From: blaisb Date: Mon, 20 Apr 2020 02:19:35 +0000 (-0400) Subject: Progress on documentation X-Git-Tag: v9.2.0-rc2~3^2~30 X-Git-Url: https://gitweb.dealii.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=522ed1db3ef2a785bc14aba7dbd9cf9c57bc0478;p=dealii.git Progress on documentation --- diff --git a/examples/step-70/doc/intro.dox b/examples/step-70/doc/intro.dox index 1bc40a2d08..81d7d4e6e9 100644 --- a/examples/step-70/doc/intro.dox +++ b/examples/step-70/doc/intro.dox @@ -14,11 +14,10 @@ In this tutorial we consider the case of two domains, $\Omega$ in -$R^{\text{spacedim}}$ and $\Gamma$ in $R^{\text{dim}}$ The domain $\Gamma$ is -embedded (i.e. non-matching) in $\Omega$ ($\Gamma \subseteq \Omega$). We want to solve a partial +$R^{\text{spacedim}}$ and $\Gamma$ in $R^{\text{dim}}$. The domain $\Gamma$ is +embedded in $\Omega$ ($\Gamma \subseteq \Omega$) and is not-matching. We want to solve a partial differential equation on $\Omega$, enforcing some conditions on the solution of -the problem *on the embedded domain* $\Gamma$ using the Nitsche method -(Freund, 1995). +the problem on the embedded domain $\Gamma$ using the Nitsche method. The technique we describe here is presented in the literature using one of many names: the immersed finite element method and the fictitious boundary method among others. The main principle is @@ -46,7 +45,7 @@ zero; for example it is a curve embedded in a two dimensional domain, or a surface embedded in a three-dimensional domain. In this tutorial program we are not interested in further details on $\Gamma$: we -take the extension $\Gamma$ for granted, assuming that the dimension of the +assume that the dimension of the embedded domain (`dim`) is always smaller by one or equal with respect to the dimension of the embedding domain $\Omega$ (`spacedim`). @@ -70,7 +69,7 @@ Nitsche method. The application of Nitsche's method will be discussed furthermore once the variational formulation has been introduced. -The weak form of the Stokes equations is obtained by writing it in vector form as +The weak form of the Stokes equations is obtained by first writing it in vector form as @f{eqnarray*} \begin{pmatrix} @@ -116,30 +115,30 @@ following modified formulation : @f{eqnarray*}{ (\nabla \textbf{v}, \nabla \textbf{u})_{\Omega} + (\nabla \cdot \textbf{v}, p)_{\Omega} - + (q, \nabla \cdot \textbf{u})_{\Omega} - (\textbf{v}\cdot \textbf{n},\textbf{u})_{\Gamma} - + \beta (\textbf{v}\cdot \textbf{n},\textbf{u})_{\Gamma} &=& -(\textbf{v}\cdot \textbf{n},\textbf{g})_{\Gamma} - + \beta (\textbf{v}\cdot \textbf{n},\textbf{g})_{\Gamma} + + (q, \nabla \cdot \textbf{u})_{\Omega} - (\nabla\textbf{v}\cdot \textbf{n},\textbf{u})_{\Gamma} + + \beta (\textbf{v}},\textbf{u})_{\Gamma} &=& -(\nabla\textbf{v}\cdot \textbf{n},\textbf{g})_{\Gamma} + + \beta (\textbf{v},\textbf{g})_{\Gamma} @f} -It can be shown (e.g. see Freund, 1995) that there exist a positive constant +It can be shown (see Freund, 1995) that there exist a positive constant $C_1$ so that if $\beta > C_1$, the weak imposition of the boundary will be consistent and stable. We note that the additional terms on the left-hand and right-hand side are equal since $\textbf{u}=\textbf{g}\text{ in } \Gamma$. It follows that : @f{eqnarray*}{ -(\textbf{v}\cdot \textbf{n},\textbf{u})_{\Gamma} -+ \beta (\textbf{v}\cdot \textbf{n},\textbf{u})_{\Gamma} &=& -(\textbf{v}\cdot \textbf{n},\textbf{g})_{\Gamma} -+ \beta (\textbf{v}\cdot \textbf{n},\textbf{g})_{\Gamma} +(\nabla\textbf{v}\cdot \textbf{n},\textbf{u})_{\Gamma} ++ \beta (\textbf{v},\textbf{u})_{\Gamma} &=& -(\nabla\textbf{v}\cdot \textbf{n},\textbf{g})_{\Gamma} ++ \beta (\textbf{v},\textbf{g})_{\Gamma} @f} We note that an alternative formulation can be used : @f{eqnarray*}{ (\nabla \textbf{v}, \nabla \textbf{u})_{\Omega} + (\nabla \cdot \textbf{v}, p)_{\Omega} - + (q, \nabla \cdot \textbf{u})_{\Omega} + (\textbf{v}\cdot \textbf{n},\textbf{u})_{\Gamma} - + \beta (\textbf{v}\cdot \textbf{n},\textbf{u})_{\Gamma} &=& (\textbf{v}\cdot \textbf{n},\textbf{g})_{\Gamma} - + \beta (\textbf{v}\cdot \textbf{n},\textbf{g})_{\Gamma} + + (q, \nabla \cdot \textbf{u})_{\Omega} + (\nabla \textbf{v}\cdot \textbf{n},\textbf{u})_{\Gamma} + + \beta (\textbf{v},\textbf{u})_{\Gamma} &=& (\nabla \textbf{v}\cdot \textbf{n},\textbf{g})_{\Gamma} + + \beta (\textbf{v},\textbf{g})_{\Gamma} @f} in which case the stability and consistency condition becomes $\beta > 0$. @@ -155,28 +154,39 @@ step-60, we still need to integrate over the non-matching embedded grid in order to construct the boundary term necessary to impose the boundary condition over $\Gamma$. +

