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不规则频率的消除及其在波浪与结构物作用的高频和多物体干涉问题中的应用
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摘要
本文使用基于高阶插值函数的边界元方法,在频域内对波浪与单个或多个物体相互作用的非线性问题进行研究,重点研究了“不规则频率”对计算结果的影响及其消除技术在实际计算中的应用。
     波浪问题本身具有非线性,线性假设条件下所得的计算结果往往不能满足实际工程的需要,很多问题都必须考虑非线性的影响。波浪与多物体的相互作用时,往往会产生一些与单物体问题不同的物理现象,此时必须同时考虑波浪与多物体的相互作用和多个物体之间的相互影响。
     在众多的数值方法中,边界元法很适合用来求解波浪与结构物相互作用的问题。一方面所关注的物理量分布在研究区域的边界;另一方面边界元法所使用的边界积分方程只需要在问题区域边界划分单元,既便于单元划分,又减少了计算量。但是,“不规则频率”问题的存在给边界元法的应用造成了很大的障碍,它使得计算结果在某些频率发生跳跃。本文在前人所提出的消除“不规则频率”的方法基础上,采用扩展积分方程计算区域的方法来消除“不规则频率”。对于扩展区域后的积分方程,使用连续元和部分非连续元对不同区域进行离散,使得计算模型中物理量的连续性与实际问题一致。对于二阶问题中的水面积分强迫项和物面积分强迫项,本文采取有效的方法进行了处理。数值积分计算过程中存在着大量的奇异积分和准奇异积分,根据实际情况本文采取了行之有效的方法消除或者减弱了奇异性。
     本文消除“不规则频率”的方法被应用于规则波与单个或多个物体的非线性问题计算当中。文中从理论分析和数值计算两方面研究了“不规则频率”对波浪力、波高和物体运动响应的影响。同时,大量的数值结果表明本文的消除方法有效地消除了“不规则频率”的影响。在分析波浪与多物体相互作用的过程中,发现了一些可能会对海上结构物安全产生重大影响的特殊物理现象。
Boundary element method which is based on higher-order shape functions is applied to the researches of interactions between regular waves and single body or multiple bodies in the frequency domain.The emphasis is put on the effects and removal of irregular frequencies in numerical computation.
     The linear assumption can not predict the effects of wave accurately in many situations because of the existence of nonlinearity.Some novel phenomena will arise when waves interact with multiple bodies because of the effect of multiple scattering.
     Boundary element method is very suitable to simulate the interactions between waves and structures because the physical quantities of primary interest are only required on the boundaries of stimulation domain.On the other hand,boundary element method only needs integration on the boundaries by Using discrete elements,which has the advantages of generating meshes easily and can reduce the burden of calculation to some extent.However, irregular frequencies become a great obstacle in the application of boundary element method. Irregular frequencies will introduce sharp jump in numerical results.Extended boundary integral equations are adopted to remove the irregular frequencies.Continuous and discontinuous higher order boundary elements are used in different sub-domains according to the continuities on different boundaries.Careful considerations are taken for forcing terms on the free surface and body surface in the integral equations for second order problem. Furthermore,different strategies for numerical integration are applied to deal with singular and nearly singular integrals.
     Present method for removing irregular frequencies is applied in the nonlinear analysis of interaction between regular waves and single or multiple bodies.The effect of irregular frequencies on exciting forces,wave elevations and motions of bodies is analyzed by theoretical and numerical method.At the same time,validity of present method for removing irregular frequencies is conformed by extensive numerical results.Some novel physical phenomena which can threaten the safety of structures are found in the analysis of interactions between wave and multiple bodies.
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