博士 / 國立臺灣海洋大學 / 系統工程暨造船學系 / 92 / Abstract
The main objective of this thesis is to develop a new and efficient numerical method for predicting the far-field and near-field blade-rate noises of marine propellers operating in a non-uniform ship wake by linear acoustic theory. In addition, both the scattering effect from the ship hull and the reflecting effect from the free surface are included.
In the present method, an exact analytic solution satisfying the linear wave equation for predicting acoustic pressure caused by unsteady sheet cavitations, unsteady thrusts and torques can be derived directly in time domain. The free surface effect is simulated by imaged method. The deriving process has no approximation about the distance between the noise source and field points. Thus, this method can be used to predict the acoustic pressure at both far and near fields, and the Doppler effect can be demonstrate in the near field evidently. The variation of the distance between the noise source and the field points will cause obvious in the near field. It is found that for computing far-field acoustic pressure induced by the unsteady sheet cavitation and thrust, noise sources on a blade can be replaced by an effective point noise source. However, in doing so, errors appear on the computation of the acoustic pressure induced by the unsteady torque force. In the near field, any kind of noise sources should be distributed on the entire propeller blades in order to avoid the errors, especially for unsteady thrust and torque.
An iterative method in time domain for computing multi-frequency waves scattered from underwater obstacles is also developed. These equations derived in the present method are expressed in some forms relative to the retarded time, and the Fourier series is used to minimize the numerical error due to time interpolations. This iterative method is an alternative to the frequency-domain boundary element method (BEM); however, this method is more efficient than the BEM, and is found very robust. Computational results of the present method and BEM show good agreements for both low frequency and high frequency cases.
To investigate the acoustic fields radiated by the propeller and scattered from the ship hull with free surface effect, a case of container ship is calculated in this thesis. Meanwhile, the pressure fluctuations induced by the propeller on the ship hull predicted by the present method is similar to the results by a higher-order panel method satisfying the Laplace equation in this case.
| Identifer | oai:union.ndltd.org:TW/092NTOU5345010 |
| Date | January 2004 |
| Creators | Jui-Hsiang Kao, 高瑞祥 |
| Contributors | 柯永澤 |
| Source Sets | National Digital Library of Theses and Dissertations in Taiwan |
| Language | en_US |
| Detected Language | English |
| Type | 學位論文 ; thesis |
| Format | 121 |
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