In the present paper, the hull roughness effects on the calm water resistance of a medium-sized high-speed catamaran are assessed using the CFD method. The simulations are carried out using both the model-scale and ship-scale geometries. The model-scale resistance is extrapolated to the ship-scale using the ITTC recommended procedure (including the resistance increment due to the hull roughness) and then compared with the data obtained from the ship-scale CFD simulation, where the hull roughness effects are evaluated using the modified wall function approach. The results show that the resistance increment predicted from the model-scale simulation is significantly larger than that of the ship-scale calculation, especially at higher speeds. Besides, it is found that the increase of the resistance is primarily attributed to the frictional resistance component. Finally, the effects of the hull roughness on the flow field are also analysed.
Ship resistance can be considerably increased by the roughness of the hull surface, which could be caused by coatings, corrosion and biofouling etc. Therefore, the hull roughness effects must be taken into account when evaluating the ship's power consumption and environmental influence. Accordingly, there have been many studies to predict the resistance increase due to hull roughness using various methods. The traditional approach is to use empirical formulas (Bowden and Davidson, 1974; Townsin and Mosaad, 1985) to estimate the resistance increment and then apply it as a concentrated point force. These formulas are usually fitted from experimental and sea-trial results and can be used for extrapolating the model test resistance to the full-scale one. Theoretically, the roughness effect can be taken into account by using the similarity law scaling procedure proposed by Granville (Granville, 1978), which, however, is limited by the flat plate simplification. An alternative method to include the roughness effect is to employ a modified wall function in the computational fluid dynamics (CFD) simulation (Speranza et al., 2019). This method has the advantage of considering the heterogeneous distribution of the shear stress on the hull surface, which cannot be accounted for by empirical or theoretical approaches.
