Yoshiaki Kodama

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.

Numerical simulation method to resolve interactions between bubbles and turbulence

A new computational method for investigating interactions between bubbles and turbulence has been developed. Both liquid and gas phases are treated as incompressible fluids and solved by a finite volume method, while the interface between the phases is resolved by a front-tracking method. The accuracy validation carried out for a problem of a single rising bubble has shown this method is capable of simulating flow around deformed bubbles with relatively small number of grid points. Then the method was applied to a direct numerical simulation of a fully developed turbulent channel flow containing bubbles. Statistics of the friction coefficient and the modulation of turbulence intensity were obtained, and they were in qualitative agreement with experiments.

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

Computation of Flows around Full Hull Forms in Oblique Towing or Steady Turning Using NICE Code

Flows around two full hull forms in oblique towing or steady turning motion have been computed using a CFD code NICE with the Baldwin-Lomax turbulence model. The computed flow in oblique towing agreed well with experiments in terms of the sway force distribution in the longitudinal direction, as well as the integrated sway force and yaw moment, and the computed hull surface pressure distribution explained well the difference in the sway force distribution on two hull forms. The modified Baldwin-Lomax model gave better results than the original model. Using the same code, flows in steady turning were also computed. Although the agreement with experiments were slightly less satisfactory than those in oblique towing, the modified turbulence model gave better results again.