the impingement wall enhances local momentum transfer rates and could explain the secondary Nusselt number peak observed only at low separation distances.

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