CURRENTS AND WAVE FORCES ON SHIPS AND MARINE STRUCTURES
Forces on a body oscillating in incoming waves and a weak current are studied. Coupling between the oscillatory wave field and the steady flow around the body is accounted for. Friction and separation effects are disregarded, and the fluid flow is modelled by potential theory. The boundary value problem for the velocity potential is transformed to an integral equation by Green's theorem, using a Green function satisfying the linear free surface condition with small forward speed. Local, small forward speed expansions for the potential and the Green function are applied in the vicinity of the body, giving two sets of integral equations for the unknown zero speed and the small forward speed potentials. There are unknowns on the wetted body surface only. The right hand side of the small forward speed integral equation involves a fast decaying integral over the free surface. There is no water line integral in the integral equations. The method is applicable to bodies of arbitrary shape. The diagonal added mass and damping coefficients are found to be functions of the current speed only through the encounter frequency. The linear exciting forces are found by generalised Haskind relations. Analytical expressions for the mean second order horizontal drift forces and the mean second order yaw moment are given, and numerical examples are presented for different body geometries. The mean drift forces are usually increased by the presence of a weak current along the incoming wave direction. For complex body geometries the wave drift damping may, however, in narrow wave number regions, become negative. The mean yaw moment may be changed by 100% by the presence of a weak current. Energy check and numerical convergence of the method are also discussed.
Intl. Symposium on the Dynamics of Marine Vehicles and Structures in Waves, 2nd; 24-27 June 1990; Brunel Univ, Uxbridge, U.K. Sponsored by Office of Naval Research (London and Washington D.C.) and RINA. Procs. Publ by Elsevier Science Pubs, ISBN 0-444-89020-3, p 167 [14 p, 17 ref, 11 fig]
Old TRIS Terms:
British Maritime Technology
Aug 14 1995 12:00AM