Effect of Air Bubbles Clinging to Models in a Towing Tank and Measures for Prevention

Abstract

Our towing tank, IHI Ship Model Basin, has been afflicted for years with air bubbles which clung to the surface of a wax model and increased unsteadily the frictional resistance with roughing the surface. The phenomenon is a kind of tank storm, that is, the "positive" storm. T. Glen described his experience on a positive storm in the discussions to K.C. Barnaby's paper and concluded that the storm was biological in cause. The water of our towing tank is not so contaminated to be bacterized, as the water is purified with filters and the tank room has no window. The investigation of our positive storm showed that the air bubbles come from the supersaturation of air in the tank water which is brought on by the rapid rise of water temperature from the beginning of spring to summer. We have used a measure to prevent bubbles clinging which is to paint a wax model and jointly to sweep them off with the water jets before each running. Air bubbles however quickly collected, when the phenomenon is heavy, on the surface of the model even after sweeping completely them off. We have taken a radical measure directly to deaerate the tank water to keep it out of supersaturation. A deaerator capable of extracting daily 1.5% of the air contained in the tank water was put in August 1981 into the line from a filter to the tank. The air content of the tank water has decreased from 103% to 95% after operation of the deaerator for half a year and we have got through the seasons of 1982 and 1983 without any air bubble clinging. It is concluded that the large yearly variation of water temperature may quite possibly set up a positive storm due to air bubbles clinging in a towing tank and the deaeration of tank water is a practicable and effective measure to break up the storm.