Space-Time Characterization of Ship Airwakes

The turbulent airwakes produced by a generic navy frigate (Simple Frigate Ship 2) was investigated in a low-speed wind tunnel. The frigate model was subjected to a headwind and immersed in a simulated atmospheric boundary layer. High-speed stereoscopic particle image velocimetry (PIV) was performed at six different spanwise planes over the flight deck. A spatio-temporal analysis of the PIV airwake measurements was conducted by applying the proper orthogonal decomposition (POD) to individual frequency bands of the airwake. Physical interpretations of the leading POD modes were substantiated by other complementary analyses. In particular, it was discovered that turbulent structures were shed from the funnel and superstructure edges. The flow recirculation region behind the superstructure exhibited a strongly three-dimensional bistable behavior. A pair of vortices near the stern also demonstrated low-frequency bistable dynamics. Notably, the bistable flow recirculation region appeared to influence or interact with the low-frequency behavior of the other major flow features.