Wake Field Prediction And Optimization Of Marine Propeller Considering Cavitation Performance

With the development of shipbuilding and shipping industries, ship performances involving safety, energy-saving and environmental friendliness etc. attract more and more attention. As the most commonly used propulsive device, propeller has a very important influence on the ship performances. Because the wake field is nonuniform, the propeller blade will meet the inflow at different attack angle during one revolution, which may result in cavitations on blades. Cavitations will cause not only blade erosion, but also propeller noise and ship vibration at stern. In recent years, on the one hand, ship becomes larger and larger in size, while the diameter of propeller cannot increase greatly due to the limited water depth of ports and waterways, which makes the propeller's load heavier; on the other hand, ships with full-form are widely used and the wake field becomes more nonuniform, which makes the operating condition of propeller worse. These two facts increase greatly the occurrence probability of cavitations and vibration. Therefore, it is necessary to take not only the propeller efficiency, but also the cavitation and vibration performance into account during propeller design.Aiming at the prediction and optimal design of the propeller's hydrodynamic and cavitation performances, this thesis has carried out research works in the following three aspects:Firstly, numerical prediction of wake field behind a ship is carried out by using CFD technology. Since the wake field has very important influences on the propeller performance, it is necessary to study the factors affecting the wake field. Taking a container ship as example, the software GMS is used as pre-processor to create the model of lines and the parametric transformations of lines are made with software NAPA. Then the CFD software PARNASSOS is used to calculate the wake fields. By comparing the calculation results with the experimental ones, the validation of the numerical method is confirmed. Through the study on wake fields at different block coefficients, length to breadth ratios of the hull, UV degrees of stern form and other parameters, the relationships between these parameters and their effects on wake field are verified. Secondly, open-water performance optimization of propeller is carried out based on support vector machine and genetic algorithm. As propeller design method based on the chart is easy to use and can provide reference for theory design method, a chart-based optimization model for open-water propeller is established first. The model is established taking the open-water efficiency of propeller as optimization object, the cavitation margin line as constraints, the advance coefficient, pitch-diameter ratio and disk area ratio as variables to be optimized. Support vector machine is used to predict the hydrodynamic performance and genetic algorithm is used to solve the model. Comparison of the optimization results with the results by using the commercial software CSPDP and those in references shows the validity of the model proposed in this thesis. This research work provides a basis for propeller optimization in nonuniform flow.Thirdly, performance optimization of propeller in non-uniform flow is carried out based on the lifting-surface method. The methods of performance prediction of propeller in nonuniform flow include the lifting-line method, lifting-surface method, panel method and CFD method. Although usually more accurate than other methods, CFD method demands relatively more computational resource and lower computational efficiency, and is not suitable for computations of numerous cases. In order to take both computational efficiency and prediction accuracy into account, the lifting-surface software ANPRO is used to predict the propeller hydrodynamic performance and cavitation performance. Comparison of the prediction results with the experimental observation shows that the lifting-surface method can predict the trend of change in cavitation extents. On this basis, by taking the propeller efficiency and the cavitation performance as optimization object respectively, pitch and camber at different radius as variables to be optimized, an optimization model is established and then solved by genetic algorithm. The comparison of the performance before and after optimization shows that the method proposed in this paper can optimize the cavitation performance while keeping the propeller efficiency unchanged, or optimize the propeller efficiency while keeping the cavitation performance unchanged.