This chapter shows the comparison of CFD calculation and tank test results for hydrodynamic property of three performances, propulsive performance, maneuverability and sea-keeping performance. Discussion is focused on the estimation accuracy of CFD code for maneuverability and examination for the future improvement of CFD code is shown.

　

3.1 Accuracy of estimation method for propulsive performance

　

 Concerning the propulsive performance, CFD calculation is already recognized as a reliable estimation tool. Therefore we mention only the outline of estimation tools' accuracy in this chapter. Correlation between calculation and tank test results at IHI ship model basin are presented here for ships with different fullness.

 At first, accuracy for estimating form factor K is discussed.

　

　

 Fig.13 shows the correlation between calculation results by NICE CFD code [3] developed by NMRI (National Maritime Research Institute) and experimental results determined at the Froude number of 0.1. Although some amount of scattering is observed, correlation between calculation and measurement is pretty good. It can be concluded from this figure that CFD code can grasp the correlation qualitatively which imply absolute value can be gained by the combination of tank test data and calculation.

 As for self-propulsion factors such as thrust deduction factor, wake factor and relative rotative efficiency, CFD calculation in self propelled condition is under the development recently. However, more practical tools at the initial design stage are introduced here.

 The first is thrust deduction factor 1-t. Since influence of viscosity on this factor is small, potential calculation might be a practical estimation tool[4]. Correlation between potential calculation and tank test results are shown in Fig. 14. As is similar in case of form factor, correlation between calculation and measurement is good.

　

　

 The next is effective wake factor (1-wE). A simple procedure is to estimate this value through nominal wake factor (1-wN), which is derived from calculated flow field by CFD calculation in towed condition.

　

　

 Fig.15 shows the comparison of 1-wN by NICE CFD calculation and 1-wE of tank test result, which also shows good correlation.

 Thus, as for propulsive performance, a number of verification results by existing estimation methods have been accumulated and already put into practical use.

　

3.2 Accuracy of estimation methods for maneuverability

　

 Recently, CFD calculation is applied rapidly to the estimation for maneuverability[5]. For the evaluation of maneuverability at the initial design stage, proper estimation of hydrodynamic force and moment will be a most important point, because directional stability is dominantly determined by these values especially for a full ship. To verify the accuracy of CFD code for estimating hydrodynamic force and moment, calculations and tank test results for a latest full ship are compared below.

 The viscous flow calculation was conducted by NICE CFD code and the experimental result was obtained by circular motion test in IHI ship model basin.

 Computed Reynolds number is 2.7e6 in accordance with experimental value of 3.5m length model ship and modified Baldwin-Lomax turbulence model is adopted here. The number of computing grid is 125 in longitudinal, 65 in girth-wise, 41 in radial direction that means 333125 grid points are used totally. Free surface effect is neglected and a mirror image condition is imposed in water surface boundary. Ship motion is completely fixed, both in experiment and calculation. Further, yawing moment: N is defined around the longitudinal center of gravity.

　

　

 Computed and measured non-dimensional lateral force: Y' and yawing moment: N' are shown in Figs. 16 to 19.