Comparison of Hydrofoil Drag Reduction by Natural and Ventilated Partial Cavitation

The theory of cavitation in an ideal fluid is utilized to design hydrofoils that have a significant increase of lift to drag ratio for a regime of partially cavitating flows. Our recently reported experiments with natural cavitation have confirmed the existence of such an increase within a certain range of cavitation number and angle of attack for the specially designed hydrofoil designated as OK-2003. For applications of such a design to engineering, it would be necessary to keep the cavitation number within this favorable range and ventilation looks to be the most promising tool for control of cavitating flows. Therefore, comparative water tunnel tests have been carried out for both natural and ventilated cavitation of the OK-2003. The general similarity between the two kinds of partial cavitation for the developed low-drag hydrofoil is proven. When validating theory with the aid of water tunnel experiments, a general issue of how to make a comparison between natural cavitation and ventilated cavitation was encountered. This issue is the difficulty to determine the pressure within partial cavities. During natural cavitation the cavity pressure can deviate from vapor pressure due to the effects of dissolved gas and possibly other water quality effects. Direct pressure measurements within the partial cavity have proved to be unstable due to the unsteadiness of the cavity. The unsteadiness effect becomes more dominant as cavitation number is increased and the cavity becomes smaller. There is a point where the measured cavity pressure becomes unusable. In the case of ventilated cavitation, the interaction of the airflow with the surface of relatively thin cavities can be significant. Finally, it was experimentally determined that different dynamics of cavity pulsation are inherent to natural and ventilated cavitation.