Fluid-structure Coupling Dynamic Characteristic Analysis Of Underwater Propeller Based On FEM And Test

The prediction of vibratory excitation forces that are induced by the propeller is one of the most important tasks for the ship engineers and researchers. These forces are transmitted to the poop of the hull; the propeller produces the excitation to the hull, so the propeller structure can be regarded as the vibration source. The dynamic characteristics of the propeller should be study at first in the semi-immense water, and especially the hull surface boundary effect. In this thesis, the emphasis on the effect of the ship hull surface acts as the flexibility and rigid boundary, which can influence the fluid-structure coupling vibration result of propeller, relative to the effects of pressure release sea-surface. Both experimental and computational means can be used to assist the client in judging the propeller design to respect to predict its dynamic characteristics.FEM was applied to solve the dynamic characteristics, prior to accepting the FE results as representative of a correct solution useful for design purposes. The finite element (FE) model of the propeller was conducted for natural frequencies computation in air and in water, based on the data measured from model-propeller firstly. And the FE model was utilized to analysis the dynamic characteristics of the underwater structure in air and in water through FE code ANSYS. The surface of the water was considered as pressure free, and the volume of fluid was assumed 10 times larger than the diameter of propeller; while the other surface as velocity zero. It can be regarded as semi-immense fluid field. The nature frequency of the propeller in water obtained to compare with that in air. This vibration of propeller was affected by the reduced space between the propeller and the stern hull, very evident in hulls with light displacement. For different boundary, based on the location description of propeller and hull plate, the propeller disc is 0.12d forward of the stern waterline at the pointed condition. The rigid hull surface is used to simulate the boundary condition; and then 2 kinds of plate (8mm and 70 mm) were used to simulate flexibility hull surface. All the nature frequency results above had been used in comparison to show the effect of hull surface.The numerical result is just one aspect to discuss the problem, while experiment should also provide the other face to aid the numerical result and amend the conclusion. Each node's information used in numerical simulation can be accepted in the test. The DASP system was introduced to analysis the vibration mode of the propeller structure. In most vibration problems, however, mode shape of the structure was interested primarily. The modes in air were tested using the DASP instrument, comparison with the numerical result as the other stand proof.The numerical results and experiment data can be used to discuss the water influence to the propeller blade, and also the different boundary condition. The conclusion or principle will be offered to aid the engineering usage. The frequency values of the semi-immense fluid field have already computed before. The conclusion contains:1. The wet mode will be discussed on the computer and experimental data;2. The principle of propeller in water compared with that in air will be mentioned;3. The boundary effect is described by the approximate relation.In a word, utilizing FEM to solve the coupling problems has not been used in this problem. FEM can be deal with the flexibility hull surface boundary compared with the BEM. ANSYS was employed to calculate the mode in air and in semi-immense water, aided by the test results, to show the still water effect. Experimental method is employed to aid the numerical results to discuss the mode and natural frequency in water and in air. Convergence of results, the nature-frequency of the propeller and the ratios of different hull surface, is demonstrated using different hull surface condition distributions; they are presented to facilitate comparison for the different complex boundary conditions and to donate the effects to the vibration of the propeller.