Design Of Propellers For Maximum Cavitation Inception Speed Or Minimum Cavitation Induced Pressure Fluctuations

As it well known that cavitation plays a significant role in controling ship vibration and noise, this thesis deals with a design and optimization method of marine propellers, in order to maximize cavitation inception speed or minimize cavitation induced pressure fluctuations but with less efficiency loss.For purpose of maximum cavitation inception speed in non-uniform wake field, the concept of Eppler method in propeller blade section design has been widely accepted. However so far, to get the proper parameters in designing Eppler profile is still a troublesome way or say, having no direct way. In this thesis, a section optimization model is formulated taking sheet, bubble cavity margins on suction side and sheet cavity margin on pressure side as the objectives and the impact of design parameters on the objectives has been investigated. Based on Genetic Algorithm (GA), a multi-objective optimization technique is set up with the software:iSIGHT. An example is given.Characteristics of sheet cavity volume variation on section at quasi-steady condition has been numerically studied and a modified parameter σ*=(σ+Cpl)/(α-αi) instead of σ/(α-αi) is proposed. The variation of cavitation volume with the modified parameter σ*can be divided into three phases:slow phase, transitional phase and sharp phase. Taking this characteristic into consideration, a flow chart for propeller design to reach minimum cavitation pressure fluctuations has been proposed. A comparative design example was carried out.The above optimization technique is successfully extended to section design for minimizing cavitating pressure fluctuations. For this purpose, a section optimization model for minimizing the amplitudes of the first three blade frequency of cavitation volume variation has been set up. A detailed research on correlation between section geometry and its corresponding sheet cavity volume was performed and regression formulae for fast prediction of the cavitation volume variation were conducted.Two criteria are recommended respectively for maximum inception speed and minimum cavitation induced pressure fluctuations for marine propeller design. Taking account of the three dimensional effect, the optimized sections are transformed into blade section formation including the blade section shape and its pitch angle. Therefore a whole design process has been scheduled. This methodology has been verified by an example and validated with experiments that show in good agreement.The achievements applied to the design optimization may provide a feasible, practical design way for marine propellers. After engineering practice, the method proposed in the thesis can be further developed to be a design tool to improve designed propeller with favorable ship vibration and noise reduction.