Identification of a Submarine Fin-tip Vortex Using POD Analysis

The meandering nature of the fin-tip vortex generated by a manoeuvring submarine can lead to a smearing of the vortex in the ensemble-averaged flow field. From stereoscopic particle image velocimetry (SPIV) measurements, it is possible to remove the smearing by shifting each instantaneous velocity field so as to produce a common centre for the vortex. However, it is difficult to identify the instantaneous vortex centre due to the presence of measurement noise and strong small-scale turbulent fluctuations. In this paper, a snapshot Proper Orthogonal Decomposition (POD) technique is used to capture the dominant large-scale coherent structures (from inspection of eigenvalue or energy distributions) to improve vortex identification and subsequent correction for meandering. The present findings suggest that, for the flow fields reconstructed from POD and subsequently corrected for meandering, the vortex flow exhibits steeper velocity gradients and is less noisy than the original results (inspection shows the reduction in turbulent fluctuation is as much as 16%).