Representation of $\Omega$ and $\Gamma$

+ +In this tutorial, both the embedded grid $\Gamma$ and the embedding +grid are described using a parallel::distributed::Triangulation. These two +triangulations can be built from deal.II GridGenerators or by reading a +mesh file produced with another application (e.g. GMSH). This is slightly +more general than what has previously done step-60. + +The main challenge in the application of the Nitsche method lies in the +integrals that are computed on $\Gamma$. As usual in finite elements we split this +integral into contributions from all cells of the triangulation used to +discretize $\Gamma$, we transform the integral on $K$ to an integral on the +reference element $\hat K$, where $F_{K}$ is the mapping from $\hat K$ to $K$, +and compute the integral on $\hat K$ using a quadrature formula. For example: ---- Here now ---- -In this tutorial program we keep things a little simpler, and we assume that the -configuration of the embedded domain is given in one of two possible ways: -- as a deformation mapping $\psi: \Gamma_0 \mapsto \Gamma \subseteq \Omega$, -defined on a continuous finite dimensional space on $\Gamma_0$ and representing, -for any point $x \in \Gamma_0$, its coordinate $\psi(x)$ in $\Omega$; +\f[ +\beta (\textbf{v},\textbf{u})_{\Gamma} = (v_j, q_\alpha)_\Gamma = \sum_{K\in \Gamma} \int_{\hat K} +\hat{\textbf{u}}(\hat x) (\textbf{v} \circ F_{K}) (\hat x) J_K (\hat x) \mathrm{d} \hat x = +\sum_{K\in \Gamma} \sum_{i=1}^{n_q} \big(\hat \textbf{u}(\hat x_i) (\textbf{v} \circ F_{K}) (\hat x_i) J_K (\hat x_i) w_i \big) +\f] + +Computing this sum is non-trivial because we have to evaluate $(v_j \circ F_{K}) +(\hat x_i)$. In general, if $\Gamma$ and $\Omega$ are not aligned, the point +$F_{K}(\hat x_i)$ is completely arbitrary with respect to $\Omega$, and unless +we figure out a way to interpolate all basis functions of $V_h(\Omega)$ on an +arbitrary point on $\Omega$, we cannot compute the integral needed for an entry +of the matrix $C$. + -- as a displacement mapping $\delta \psi(x) = \psi(x)-x$ for $x\in \Gamma_0$, -representing for any point $x$ the displacement vector applied in order to -deform $x$ to its actual configuration $\psi(x) = x +\delta\psi(x)$. -We define the embedded reference domain $\Gamma_0$ `embedded_grid`: on -this triangulation we construct a finite dimensional space (`embedded_configuration_dh`) -to describe either the deformation or the displacement through a FiniteElement -system of FE_Q objects (`embedded_configuration_fe`). This finite dimensional -space is used only to interpolate a user supplied function -(`embedded_configuration_function`) representing either $\psi$ (if the -parameter `use_displacement` is set to @p false) or $\delta\psi$ (if the -parameter `use_displacement` is set to @p true). The Lagrange multiplier $\lambda$ and the user supplied function $g$ are defined through another finite dimensional space `embedded_dh`, and through @@ -200,40 +210,6 @@ Q_h(\Gamma) = \text{span} \{q_i\}_{i=1}^m respectively, where $n$ is the dimension of `space_dh`, and $m$ the dimension of `embedded_dh`. -Once all the finite dimensional spaces are defined, the variational formulation -of the problem above leaves us with the following finite dimensional system -of equations: - -\f[ -\begin{pmatrix} -K & C^T \\ -C & 0 -\end{pmatrix} -\begin{pmatrix} -u \\ -\lambda -\end{pmatrix} -= -\begin{pmatrix} -0 \\ -G -\end{pmatrix} -\f] - -where - -@f{eqnarray*}{ -K_{ij} &\dealcoloneq& (\nabla v_j, \nabla v_i)_\Omega \qquad i,j=1,\dots,n \\ -C_{\alpha j} &\dealcoloneq& (v_j, q_\alpha)_\Gamma \qquad j=1,\dots,n, \alpha = 1,\dots, m \\\\ -G_{\alpha} &\dealcoloneq& (g, q_\alpha)_\Gamma \qquad \alpha = 1,\dots, m. -@f} - -While the matrix $K$ is the standard stiffness matrix for the Poisson problem on -$\Omega$, and the vector $G$ is a standard right-hand-side vector for a finite -element problem with forcing term $g$ on $\Gamma$, (see, for example, step-3), -the matrix $C$ or its transpose $C^T$ are non-standard since they couple -information on two non-matching grids. - In particular, the integral that appears in the computation of a single entry of $C$, is computed on $\Gamma$. As usual in finite elements we split this integral into contributions from all cells of the triangulation used to