Comparative study of numerical simulation and empirical methods for air resistance analysis of a ship

Abstract

In confirming whether the guarantee speed of ship is satisfied or not, the effect of estimation result of ship's air resistance due to wind is considerably high. The air resistance could be generally determined from the wind tunnel test, data set of similar vessels or empirical methods according to ISO15016:2015. The wind tunnel test for a ship is performed by using a model-ship less than 2 meters in length due to very sensitive blockage effect. Wind speed in tunnel is not high enough to meet the Reynolds number of full scale ship. However the turbulence intensity and wind profile in test section are simulated for similarity of actual ocean wind. Data set of similar vessels and empirical formulas are used as economic options but these are less accurate because it is difficult to take into account the exact geometrical characteristics of the target ship. Alternatively, full scale CFD can be applied to determine the air resistance of a ship for geometrical and physical similarity. In this paper, full scale CFD results of two general cargo ships for air resistance are compared with Fujiwara formula and the data set of ISO15016:2015. In case of CFD computation, atmospheric boundary layer is functionally simulated based on Frøya wind profile for wind directions of 0？ to 180？. Comparative analysis between model and full scale is also carried out to examine the scale effect. Finally, the difference of resistance increase due to wind is based on ISO15016 is investigated according to the results of CFD, data-set and empirical formulas for air resistance. The difference of air resistance results by each method shows that the uncertainty can be increased in the ship speed-power correction. This study confirms that the application of more accurate data for air resistance is an important factor in the verification of ship guarantee speed. Copyright ？ 2018 by the International Society of Offshore and Polar Engineers (ISOPE)