Prediction of tip vortex cavitation for ship propellers

An open propeller is the conventional device providing thrust for ships. Due to its working principles, regions with low pressure are formed on its blades specifically at the leading edge and in the tip region. If this pressure is becoming lower than the vapor pressure, the cavitation phenomenon is initiated, i.e. in these low pressure regions liquid turns into vapor. Thus, cavitation results in regions filled with vapor, present on and close to the propeller blades. This causes nuisance effects like noise, vibrations, erosion and even thrust break-down in its most severe cases. Due to these unwanted features of cavitation, the design of propeller blades has to account for cavitation. This requires a computational tool to predict propeller cavitation, already in the design stage of the propeller. Such a computational tool should be suited for prediction of the cavitating flow around model-scale and full-scale propellers. Model-scale tests, as a form of studying cavitation phenomena, have been used from the beginning of recorded history on cavitation. This procedure is well established and fine-tuned with well-recognized capabilities. However, the main drawbacks of the method based on model-scale tests are: the scaling of the tip vortex cavitation inception and the scaling of the performance of the cavitating propellers. Therefore, in the last decades there has been an increasing effort in using numerical simulation methods to predict cavitation on propellers. These methods range from potential-flow methods to Large-Eddy Simulation methods. In the present study, the focus is on using a high-fidelity mathematical model of the flow, implemented in a relatively fast numerical method, such that the method can be used within industry. The approach based on the Reynolds-Averaged Navier-Stokes (RANS) equations has been selected. This method is used in achieving the goal of the thesis: the prediction of the inception of tip vortex cavitation and the prediction of the development of the tip vortex. For this purpose, a number of benchmark test cases have been used to validate the chosen approach and to investigate the basic flow characteristics of cavitation. Subsequently, the method has been applied to a number of open propellers.