Experimental Characterization of Cavitating Flows using Velocimetric Techniques and Erosion Tests

Cavitation erosion is one of the most damaging effects of cavitation, as it results in material removal from the surface. Propeller designers put all their efforts in avoiding cavitation erosion in design conditions of propeller, by eliminating all those cavitation phenomena involving some risk of erosion, such as bubble cavitation, cloud cavitation and large unstable sheet cavitation. This approach is not adequate for high powered ships, as the full avoidance of these cavitation phenomena may result in unoptimised propeller design in terms of efficiency and cavitation inception speed. To further improve the propeller design, an optimal compromise between efficiency and erosion avoidance is made, such that cavitation is not aggressive enough to initiate the surface erosion. In this framework, propeller designers need reliable predictive tools to assess the probability of erosion occurrence in full scale, in order to decide if a given geometry can be accepted or corrective actions are needed for improvements. The aim of this thesis is development of experimental method to study the cavitation erosion on model scale marine propellers, and to study the dynamics of cavities causing the cavitation erosion. The method will be validated based on full scale results of propellers encountering cavitation erosion during their operational life. In order to develop the method, three different model propellers with different cavitation extents, working in twin screw configuration has been used. The working propellers in twin screw configuration generates complex flow field due to presence of shaft inclination and appendages. The effect of shaft inclination on the flow field has also been studied using velocimetric measurements (Laser Doppler Velocimetry, LDV) in this thesis with an aim to study the effect of propeller loading on the flow field, and physics of flow around inclined shaft. The aggressiveness of cavitation erosion is directly related with the dynamics of cavities. The complex shape of propeller blades make it difficult to study detailed dynamics of cavities on propellers models. To overcome this, simplified two dimensional hydrofoil has been chosen as a test case to study the dynamics of cavities causing erosion. The generation, shape and collapse of cavities has been studied using the hydrofoil model. The experimental velocimetric analysis (Particle Image Velocimetry, PIV) has been carried out for cavitation on hydrofoil, with a main aim to obtain data, knowledge and understanding of phenomena for propellers working in twin screw configuration.