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动力问题的非时间步参数时间有限元法及弹性波传播数值模拟的格子法
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摘要
振动问题和波动问题是动力学研究领域的两大方面。由于结构形式和介质
    的复杂性,对于这两个方面的问题所从事的解析研究取得的进展不是很迅速,
    大部分问题还是要依靠数值方法来解决。合理、有效、计算精度高、耗时少,
    且易于编制通用程序的数值计算方法的研究一直是动力学研究领域的热点。
     本文基于弹性动力学问题中位移型Gurtin变分原理的思想,利用拉普拉斯
    变换,将具有初值—边值的动力学问题变换成拉普拉斯空间上等价的边值问
    题;并类似于原真实空间中解决边值问题的最小势能原理,在拉普拉斯空间构
    造了等价边值问题的泛函。然后,将此拉普拉斯空间上的泛函恢复到原真实空
    间;给出了适合于构造时间有限元方法的只含单重卷积的简单泛函表达式。这
    种表达形式的简单泛函为时间有限元的构造创建了一个“平台”,由此可开展
    时间有限元方法的一系列研究工作。
     本文针对由Gurtin变分原理建立的时间有限元方法容易产生数值计算不
    稳定这一问题,提出了非时间步参数的概念。在对时间域进行离散时,针对时
    间域离散所选取的各种插值函数形式并采用一个或几个非时间步参数来配合,
    能够有效地控制数值计算的稳定性,从而解决了基于Gurtin变分原理构造出的
    时间有限元方法的稳定性不易实现这一问题。这种思想方法非常适用于结构动
    力分析问题中的各种无条件稳定计算格式的构造。
     本文利用给出的简单泛函表达式,并结合在时间域离散时加入非时间步参
    数的思想,对一些时间域插值函数形式的时间有限元方法进行了研究。给出了
    一些解决初值—边值混合动力学问题的时间有限元的方法。获得了一些有意义
    的结果。
     本文提出了一种对弹性波传播进行数值模拟的新的全离散方法—格子法。
    在半无限介质中,该方法可以精细模拟任意复杂形状的自由表面和任意复杂的
    内部介质界面,灵活地处理自由表面问题,自由表面条件自然引入,同时又不
    需要特殊处理自由表面稳定性条件。该方法解决了其它差分方法在弹性波传播
    数值模拟中较难处理的任意复杂形状的自由表面和内部介质界面问题。在空间
    离散方面与有限元法类似,可以按照连续介质的形状及内部分界面任意剖分网
    格,具有网格剖分的灵活性,这是一般规则网格差分方法不可比拟的,但是格
    子法的计算量比有限元方法的计算量要少。格子法是一种显式计算方法,对计
    算机内存需求的少,计算效率高,能够对更大范围的介质区域中的弹性波传播
    进行数值模拟,这些方面比弹性波传播的有限元法要有优势。格子法是将节点
    的运动微分方程进行积分处理,给出了被研究节点的运动方程的积分形式,该
    方法不同于一般的用于弹性波传播数值模拟的有限差分法,一般的有限差分方
    法是基于节点的动力学的运动微分方程。
     在格子法思想的基础上,本文对三角形格子法进行了详尽的研究。对其计
    
