Air cavity with variable length under a model hull

Abstract

The application of air-ventilated cavities for marine vessels can result in substantial drag reduction. Experimental and modelling studies of air cavities formed under a model-scale hull are reported. Experiments were conducted with a simplified stepped hull on the water surface in a recirculating water channel. Stable cavities with different lengths and cavitation numbers were generated at fixed water flow velocity and model position. No air leakage from cavities was observed in a range of cavity lengths. The maximum length of a stable cavity with air leakage was identified. Significant enhancement of the maximum cavity length was achieved by placing an inclined plate under the cavity and supplying a large amount of air during a short period of time. This augmentation of the cavity presents an effective practical means for lengthening air-ventilated cavities and controlling cavity properties. Numerical studies simulating an air-cavity flow were conducted using a linearized potential flow method. Calculations show a qualitative agreement with trends of experimentally observed lengths of air cavities and confirm a possibility of increasing the cavity length by placing a vortex underneath the cavity. A quantitative agreement is obtained for maximum lengths of non-augmented cavities when non-linear and viscous effects are minimal.