Modification of Air Cavity Flow Under Model Hull With Hydrodynamic Actuators

Abstract

Air cavities employed under ship hulls can result in significant decrease of the water frictional drag by reducing the hull wetted area. However, these cavities usually perform well only in a limited range of the ship speed and attitude. In off-design states and in the presence of sea waves, efficient air cavities covering large areas of the hull are difficult to form and maintain. This problem can be potentially addressed with help of hydrodynamic actuators, such as compact hydrofoils, tabs, and spoilers, which can assist with forming and maintaining air cavities under ship hulls. In this study, exploratory tests have been conducted with a simplistic small-scale hull having a bottom recess. Air was supplied into the recess to produce an air cavity, and several actuators were implemented and manually controlled during the tests. Subjected to external water flow, the air cavity under the hull was found to be responsive to variable positions of the actuators. Positive effects on the air cavity produced with specific actuator settings are identified and discussed in the paper. A series of experimental photographs of the air-water interface are shown for various actuator settings. The air flow rates needed to establish and maintain a large air cavity under the model hull are also reported.