    
     os
    一
    算方法、稳定性和数值计算的频散特性做了全面的研究,同时给出了数值算例,
    结果表明这一方法程序实现简单,计算精度高、耗时少,所需计算机内存较少,
    且具有频散小,稳定性好,可适用于高泊松比材料的特点。该方法的计算耗时
    和对计算机内存的需求与二阶规则网格交错差分法相当,但三角形格子法能精
    细刻划和灵活处理自由表面和内部介质界面问题。通过对自由表面问题的具体
    研究,得知格子法的自由表面边界多J牛是自然引入的,并且自由表面稳定性条
    件是自动满足的,无须另外处理,这是格子法的一个显著特点一即可精细模拟
    任意复杂形状的自由表面问题,又不需要特殊处理自由表面稳定性条件。
     三角形格子法适用于高泊松比介质,但不太适用于液体介质,为了克服这
    一不足,同时为了进一步研究计算效率高于原三角形格子法的方法,本文提出
    了一种三角形和四边形的混合格子法。本文主耍是针对横观各向同性介质中弹
    性波传播数值模拟进行了研究。在该方法中,对四边形格子内的应力作了均匀
    性假定,只需要由四边形格子中心处场变量的空间导数计算每个格子中心处的
    应力,在本构关系中并不含对空间变量的导数,因此,该方法适用于任意各向
    异性介质模型惰况。这种混合格子法为复杂各向异性介质中弹性被传括数值模
    拟提供了一种有力的工具。混合格子法中的四边形格子适用于模拟液体介质中
    的弹性波传播,因此,混合格子法可用于模拟固-液混合体中的弹性波传播
    题;同时四边形格子的选用使得混合格子法能进一步减少计算耗时和对计算机
    内存的需求。混合格子法同样可以精细刻划和灵活地处理自由表面问题,并且
    自然满足复杂几何边界的自由表面稳定性条件。
     本文最后基于格子法的思想,将格子法的研究工作推广到研究具有任意自
    由表面形状的三维非均匀介质空间模型。文中给出了空间四面体和空间平行六
    面体格子法以及由这两种格子法组合成的三维混合格子法,对空间四面体格子
    法和空间平行六面体格子法的计算方法、稳定性条件以及相速度频散都做了详
    尽的研究。采用三维混合格子?
Vibration and wave motion are the two research fields of the elastodynamics. The
     development in the research of analytical solutions is not very rapid because of the
     complexity of the structural patterns and the media, so moat of the problems have to be
     investigated by numerical methods. Research of numerical simulation methods that are
     reasonable, effective, high accuracy, low computational cost and ease to program is the
     highlight field of the elastodynaniics.
    
     Based on Gurtin variational principle of displacement model and making use of
     Laplace transform, the initial value problems of elastodynamics can be change into the
     ~equivalent?boundary value problems in the Laplace space. Similar to the principle of
     minimum potential energy used for the conventional boundary value problems, A kind
     of functional is constructed in the Laplace space. Then, a simple expression of
     functional with single convolution integral is given by returning functional in the
     Laplace space back into the real space. Such a simple functional establishes a
     損latform?for the constructions of time finite element methods, so a series of research
     of time finite element methods could be developed.
    
     A concept of non-time-step-parameter is put forward in dealing with the stability
     problem that often arises in the numerical calculation for the initial value problem of
     dynamics. One or several non-time-step-parameter can be used in the interpolation
     function for the discretization in time domain, so the stability in the numerical
     calculation could be controlled effectively. This idea can also be applied to the
     construction of other unconditional stable algorithms for the structural analysis of
     dynamic response.
    
     Based on the simple expression of functional and combining the idea of
     non-time-step-parameter used in the interpolation function for the discretization in time
     domain, some time finite element methods are studied by using some type of
     interpolation function in time domain. Some time finite clement methods that could be
    
    
    
    
     U?
    
    
    
    
     used to solve the initial-boundary value problems of dynamics are put forward and
     obtain some significant conclusions.
    
     A new fully numerical modeling algorithm for elastic wave propagation in
     heterogeneous media is presented. The scheme is called grid method. The method can
     accurately model the surface topography and inner curved interfaces by using an
     unstructured mesh and it is flexible in dealing with the free-surface problem for the
     semi-infinite media; The complex geometrical free-surface boundary conditions are
     satisfied naturally and they are stable. Grid method can solve the surface topography
     and inner curved interface problems that the other finite difference methods give rise
     difficulti s in incorporating surface topography and curved interfaces by using regular
     grids for the numerical modeling of elastic wave propagation. Similar to the finite
     element method in discretization of numerical mesh, the grid method is very flexible in
     incorporating surface topdgraphy and curved interfaces. The conventional regular grid
     finite-difference methods are not comparable to the grid method in such an aspect. On
     the other hand, the grid method need not requirements to calculate and store the global
     stiffness matrix like the finite element methods and it is a kind of explicit scheme, so
     the memory requirements and computational cost for the algorithm are much less than
     that of finite e
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