Aerodynamics of realistic frigate geometries: Conditions for bi-stable wake dynamics

The aerodynamics of realistic frigate geometries is characterized by pressure and force measurements at the base of the three aft-facing steps that make up the frigate. The base model is a Lafayette class frigate. The conditions for bistable wake flow dynamics downstream of the command post, hangar and stern, are studied by changing elements of the geometry. The wake flow dynamics were found to be bi-stable only aft of the hangar, which can be detrimental to helicopters landing or taking off from the ship's deck. The bow level in front of the command post plays a key role in the ship's aerodynamics. It was found that bi-stability was only recovered when the ship's geometry was changed, by doubling the height of the parallelepiped command post and raising the bow to the level of the T-shaped structure. The wake flow at the stern was only bi-stable when the stern section was rectangular and straight. Finally, The bi-stability of the wake flow is eliminated on the hangar by adding a flap of sufficient depth in the extension of the hangar roof.