Measurement of the Velocity Field in the Wake of a Ship Double-Model in the Wind Tunnel with a Laser Velocimeter

Abstract

The flow field that develops around the hull of a ship advancing at constant speed on a straight course presents an extremely complicated pattern of velocity distribution. The problem is even worse if the ship maneuvers in water of finite depth or confined by solid boundaries as in a harbour. The following remarks apply as long as the flow field is stationary (constant speed, no change of direction). The flow may be described by the methods of boundary layer theory (except in the stern region) since the Reynolds number is very high (order of magnitude: 10⁹). Modern methods of computational fluid dynamics are well suited to perform the calculation of the so called outer flow neglecting the viscosity of the water. One method is to solve this boundary value problem, now in the frame of potential theory, by so called panel methods. The boundary, here the surface of the underwater ship hull, is divided into a number of elements and singularities (e.g. sources and sinks) are located on these elements. The boundary value problem may be solved numerically yielding detailed knowledge about that flow field that would develop if the surrounding medium were a perfect fluid. In fact the problem is severely complicated by the presence of the free surface. To a certain degree this may be taken into account by the calculation method. The resultant potential theoretical velocity field and the pressure distribution induced on the hull surface may be taken as the input for a subsequent boundary layer calculation.