A Conditional Perspective of the Ship Airwake

The airwake of a simplified frigate model was measured using a synchronous, dual-plane, stereoscopic particle image velocimetry (PIV) approach. Four pairs of orthogonal streamwise and spanwise plane combinations were surveyed over the flight deck. Synchronous PIV measurements allowed the three-dimensional airwake structure to be examined during two flow events likely relevant to rotorcraft operations. The flow events were the vertical oscillations of the reattaching shear layer aft of the superstructure and the bistability of the recirculation region. Conditional averages were obtained by averaging flow snapshots meeting specific criteria for each flow event of interest. The conditional averages showed that vertical flow oscillations of the shear layer were associated with large-scale, three-dimensional regions of organized fluctuations. These large scales encapsulated significant flow gradients across the streamwise and spanwise extent of the flight deck. The influence of the shear layer on the recirculation bubble and the flow angularity above the deck were also established. In addition, examining the bistable recirculation region through conditional averages revealed that each stable state influenced nearly the entire flowfield over the flight deck. Specifically, the location and size of the horseshoe and flight deck vortices were altered depending on the predominant stable state. The time dynamics of the airwake as the flow transitioned between the two stable states were subsequently established.