Experimental study on drag reduction by air cavities on a ship model

This paper describes the experimental study on the drag reduction of an inland waterway ship scale model by a system of air cavities. The cavities were generated underneath the flat bottom of the ship model. As the cavity length grows with the velocity squared, a system with a variable number of cavities was suggested in order to ensure a high efficiency of the drag reduction at different velocities. Underwater cameras were used to visualize the cavities during the test. The video recordings provide the data on the contours of the cavities and their dynamics in the bottom plane. It was observed that there is an effect of the flow around the ship model on the cavities. The cavities at the forward side of the bottom are affected, most likely, by the pressure and velocity nonuniformity generated by the bow. This effect is expressed in the extended cavity length and thickness. The local flow characteristics around the ship might significantly influence the cavity parameters. The drag measurements show the efficiency of the drag reduction by the suggested system for a representative ship model in calm water and in head waves. In addition, the stability of the of air cavities system is discussed.