Hideki Kawashima

Experimental study on microbubbles and their applicability to ships for skin friction reduction

Microbubble experiments were carried out using a circulating water tunnel specially designed for microbubble experiments. The tunnel has a long test section, which enables measurements on the persistence of the skin friction reduction effect by microbubbles in the streamwise direction. It also has a damp tank, which enables continuous testing of microbubbles. Skin friction was measured using a skin friction sensor, which is a force gauge type of 250 N/m 2 full scale, and skin friction reduction by microbubbles up to 40% was obtained. The local void ratio in the bubble condition was measured by putting a suction tube in the test section, and it was obtained that the local void ratio close to the wall has strong correlation with skin friction reduction. The scale effect and the applicability of microbubbles to full scale ships was discussed, based on experimental results using a long flat plate.

Experimental Investigation on Effects of Surface Roughness Geometry Affecting to Flow Resistance

Experimental investigation on effects of surface roughness geometry affecting to flow resistance has been carried out. The concentric cylinder device composed of outer cylinder and inner test cylinder was employed to the experiment. We prepared 24 different roughness models having various skewness of roughness profile as test inner cylinders. Surface of test cylinder has ridge and valley roughness whose shapes are isosceles right triangle V-shape. These ridge and valley are arranged at equal intervals. Therefore, RMS roughness of the surface and skewness of the surface roughness profile can be evaluated. In the experiment, inner cylinder is rotated but outer cylinder is stationary, torque of rotating inner cylinder was measured. Based on the torque measurement, we investigated the effect of skweness of the surface roughness on flow resistance. As a result, when the roughness profile has Gaussian distribution (skewness = 0), friction coefficient increases with increasing RMS roughness. Moreover, friction coefficient also increases with increasing skewness of surface roughness under same RMS roughness. In order to predict the friction coefficient from the geometric information of the surface, we estimated the equivalent sand grain roughness from surface roughness parameters. Results showed that it was clarified the relation among skewness of roughness profile, equivalent sand grain roughness and the root mean square of surface roughness.Copyright

A Research Project on Application of Air Bubble Injection to a Full Scale Ship for Drag Reduction

This paper is a progress report of a research project toward practical use of air bubble injection as a drag reduction device for ships. Air bubbles injected into the turbulent boundary layer in water flow are well known to have significant skin friction reduction effect. The current research project will last for three years, starting in April 2005. The project aims at obtaining 10% net energy-saving by air bubble injection, taking into account the work needed for injecting air bubbles. A full scale experiment is scheduled in September 2007. The photo and principal particular of the ship used for the full scale experiment are shown in Figure 1 and Table 1. The ship has a wide and flat bottom. Therefore, once air bubbles are injected at the bottom near the bow, they are expected to cover the entire bottom surface efficiently. The air bubbles must be injected against the hydrostatic pressure at the point of injection. Estimation of the rate of drag reduction per unit amount of injected air at full scale is extremely difficult if it is based on small model-scale experiments, because the scale ratio of air bubbles to boundary layer length scales is very different between model and full scale experiments. Therefore we carried out experiments using a flat plate (L = 50m, B = 1m) in the 400m towing tank of the institute. The plate was towed at 6.2m/s (12kt), the cruising speed of the ship for a full scale experiment. Air bubbles were injected at 3m from the bow. Both the total drag of the flat plate and local skin friction were measured. Recently we attached end plates almost along the entire length, in order to prevent air bubbles from getting lost from the sides, and obtained significant improvement in drag reduction. Injected air bubbles are expected to go into the propeller operating at the stern and the propeller performance may deteriorate. Therefore we carried out tests of a model propeller working in bubbly flow. So far we found that the degradation of the propeller performance due to bubbles is small and tolerable. The project is carried out in collaboration with Osaka Univ., Hokkaido Univ., Tokyo Univ., Mitsui Engineering & Shipbuilding CO., LTD. and Azuma Shipping CO., LTD.. The project is funded by NEDO (New Energy and Industrial Technology Development Organization), Japan.Copyright