Basic Study on the Scale Effect of the Viscous Flow around a Marine Propeller

Abstract

This paper describes the mechanism of the scale effect on the viscous flow around a marine propeller, which are regarded as a very important problem from engineering point of view due to its close relation to the full-scale powering estimation using the model-scale experiment data. Using Reynolds Averaged Navier-Stokes code developed by the author, a series of turbulent flow computations are made around a three-bladed marine propeller at four Reynolds Numbers in the range between a model and a full scale condition. The computed results show the thrust coefficient (K_T) tends to increase as Reynolds number higher and vice versa for the torque coefficient. The mechanism of the scale effect on K_T is further investigated by decomposing it into pressure (K_<TP>) and friction component (K_<TF>). The scale effect on K_<TP> is shown to have the same order as that of K_<TF> and it reveals the importance of the scale effect on K_<TP>, which are neglected in the method of powering estimation used at the design process. By comparing the computed results of surface pressure and displacement thickness distributions at model and full scale Reynolds number, the mechanism of the scale effect on K_<TP> is understood as the increase of the effective camber of the blade section due to the decrease of the displacement thickness as Reynolds number higher. Decomposition of K_<TP> into the spanwise direction shows the magnitude of the scale effect is much larger at the tip than at mid-span sections, which suggests the close relation with three-dimensionality of the flow structure. Improvement of turbulence modelling including transition and surface roughness effect is thought indispensable for the quantitative estimation of tha scale effect on this important problem.