Time-Resolved Ship Airwake Measurements in a Simulated Atmospheric Boundary Layer

Abstract

The unsteady flows produced over the stern of a Simple Frigate Shape 2 ship model are studied in a low-speed wind tunnel. Time-resolved particle image velocimetry (TR-PIV) measurements were performed in several streamwise and cross planes. Two grids of Cowdrey rods were used to simulate the basic characteristics of an atmospheric boundary layer in terms of upstream velocity profile and turbulence intensity. The TR-PIV measurements showed large regions of unsteady flow separation, dominant vortical flows, and significant wall-normal flows over the flight deck regions caused, in part, by the irregular shedding of vorticity and turbulence from the upstream funnel and superstructure of the ship. Turbulence intensities were found to be particularly high over the flight deck. The measured flows also suggested the existence of asymmetric, intermittent flow in the near-wall regions of the deck, and time-varying fluctuations in the recirculation region behind the hangar. The crossplane measurements showed the development of shear layers at the corners of the flight deck on both the port and starboard sides, and sets of counterrotating vortices at the edges of the flight deck. An energy spectrum analysis showed dominant frequencies in the regions where the shear layer was developed behind the funnel and above the flight deck. Spectral proper orthogonal decomposition revealed that the physically relevant coherent structures in the airwake were low-frequency modes near the flight deck and at large-scale of the order of the length of the deck.