On the Characteristics of a Hydrofoil Section Running at a Constant Advance Speed in Calm Water

Abstract

Free water surface effects on the hydrodynamic characteristics of a 2-dimensional foil section running with zero attack angle are examined through model experiment and numerical calculation. The foil section is used as an anti-pitching foil of a catamaran ship, which has shown remarkable results in the tank test. The catamaran ship equiped with the anti-pitching foil between its twin hulls, which is obviously effective as a transverse strength member, has fairly good propulsive performance as well as good seaworthiness in rough seas. The model experiment of the foil is performed in a few conditions of submergence and advance speed corresponding to the working range of the anti-pitching foil. Surface pressure distribution is measured by means of the SCANI valve & differential pressure gauge system, and the hydrodynamic forces and moment by a load cell. It is known by the measurement that free water surface effects on the lower surface of the foil is very small and that there is little change in the pressure distribution on it by the change of running conditions. Pressure distribution on the upper surface varies according to the change of submergence and advance speed. In the region where Fn > 1.0, pressure distribution and observed flow field over the foil are smooth. The lift coefficient varies smoothly and the drag coefficient gives almost constant value in the high speed region. In the low speed region, pressure distribution, flow field and hydrodynamic forces show drastic changes to small changes of the running conditions. When the submergence is very small, an adverse pressure gradient or a negative lift coefficient is seen in the low speed region. Free water surface effect on the pressure distribution is negligiblly small for the condition f/c > 1.0 and Fn > 1.0. The effect to the lift coefficient, however, is still seen even when f/c= 2.0. Numerical calculation of the linearized problem by means of the surface vorticity distribution method is performed. Wholly speaking, the comparision of the surface pressure distribution between measurement and calculation shows good agreement throughout the range of present measurement. Some discrepancies are seen in the case where the submergence is small and Fn <1.0. The separation of flow on the upper surface near the trailing edge is also observed when Fn < 1.0 for deeply submerged condition.