基于滑动Kriging插值的EFG-SBM求解含侧边界的稳态热传导问题
Element-Free Galerkin Scaled Boundary Method Based on Moving Kriging Interpolation for Steady Heat Conduction Analysis with Temperatures on Side-Faces
通讯作者: 陈灯红,男,副教授;E-mail:d.chen@ctgu.edu.cn.
责任编辑: 陈晓燕
收稿日期: 2020-07-8
基金资助: |
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Received: 2020-07-8
作者简介 About authors
王峰(1987-),男,山东省莱阳市人,副教授,从事无网格法及比例边界有限元法研究.
采用基于滑动Kriging插值的无单元伽辽金比例边界法(EFG-SBM)求解侧边界有温度载荷的稳态热传导问题,该方法通过无单元伽辽金法(EFG)和滑动Kriging插值离散环向边界.由于滑动Kriging插值形函数具备Kronecker delta函数插值特性,克服了移动最小二乘逼近难以直接准确施加本质边界条件的不足.作为一种新型的边界型无网格法,EFG-SBM兼有EFG法和比例边界有限元法(SBFEM)的优点.该方法继承了SBFEM的半解析特性,通过引入比例边界坐标系,可将偏微分控制方程环向离散,径向上解析求解.与传统的SBFEM相比,环向边界通过节点进行离散,前处理和后处理简便.通过数值算例可以看出,相比基于拉格朗日多项式的SBFEM,基于滑动Kriging插值的EFG-SBM计算精度更高.相比有限元法(FEM),该方法能更好地反映尖角处热奇异性以及无限域温度分布状态.
关键词:
The element-free Galerkin scaled boundary method (EFG-SBM) based on moving Kriging (MK) interpolation is used to solve steady heat conduction problems with temperature loads on side-faces, in which the circumferential boundary is discretized based on MK interpolation and the element-free Galerkin (EFG) method. As the shape functions constructed from the MK interpolation possess the Kronecker delta interpolation property, the MK shape functions overcome the shortcomings of moving least squares (MLS) approximation which is difficult to impose essential boundary conditions directly and accurately. As a new boundary-type meshless method, EFG-SBM has advantages of the EFG and scaled boundary finite element method (SBFEM). This method inherits the semi-analytical property of SBFEM by introducing the scaled boundary coordinate system, in which the governing differential equations are weakened in the circumferential direction and can be solved analytically in the radial direction. Unlike the traditional SBFEM, the preprocessing and postprocessing processes of EFG-SBM are simplified since only the nodal data structure is required in the circumferential direction. Numerical examples show that the EFG-SBM based on MK interpolation can obtain a higher accuracy than the SBFEM based on Lagrange polynomials. Compared with the finite element method (FEM), this method can better characterize the thermal singularity at the sharp corner and the temperature distribution of the infinite region.
Keywords:
本文引用格式
王峰, 陈佳莉, 陈灯红, 范勇, 李志远, 何卫平.
WANG Feng, CHEN Jiali, CHEN Denghong, FAN Yong, LI Zhiyuan, HE Weiping.
稳态传热是指系统各点温度仅随空间位置变化,不随时间发生改变[1].在水利工程中,拱坝封拱温度计算下游以年平均气温、上游以年平均水温作为边界条件,此时求出的坝体温度场为稳定温度场.
比例边界有限元法(Scaled Boundary Finite Element Method, SBFEM)是1997年由Song等[9]基于坐标变换提出的一种半解析半数值的方法,能精确高效地求解热奇异性、无限域热传导问题[10,11].该方法在计算域引入相似中心,建立含有径向和环向的比例边界坐标系.经过相似变换,物理变量转化为域边界点和相似中心所连径向坐标变化表示,偏微分控制方程转化为一阶常微分方程组.只需通过有限元法离散物理域边界,空间求解维数降低一维.该方法集成了有限元法和边界元法的优点,在环向可以获得有限元法精度解,相比边界元法,无需基本解.然而,比例边界有限元法的计算精度受环向离散网格影响,传统的拉格朗日多项式基函数可能导致相邻节点之间的低阶连续性.为此,Lobatto 多项式[12]、非均匀有理B样条[13]已经被用于构造环向形函数,计算精度和效率明显提高.
