A Validation, Comparison and Automation of Different Computational Tools for Propeller Open Water Predictions

Abstract

Open water behaviour of a propeller is an important indication of the propeller performance. This Master’s Thesis report describes the procedure for computing open water characteristics using four different methods, how to measure velocity fields and how to predict cavitation for a propeller in uniform inflow. The four methods were Computational Fluid Dynamics (CFD), boundary element method, lifting line method and Wageningen Series. Further the report describes how the methods were automated in means of computing open water characteristics and how they compare to model test results. The objective with the project was to gain knowledge about when the methods are preferred to use, what limitations they have and how to minimize the work effort by means of automation of the tools. The benefit of setup automation is not solely the time savings, but also the security in standardized setup methods. This reduces the risk of setup errors in the results. Automatic post processing was developed to some extent for the tools as well. For the project, three different propeller geometries were used; one designed to generate a tip vortex, one designed to reduce pressure pulses and one with a more conventional design. The propeller designed to generate a tip vortex was part of the SMP’11 Workshop on Cavitation and Propeller Performance. The SMP’11 Workshop was intended to give research groups the possibility to validate their computational tools against both model tests and other software, set up by different users. The workshop provided model test results of open water characteristics, velocity field measurements and cavitation patterns. These test results were predicted in this project and will also be included in the workshop. The other two propellers were used to automate the four methods and validate them against model test results. The CFD analyses were performed with the open source CFD toolbox OpenFOAM. Steady Reynolds-Averaged Navier-Stokes (RANS) simulations with k − ωSST turbulence model and wall functions in combination with Multiple Reference Frames (MRF) were used for the CFD simulations. The grids and automatic CFD pre-processing were performed with the commercial meshing software ANSA. The boundary element method predictions and grid generation were performed in the CRS developed tool PROCAL. The lifting line method predictions were performed in an Excel workbook with macros. The Wageningen Series predictions came directly from the Wageningen polynomials. The three latter methods were automated using visual basic and Excel. One important conclusion is that CFD gives the most accurate predictions, but requires many CPU-hours. When results are needed quickly, the boundary element method is useful and accurate enough. The lifting line method generates less accurate results than the other methods. The Wageningen Series is useful to give an indication of the predicted results validity. The automated codes save hours of work and results in consequent setups.