機械振動引致船艦水中輻射噪音之聲──振耦合分析

VIBRO-ACOUSTICS ANALYSIS FOR THE MACHINERY EXCITED UNDERWATER RADIATED NOISE OF SHIPS

就特殊船舶而言，因應船舶振動而引致的水中噪音輻射相當重要，如海洋研究船在漁業生態之實況測量，在水下部分必須嚴格控制自船體輻射出去之噪音，以減少環境之干擾；軍事艦艇為提高隱密性，必須嚴格控制自船體水下部份輻射出去之噪音，以減少被聲納偵測之機率，達到隱匿避敵之目的。所以如何正確評估船體輻射至水中噪音的大小及聲壓分佈，實為發展高值船舶之重要課題。本文嘗試以耦合的有限元素法／邊界元素法（FEM/BEM），來計算船（艦）體結構受到主機激振力激振時，所輻射出至水中的噪音量，及所產生聲場壓力的分佈。船（艦）體結構以有限元素法來表示，而聲音介質對結構的作用，則以邊界元素法來表示。本文先以一半沒水之薄球殼為例，以本文數值解與解析解比較，藉以驗證本文之耦合有限元素法／邊界元素法之正確性。最後以一船舶中心設計之警巡艦為例，計算在主機不平衡力∕力矩激振時，船艦體浸水表面的法向壓力、位移反應及所輻射出至水中的噪音，並與實船海上量測值相互比較、驗證。 The estimation of the underwater radiation energy and pattern by ships is a very important subject for some of the special purpose vessels, such as ocean research vessels and naval vessels. The underwater radiation noise that induces by ship vibration may deeply affect not only the results of onboard measurement for ocean research vessel but also the characteristics of stealth for naval vessels. This paper presents a coupled FEM/BEM for solving fluid/structure interactions under mono-frequency external loadings. The elastic structure is modeled by finite element method, and the acoustic loading is represented by a boundary element method. Firstly, a semi-submerged thin shell sphere was selected as a numerical example. The numerical results were compared with the analytical solutions to verify the present method. And then, an existing vessel was selected to compute the responses of wetted surface acoustic pressures and normal velocities and radiated underwater noise caused by the machinery excited hull structural vibration. The results were also compared with measured data under the ship sea-trial and good correlations were obtained.