Numerical And Experimental Study Of The Correction Method Of Blockage Effect In The Towing Tank

Abstract:The tank blockage effect is an unavoidable problem in the resistance test.The lager the ship is,the more significant impact of blockage effect will be.The tank boundary effect can cause the resistance is greater than that in the infinite domain under the same Froude number.Tank blockage effect is the main component of the boundary effect which can divide into the blockage effect,the side-wall effect and the shallow water effect.Therefore,to explore the role of the blockage effect,and put forward a reasonable correction method has practical significance.At present,ITTC recommended empirical formulas are adopted to correct the blockage effect by each ship model laboratory,but with the increase of model scale and speed,and development of special ships,the accuracy of the correction method is not enough,so we must adopt more accurate method to modify the blockage effect.Based on the object of KCS and DTMB 5415,adopting the technique of CFD and the towing tank tests,the mechanism of the blockage effect and the blockage effect correction method was researched in the paper.The resistance of KCS was calculated under the infinite domain and 1.5%,1.8%,2.2%,3.0%blockage ratio with the consideration of heave and pitch movement,the DTMB 5415 calculated under the infinite domain,0.6%,1.5%,2.2%and 3.0%blockage ratio.In addition to explore the different width to depth ratio(b/h)of the towing tank on the blockage effect,two modals' blockage ratio was fixed at 1.5%.Respectively,ship model resistances were obtained under the(b/h)for 3/1,2/1 and 3/2,which show that the width of tank has greater influence on the total resistance.The blockage effect correction need to consider the width to depth ratio of the towing tank.Through comparing the total resistances which were corrected by different formulas,it is can be seen that the formula precision is not enough especially when the blockage ratio and Froude number was greater.When the(b/h)is 2/1,by comparing the effect of correction,the Scott formula and the mean flow correction formula based on the average cross sectional area are recommended as the main correction means for the full-formed ships,and the Simplified Tamura formula and the mean flow correction formula are recommended as the main correction means for the slender ship.Then the formula is improved by adopting a factor to correct the reflow velocity according to comparing the modified results of different formulas.The effect of blockage on free surface,pressure distribution and flow field around a ship model was studied to research the mechanism of the blockage effect,and the differences of between different ships were researched as well.Finally,three KCS ship models with different scale were done resistance experiments in Harbin Engineering University ship model towing tank,according to the data,the improved correction method was verified.It proved that second revised to the formulas got better correction effect.