From: bonito Date: Wed, 5 Jan 2011 20:20:33 +0000 (+0000) Subject: Step-38: correct surface formulas X-Git-Url: https://gitweb.dealii.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=e086ece3b32ec84a896e5dacbf051840c616eb95;p=dealii-svn.git Step-38: correct surface formulas git-svn-id: https://svn.dealii.org/trunk@23126 0785d39b-7218-0410-832d-ea1e28bc413d --- diff --git a/deal.II/examples/step-38/doc/intro.dox b/deal.II/examples/step-38/doc/intro.dox index 1dd7ace34d..efac20be85 100644 --- a/deal.II/examples/step-38/doc/intro.dox +++ b/deal.II/examples/step-38/doc/intro.dox @@ -53,8 +53,9 @@ where $\tilde v$ is a "smooth" extension of $v$ in a tubular neighborhood of $\G $\mathbf n$ is the normal of $\Gamma$. Since $\Delta_S = \nabla_S \cdot \nabla_S$, we deduce @f[ -\Delta_S v = \Delta \tilde v - \mathbf n^T \ D \tilde v \ \mathbf n - (\nabla \tilde v)\cdot \mathbf n (\nabla \cdot \mathbf n). +\Delta_S v = \Delta \tilde v - \mathbf n^T \ D \tilde v \ \mathbf n - (\nabla \tilde v)\cdot \mathbf n (\nabla \cdot \mathbf n - \mathbf n \ D \mathbf n \ \mathbf n ). @f] +Worth mentioning, the term $\nabla \cdot \mathbf n - \mathbf n \ D \mathbf n \ \mathbf n$ appearing in the above expression is the total curvature of the surface (sum of principal curvatures). As usual, we are only interested in weak solutions for which we can use $C^0$ finite elements (rather than requiring $C^1$ continuity as for strong @@ -147,9 +148,10 @@ We produce one test case for a 2d problem and another one for 3d: solution function. There are (at least) two ways to do that. The first one is to project away the normal derivative as described above using the natural extension of $u(\mathbf x)$ (still denoted by $u$) over $\mathbb R^d$, i.e. to compute @f[ - -\Delta_\Gamma u = \Delta u - \mathbf n^T \ D u \ \mathbf n - (\nabla u)\cdot \mathbf n (\nabla \cdot \mathbf n). + -\Delta_\Gamma u = \Delta u - \mathbf n^T \ D u \ \mathbf n - (\nabla u)\cdot \mathbf n \kappa, @f] - Since we are on the unit circle, $\mathbf n=\mathbf x$ so that + where $\kappa$ is the total curvature of $\Gamma$. + Since we are on the unit circle, $\mathbf n=\mathbf x$ and $\kappa = 1$ so that @f[ -\Delta_\Gamma u = -8 x_1x_2. @f] @@ -179,7 +181,7 @@ We produce one test case for a 2d problem and another one for 3d: In 3d, the domain is again half of the surface of the unit ball, i.e. a half sphere or dome. We choose $u(\mathbf x)=-2\sin(\pi x_1)\cos(\pi x_2)e^z$ as the solution. We can compute the right hand side of the - equation, $f=-\Delta_\Gamma u$, in the same way as the method above, yielding an + equation, $f=-\Delta_\Gamma u$, in the same way as the method above (in that case $\kappa = 2$), yielding an awkward and lengthy expression. You can find the full expression in the source code. diff --git a/deal.II/examples/step-38/step-38.cc b/deal.II/examples/step-38/step-38.cc index ab1bcd485f..56e1e6c102 100644 --- a/deal.II/examples/step-38/step-38.cc +++ b/deal.II/examples/step-38/step-38.cc @@ -274,7 +274,7 @@ RightHandSide<3>::value (const Point<3> &p, normal /= p.norm(); return (- trace(hessian) - - (2-3-1) * (gradient * normal) + + 2 * (gradient * normal) + (hessian * normal) * normal); }