Measurements of the Water Surface Contour Behind a Hydrofoil of Moderate Aspect Ratio

Abstract

An experimental program has been carried out for the measurement of the water surface contour due to a submerged hydrofoil of finite span. Because of the hydrofoil downwash, the water surface has a rather pronounced depression in the form of a long, narrow trough which extends many chords aft the hydrofoil. When the trailing vortex cores becomes sufficiently close to the water surface depression, flash ventilation of the vortices and the entire upper surface has been observed to occur abruptly. The model used here was a hydrofoil with a NACA 16-206 section and a rectangular plan form, mounted on a NACA 16-006 strut. The hydrofoil has a chord of 3 inches and an aspect-ratio of l.33. It has been found that the length and depth of the surface depression, and the location of the trough bottom are well defined functions of the Froude number and of the ratio of chord-to-submergence depth. It has also been observed that the distance between the trailing vortex core and the lowest points of the depression is an important parameter in effecting the onset of ventilating flow. This investigation covers a range of flow velocity, angle of attack, depth of submergence, and the flap angle deflection.