为了避免网格影响,学者们提出了无网格法[14,15],该方法非常适合求解涉及网格重构和畸变等问题.其中具有代表性的是无单元Galerkin法(Element-Free Galerkin Method, EFG)[16]、无网格局部Petrov-Galerkin (Meshless Local Petrov-Galerkin, MLPG) 法[17]、径向基点插值法(Radial Point Interpolation Method, RPIM)[18].EFG法是借助于移动最小二乘 (Moving Least Squares, MLS) 法与Galerkin弱形式,构造近似场函数的一种离散方法.相比有限元法,计算精度和收敛速度明显提高,无需后处理便可获得光滑梯度场.将EFG法引入到比例边界坐标系环向边界,可获得新的边界型无网格法—无单元伽辽金比例边界法(Element-Free Galerkin Scaled Boundary Method, EFG-SBM)[19].该方法只用节点离散环向边界,前处理简单.相比基于拉格朗日多项式的SBFEM,该方法形函数更加光滑连续,具有更高的计算精度和更快的收敛速度.
然而,与EFG法类似,EFG-SBM中的形函数通常是由移动最小二乘近似构造的,不具有Kronecker delta插值特性,不能直接施加本质边界条件,故形函数转化矩阵法[19]被提出,该方法将虚节点值转换为实际节点值,方便了本质边界条件的施加.另外一种处理本质边界条件的方法是选取具有Kronecker delta插值特性的形函数,例如插值型移动最小二乘法[20]、滑动Kriging插值[21].滑动Kriging插值(Moving Kriging, MK)是基于变异函数理论和结构分析的线性无偏插值,直接通过节点进行插值,建立的形函数具有Kronecker delta插值性质,能直接施加本质边界条件.目前已被广泛应用于无网格法中,能显著提高计算效率[22,23,24,25,26].由于MK插值模型参数取值直接影响计算精度,Zheng等[27]提出了模型相关参数与节点间距之间关系表达式.
本文将MK插值的EFG-SBM法扩展用于二维含侧边界的稳态传热问题求解,文中所有算例均无量纲.环向边界用节点进行离散,通过MK插值来构造形函数,计算域边界依据端点分段为不同的光滑线段.由于MK插值构建的形函数具有Kronecker delta插值性质,能直接施加本质边界条件.
1 MK插值
在EFG-SBM中,为了不增加角点试函数的光滑性,MK插值是沿着每条线段而不是整个域边界实现的.设问题域边界Γ由M条光滑边组成,N个离散节点si(i=1,2,…,N)分布在一条边上.设点s插值域内有n个节点sk(k=1,2,…,n),则点s处温度近似函数Th(s)通过MK插值逼近为
式中:给定的n个节点处的温度值向量为T=
形函数Φ(s)=
p(s)为m次完备的单项式基,即
对于线性基,
对于二次基,
P为节点处基函数值矩阵;R为对角线为1的对称相关矩阵;I为n阶单位矩阵.
矢量r(s)表达式为
R(si,sj)为任意两节点si和sj之间的对称相关矩阵,一般采用Gaussian型函数
式中:θ>0,为函数相关参数;rij=‖si-sj‖.
图1
图2
一维空间[0,1]通过6个节点(0,0.2,0.4,0.6,0.8,1)进行离散,图3所示为采用二次基时一维MK形函数,很明显,MK形函数具有Kronecker delta插值性质,即
同时也满足单位分解特性,即
对于温度函数T(s),在曲线域进行插值,插值域半径ri为
式中:β为系数;ds为点s附近节点距离.
图3
图3
采用二次基时一维MK形函数
Fig.3
One-dimensional MK shape functions with quadratic basis
2 侧边界含温度荷载的EFG-SBM方程
2.1 二维稳态热传导控制方程
设问题域边界Γ由温度边界ΓD、热流密度边界ΓN及对流换热边界ΓR组成,当不考虑内热源,二维稳态热传导控制方程为
相应的边界条件为
式中:Ω为计算域;
2.2 侧边界施加温度荷载求解
如图4所示,引入相似中心为(x0,y0)的比例边界坐标系oξs.ξ为径向坐标,从相似中心连至边界点,相似中心ξ值为0,边界上ξ值为1.s为环向坐标,表示沿着边界Γ从s0开始逆时针旋转的弧长,终点为s1.由式(16)可得
图4
式中:
边界(ξ=1)上Jacobian矩阵为
Jacobian行列式为
(xs(s),ys(s))为边界ξ=1上点的笛卡尔坐标.
