Hydrodynamics of a Displacement Air Cavity Ship

Paper in proceeding, 2012

To study the resistance reduction of an air cavity for a displacement vessel, a simplified model of a single cavity is tested in a cavitation tunnel. The drag force acting on the cavity and the aft plate were measured and the water-air interface was monitored in different conditions. Behavioural changes to the free surface were observed by changing the geometry and flow characteristics like air pressure, air flow rate and water flow speed. Computational model for different cases with the same geometry and conditions were simulated using computational fluid dynamics(CFD). The length of the cavity was designed to include approximately 2.5 times the expected wavelength of the water-air interface with Froude number based on the ship length of 0.19 which approximately represents a displacement ship with multi-wave air cavity at its cruising speed. Parameters like cavity pressure play an important role for the wave’s shape and the stability of the free surface inside the cavity. Both computation and experiment show that the amplitude of the wave is sensitive to the air pressure in the cavity and the re-attachment of the water to the rear end of the cavity has a close correlation to the inlet air pressure and water velocity. The ultimate goal of this investigation is to minimize effective power including air supply while reducing resistance in/around the cavity, but equally important is it to gain amore basic understanding of the air-cavity’s behaviour and possible additional resistance components.