A study on the modeling of marine propeller tip flows using BEM

It has been recognized [1] that Boundary Element Methods (BEM) are able to predict the pres-sure distribution in the steady case with reasonable accuracy for the major part of a marine propeller blade. However, there are still some difnculties in obtaining a reliable solution close to the blade tip with the so-called conventional grids, especially if fine discretizations are ap-plied at the tip. In thiscase, the solutions appear to converge forthemajorpart of the propeller blade, but an irregular (non-smooth) behaviour of the pressure distribution near the tip is often seen. These difficulties give origin to divergence of the iterative scheme used to impose the Kutta condition of pressure equality on both sides of the propeller blade at the trailing edge or large pressure peaks at the trailing edge. Results of BEM calculations for a new panel arrangement with a "hydrodynamic tip" are pre-sented and compared with the conventional grid for two tested propellers: DTRC propeller P4119, without skew, and a modified propeller based on DTRC propeller P4842. The "hydro-dynamic tip" is defined as the location on the trailing edge where the vortex wake ends and no longer coincides with the geometrical tip. A rigid wake model and a wake alignment model with roll-up are considered. The results show that the use of "hydrodynamic tip" grids reduces the pressure oscillations edge inboard of the "hydrodynamic tip". The use of the present wake alignment model with roll-up in combination with the conventional grid introduces distur-bances in the pressure distribution near the tip due to the proximity of the tip vortex from the blade suction surface. With the "hydrodynamic tip" grids the rolled-up wake at the tip leaves the blade smoothly. In these cases, the effect of wake roll-up on the pressure distribution near the tip appears to be small.