相似中心和起点s0、终点s1的连线为侧边界,当侧边界s=s0或s=s1上温度不为0时,此时温度函数T(ξ)分解为约束温度函数
式(29)的第一行方程可得
假设
式(30)为非齐次常微分方程,该方程的通解是对应齐次线性方程的通解加非齐次线性方程的特解.式(30)的特解也可以设置成幂级数的形式,即
代入式(30)可得
对于有限域,式(33)的解表示为
式中:λi为特征值分解所得特征值;c1i为积分常数.
对于有限域,对偶变量q(ξ)可以分为
在边界ξ=1处,将式(34)代入式(35)可得
式中:
此时qu(ξ)的通解为
在边界ξ=1处,
其中,
由式(42)可得
代入式(43)可得
同样,对于无限域,可得
3 数值算例
3.1 矩形板稳态热传导问题
该问题的温度解析解为
图5
图5
矩形板热传导边界条件
Fig.5
Boundary conditions of heat conduction for rectangular plate
图6
图6
矩形板相似中心和节点分布
Fig.6
Scaling center and nodes distribution for rectangular plate
图7
图8
表1 矩形板稳态热传导问题理论解与数值解比较
Tab.1
计算点 (x,y) | 理论解 | SBFEM解 | EFG- SBM解 | SBFEM 误差/% | EFG-SBM 误差/% |
---|---|---|---|---|---|
(5,1) | 9.927 | 9.971 | 9.927 | 0.45 | 0 |
(5,2) | 20.841 | 20.823 | 20.842 | 0.09 | 0.005 |
(5,3) | 33.830 | 33.804 | 33.830 | 0.08 | 0 |
(5,4) | 50.184 | 50.155 | 50.185 | 0.06 | 0.002 |
(5,5) | 71.533 | 71.509 | 71.533 | 0.03 | 0 |
3.2 L型板稳态热传导问题
图9
图10
图10
L型板相似中心和节点分布
Fig.10
Scaling center and nodes distribution for L-shaped plate
图11
图11
Fig.11
Temperature isothermal line of L-shaped plate at
图12
图12
Fig.12
Temperature isothermal line of L-shaped plate with V-shaped reentrant corner at
图13
图13
Fig.13
Flux isothermal line of L-shaped plate with V-shaped reentrant corner at
图14所示为
图14
图14
Fig.14
Temperature isothermal line of L-shaped plate at
3.3 半无限域稳态热传导问题
考虑如图15所示的半无限域,侧边界温度为50,边界温度为
图15
图16
图16
半无限域相似中心和节点分布
Fig.16
Scaling center and nodes distribution for semi-infinite domain
图17
4 结论
(1) 传统移动最小二乘法形函数缺乏插值特性,将滑动Kriging插值法引入到EFG-SBM进行改进.由于滑动Kriging插值形函数具有Kronecker delta函数插值性质,在本质边界条件施加上无需其他处理.同时相比SBFEM中常用的拉格朗日多项式,滑动Kriging插值建立的形函数更加连续光滑.
(2) 将基于滑动Kriging插值的EFG-SBM用于求解二维含侧边界的稳态传热问题,计算域边界根据端点分段为不同光滑线段,每条线段通过滑动Kriging插值来构造弧长以及温度场函数.通过3个典型算例来说明EFG-SBM的特点,首先,在同等自由度离散情况下,相比SBFEM,EFG-SBM计算精度更高.其次,对于V型凹角结构,相比有限元法,EFG-SBM能更好地反映尖角处热奇异性(温度或热流奇异).最后,通过对半无限域稳态传热问题进行求解,有限元法截断边界对温度分布状态具有较大影响,实际问题求解应予以重视,而EFG-SBM可以很方便地刻画无限域.
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