平板在水中振動的自然頻率與它在空氣中振動的自然頻率不同，此乃水的影響所致。平板在水中振動時，有一部分的水會隨著平板振盪，此現象有如平板攜帶著許多均勻分佈的集中質量一齊振動。本文利用現有文獻理論，自行設計電算程式，先求出水平矩形平板位於剛性牆孔洞中，單邊接觸水面時的附加質量係數 。由這些附加質量係數，吾人即可估算出矩形平板之流體附加質量(fluid -added mass) ，並將其平均分佈至整個矩形平板之有限元素模型相關元素的節點上，最後，利用有限元素法即可求出矩形平板的自然頻率及振態，並與現有的理論數據相比較。本文重點在探討位於剛性牆孔洞中，單邊與水面接觸的矩形平板，在四邊支撐情況、尺寸(面積) 大小及邊長比不同時，對自然頻率係數的影響。此外，對附加質量係數(added virtual mass incremental (AVMI) factor)和無因次附加質量(non-dimensionalized added virtual mass incremental (NAVMI) factor)間的關係，本文也加以研究。

　　Either in contact with or immersed in water, the natural frequencies of a plate change significantly due to presence of water. In this paper, a computer program that based on exiting theory was developed. By using the last computer program, the added mass coefficients for a horizontal rectangular plate placed in the aperture of an infinitely rigid plane wall in contact with fluid on one side were determined. Use of foregoing added mass coefficients, the fluid-induced added mass was estimated and replaced by a multitude of concentrated ones uniformly distributed at the associated nodes of the finite element model of the rectangular plate. Finally, based on the last finite element model, the lowest five natural frequencies and associated mode shapes were calculated and compared with the exiting theoretical results. The purpose of this paper is to studied in different of boundary condition, size and aspect ratio, a horizontal rectangular plate that the natural frequencies is effected placed in the aperture of an infinitely rigid plane wall in contact with fluid on one side. Furthermore, the relationships between added virtual mass incremental factor and non-dimensionalized added virtual mass incremental factor were also presented.

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