Dynamic Modeling of Air Cushion Vehicles

The motion of an Air Cushion Vehicle (ACV) is a complex process involving the nonlinear dynamics of the ship, free surface waves, air cushion cavity, skirt, and air flow hydraulics (e.g. orifice behavior of the bag feed holes). The overall system is tightly coupled, with the loading of the ship dependent on the pressure field within the cavity, and the dynamics of the cavity dependent on the motion of the ship, free surface, and skirt. Principle excitation to the system is through the free surface motion and the fan flow. The large dimensions of the system introduce low frequency acoustic and mechanical resonances, which lead to complex and coupled system dynamics. The focus of this paper is on analytical modeling of an ACV and its physics to enable verification of a numerical model which is under development. The initial focus is on the dynamics of the air cushion cavity, with emphasis on its resonant frequencies and mode shapes. The mode shapes are important because they define the nature of the dynamic pressure distribution acting on the ship, and associated heave, pitch, and roll excitation. The strong dependence of the cavity resonant characteristics on the impedance of the skirt, which bounds the cavity, is first demonstrated by assessing limiting cases of a high impedance skirt (e.g. massive or rigid) and of a low impedance skirt (e.g. light or soft). The changes in resonant frequency and dynamic pressure distribution associated with the changes in skirt impedance are illustrated. Because the actual skirt impedance will lie between these two idealized cases, we also develop a lumped parameter model of the skirt dynamics. Initial parametric studies with this model, which investigate variations in the skirt mass, further demonstrate the strong dependence of the resonant frequencies and pressure distributions on the skirt impedance.© 2007 ASME