Validation & Verification of Hydro-Elastic Analyses for Marine Propellers

Abstract

A fairly recent development in the maritime industry is the rising interest in composites, as they have great potential to outperform conventional metallics. They offer good corrosion resistance, fatigue resistance, a low magnetic signature, and a high strength to stiffness ratio. In case of a propeller, the latter may be utilized by adapting the geometry passively to suit the loading condition more optimal. A possible is to mitigate cavitation by utilizing the relatively large deformations when subjected to loads. The hydrodynamic response of a flexible propeller in a flow field can be predicted by the use of Fluid-Structure Interaction (FSI) software, which is currently being developed at MARIN. The project is called ComPropApp, and combines existing fluid and structural solvers. As the ComPropApp is still under development it needed to be verified and validated, which is the main objective of this thesis. As an initial step in the verificatio, a falsification study was applied on the procedures followed in the ComPropApp. This led to the discovery of several errors, to which corrections have been applied to improve the program. With an updated version, computations with a number of different materials were performed to finalize the verification. Then, a model size polyurethane propeller has been manufactured at MARIN to be used in the experimental validation. Experiments were performed in the cavitation tunnel testing facility at MARIN. Here the propeller was tested in several operating conditions, which was then compared with ComPropApp simulations. Lastly, it was investigated whether the application of a composite can reduce cavitation. This was for theoretical research only, since the testing propeller would fail far before reaching cavitating conditions. With the fluid solver, a requirement for twist deformation was set up. Based on these requirements, a range of composite materials was defined, and with it, ComPropApp computations were performed. The resulting displacements and pressure distributions were then compared for rigid and composite propellers. With the presented verification study, it can be concluded that the FSI software is capable of providing realistic computation results. The validation study has led to conclude that the unsteady FSI module is capable of qualitatively predicting the bend deformations in open water conditions. However, due to the large uncertainties arising from the testing material properties and questionable machining quality, the measurements cannot be utilized to define the accuracy of the FSI software. In wakefield conditions the additional uncertainty of the wake velocity distribution meant that these measurements are inconclusive, hence the validation was only performed for open water conditions. The material study with the purpose of mitigating cavitation has shown potential in the application of anisotropic materials. Composites with a specific ply orientation sequence have the possibility of realizing bend twist coupling motions, such that the propeller would unload itself in the vicinity of the ship hull, with reduced cavitation as an expected result.