Combined EFD and CFD Approach using the Form Factor for Full Scale Ship Powering Prognosis

Abstract

In this study, the investigation of the form factor determination as part of the 1978 ITTC power prediction method is performed with the objective to find suggestions to improve the ship power prediction. This study follows a combined CFD/EFD approach, where the extrapolated resistance from the towing tank tests is calculated with the form factor obtained from a CFD based method. In the present thesis two ship models are analysed, the MO5500 (a multipurpose vessel) and MO4500 (a container ship), these models correspond to HSVA benchmark models from which the experimental resistance is obtained from towing tank tests performed at the HSVA facilities.

The CFD based form factors are obtained from the viscous resistance of double body numerical simulations divided by the frictional resistance of a flat plate. In this study, numerical simulations are used to obtain the flat plate frictional resistance curve, using two different turbulence models, the k-omega standard and the k-omega SST. The obtained numerical friction curves predict frictional resistance coefficient values at low Reynolds below the ones predicted analytically, following the same behaviour of other existing numeral friction lines, where for example the difference between them decreases as the Reynolds number increases.

In order to have a better understanding of the behaviour of the form factor, it was determined for both ship types at model and full scale. The results confirm the dependence of the form factor on Reynolds number because of the different values obtained at both scales. The difference between the numerical form factors at different scales present an average percentage of 10.4% for the ones obtained with the ITTC'57 correlation line. While the average difference with the numerical line of k-omega SST is 6.4%, these results corroborate the scale effects of the ITTC'57 line and the decrease with the use of numerical friction lines.

Full scale resistance predictions were performed following the ITTC'78 method (which applies the ITTC'57 correlation line) using the model resistance results from the towing tank tests, and also extrapolating from the results of the CFD model scale simulations. The extrapolation process was done with 4 different form factors, the experimental one and 3 based CFD form factors obtained with the ITTC'57 curve, and with the 2 numerical friction lines derived in this study. In addition, the power predictions for the two ship models were also performed following the standard HSVA correlation method which does not contain form factors.

The full scale resistance predictions for the 2 ship models were compared with the results of the CFD resistance simulations at full scale. It was found that the extrapolation made with the form factor derived from the numerical friction line k-omega SST presented the smallest difference from the CFD full scale resistance values, while the highest difference was obtained with the form factor based on the ITTC'57 curve.