ANALYSIS OF DRAG SOURCES IN A FULLY SUBMERGED WATERJET

Abstract

A parametric study is conducted to optimise the drag performance of a novel concept of ram inlet waterjet. The latter being an outboard device housing the whole propulsion system. The study follows the guidelines from aero-engines literature which account for the pre-entry streamtube drag in the computation of external hydrodynamic resistance. Flow field data are obtained by solving Reynolds-Averaged Navier-Stokes equations, with k − ω SST turbulence model, on a 2D axisymmetric domain. Reynolds number, based on propulsor’s highlight diameter and free stream velocity, is varied between 1 . 6 · 10 6 and 2 . 7 · 10 6 . For each far-field condition, 9 different mass flow rate values are imposed on the internal boundaries corresponding to the pump sections and resulting in an overall range of the machine flow coefficient of 0.32. Comparing performance among three geometries shows that walls’ negative gradients drastically affect the drag coefficient in the whole operating envelope. In particular, reduced cross-sectional dimensions and longer shapes favour smoother external outlines. These prove to reduce the system’s resistance, despite the augmented axial length. Contextually, the analysis of the capture streamtube through the mass flow capture ratio shows that this parameter is less effective in enhancing performance, especially when the internal geometry is kept unchanged.