A Preliminary Experimental Study of Vertical Hydrofoils of Low Aspect Ratio Piercing a Water Surface

Abstract

Most types of problems which arise in connection with the use of a hydrofoil operating in water can be solved simply by treating it as an airfoil operating in air. For this purpose, use can be made of the great wealth of theoretical information and experimental data which can be found in the literature. There are, however, regimes of operation of the hydrofoil which are not duplicated by the airfoil excepting possibly under very special conditions. These regimes are identified by one of the following: (a) cavitation (b) ventilation (c) proximity to a free surface Cavitation is characterized by the presence of water vapor bubbles at regions in the flow where the pressure is less than the vapor pressure corresponding to the existing water temperature. Although most commonly observed on the blades of propellers or on the vanes of axial flow pumps, cavitation can also be present on fins used to stabilize high speed underwater missiles, on hydrofoils used as lifting surfaces, or on support struts of various kinds. Allied to this problem is ventilation, a condition which is like cavitation in that it results in discontinuities in density in the fluid surrounding the hydrofoil, although the initiating mechanism is fundamentally different and the lighter medium is air or gas instead of water vapor. A third type of flow regime which may be very important is that associated with a hydrofoil which approaches or intersects a water-vapor or water-gas interface. In this case the flow must satisfy the constant pressure boundary condition on that interface. The effect of gravity may or may not be important, and the hydrofoil can be oriented in any direction. A lifting hydrofoil would most likely be parallel, or nearly parallel, to the water surface, whereas a support strut or a stabilizing fin would inter sect the water surface nearly at right angles. It is this last mentioned type of ope ration which is investigated in this report, and which, as will be seen later, also implies a study of the effects of air ventilation. Among the specific fundamental questions which arise in considering a vertical hydrofoil piercing a flat water surface and which is at an angle of attack to the flow, are the following: (a) How does the presence of the air-water interface affect the apparent aspect ratio of the hydrofoil as compared with its geometrical value? (b) What is the effect of air ventilation on the value of cross-force developed by the hydrofoil, and what observations can be made regarding the inception of this phenomenon? Since no previous hydrofoil studies had been performed in the Free Surface Water Tunnel, it was also of interest to determine the suitability of that facility and its associated equipment for doing work of this kind. On the other hand, the investigation was intended only as a preliminary one and was, therefore, undertaken with limited resources.

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