Abstract
This paper describes the effects of drag reduction by microbubbles injection on very long ship models with flat bottom. The local skin friction and the total drag of 20 m and 40 m long slender ship models were measured in a towing tank with injecting microbubbles from porous plates on the flat bottom. Three cases of injecting position, bow, midship and both, were tested. Microbubbles were observed by CCD camera.
The drag reductions by injecting microbubbles were obtained in all experimental cases, and the maximum reduction rate was about 14% in total drag, and about 60% in frictional drag on the part of flat bottom covered by microbubbles. It became clear that the reduction of local skin friction persists for about 40m downstream from the injection point, although the reduction rate decreases with distance. The relation between drag reduction rate and air flow rate, velocity, injection position, etc., are illustrated, and the possibility of energy saving by microbubbles injection is discussed.
