Abstract
A general unsteady vortex-lattice technique, called FINS, is developed to model the flowfield around a fin attached to the hull of a moving ship. The fins can have any shape and movable flaps and be at any angle of attack below those where stall and/or cavitation occur. Then a method to combine this model with such existing motion-predicting programs as LAMP or FREDYN is developed. The combined programs, working in concert, can be used to predict the motion of ships with fins attached to the hull in the time domain. Because of the great disparity in the dimensions of the hull and fins, different grid sizes and different time steps are required to produce the same accuracy. Moreover, LAMP and FREDYN are based on surface distributions of sources, whereas FINS is based on a lattice of discrete vortex lines. Consequently, the final combination of the two programs (LAMP and FINS in the present work) is a single program that simultaneously uses distributions of sources and vorticity over grids of vastly differing sizes while using different time steps for the different grids. To provide confidence in the new model of flow over the fins, it is shown in several examples that the numerical and experimental results are in close agreement. In a final example, the program with the combined models is used to predict the motion of a ship in a seaway. The results show a 50% reduction in the computed peak amplitudes of the roll motion of a CG47 cruiser in a random beam sea; the sea state is five.
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