From: Wolfgang Bangerth Date: Sun, 2 Jul 2023 17:15:18 +0000 (-0600) Subject: Start with the introduction. X-Git-Url: https://gitweb.dealii.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=d87d6b42d8bdf2ce131aae0db5162f392c7bf10c;p=dealii.git Start with the introduction. --- diff --git a/examples/step-86/doc/intro.dox b/examples/step-86/doc/intro.dox index c59cfdf609..fe8c4689f6 100644 --- a/examples/step-86/doc/intro.dox +++ b/examples/step-86/doc/intro.dox @@ -1,2 +1,130 @@ +
+ + +This program was contributed by Wolfgang Bangerth, Colorado State University. + +This material is based upon work partially supported by National Science +Foundation grants OAC-1835673, DMS-1821210, and EAR-1925595; +and by the Computational Infrastructure in +Geodynamics initiative (CIG), through the National Science Foundation under +Award No. EAR-1550901 and The University of California-Davis. + +Stefano Zampini (King Abdullah University of Science and Technology) +contributed the results obtained with the PETSc variant of this program +discussed in the results section below. + +
+

Introduction

+ +step-26 solved the simple heat equation, one of the prototypical examples +of time dependent problems: +@f{align*}{ + \frac{\partial u(\mathbf x, t)}{\partial t} + - + \Delta u(\mathbf x, t) + &= + f(\mathbf x, t), + \qquad\qquad && + \forall \mathbf x \in \Omega, t\in (0,T), + \\ + u(\mathbf x, 0) &= u_0(\mathbf x) && + \forall \mathbf x \in \Omega, + \\ + u(\mathbf x, t) &= g(\mathbf x,t) && + \forall \mathbf x \in \partial\Omega, t \in (0,T). +@f} +While that program showed a number of advanced techniques such as +using adaptive mesh refinement, it did not address one big issue: +It hand-rolls its own time stepping scheme, which in that program +is the simple +Crank-Nicolson +method with a fixed time step. This is neither accurate nor efficient: We +should be using a higher-order time stepping algorithm, and we should +use one of the many ways to efficiently and automatically choose the +length of the time step in response to the accuracy obtained. + +This would of course require quite a lot of development effort -- unless, +of course, you do what we always advise: You build on what others have +already done and have likely done in a way far superior to what one can +do by oneself. In the current case, deal.II has interfaces to two +such libraries: SUNDIALS, the *SUite of Nonlinear and DIfferential/ALgebraic +equation Solvers* (and here specifically the Runge-Kutta-type solvers +wrapped in the SUNDIALS::ARKode class), and PETSc's TS sub-packaged +(wrapped in the PETScWrappers::TimeStepper class). + +Both of these require that we first write the partial differential equation +in the form of an ordinary differential equation. To this end, let us turn +around the approach we used in step-26. There, we first discretized in time, +obtaining a PDE to be solved at each time step that we could then discretize +using the finite element method. This approach is called the "Rothe method". +Instead, here, we use what's called the "method of lines" where we first +discretize in space, obtaining a system of ordinary differential equations +to which we can apply traditional time steppers. (There are some trade-offs +between these two strategies, principally around using dynamically changing +meshes; we will get back to this issue later on.) + +To get this started, we take the equation above and multiply it by a test +function $\varphi(\mathbf x)$ and integrate by parts to get a weak form: +We seek a function $u(\mathbf x, t)$ that for all test functions +$\varphi \in H^1(\Omega)$ +satisfies +@f{align*}{ +\left(\varphi(\mathbf x), + \frac{\partial u(\mathbf x, t)}{\partial t} \right)_\Omega + + +\left(\nabla \varphi(\mathbf x), + \nabla u(\mathbf x, t) \right)_\Omega + &= +\left(\varphi(\mathbf x), + f(\mathbf x, t) \right)_\Omega, + \\ +\left(\varphi(\mathbf x), u(\mathbf x, 0)\right)_\Omega &= +\left(\varphi(\mathbf x), u_0(\mathbf x)\right)_\Omega && + \\ + u(\mathbf x, t) &= g(\mathbf x,t) && + \forall \mathbf x \in \partial\Omega, t \in (0,T). +@f} + +We then discretize by restricting ourself to finite element functions +of the form +@f{align*}{ +u_h(\mathbf x,t) = \sum_j U_j(t) \varphi_j(\mathbf x), +@f} +which leads to the problem of finding $u_h(\mathbf x, t)$ that for all +discrete test functions $\varphi \in V_h(\Omega)$ satisfies +@f{align*}{ +\left(\varphi_i(\mathbf x), + \frac{\partial u_h(\mathbf x, t)}{\partial t} \right)_\Omega + + +\left(\nabla \varphi_j(\mathbf x), + \nabla u_h(\mathbf x, t) \right)_\Omega + &= +\left(\varphi_i(\mathbf x), + f(\mathbf x, t) \right)_\Omega, + \\ +\left(\varphi_i(\mathbf x), u_h(\mathbf x, 0)\right)_\Omega &= +\left(\varphi_i(\mathbf x), u_0(\mathbf x)\right)_\Omega && + \\ + u_h(\mathbf x, t) &= g_h(\mathbf x,t) && + \forall \mathbf x \in \partial\Omega, t \in (0,T), +@f} +where $g_h$ is an interpolant of the function $g$ on the boundary. + +This equation can be rewritten in matrix form in the usual way, by +expanding $u_h$ into its coefficients times shape function form, +pulling the sum over $j$ out of the integrals, and then considering +that choosing test function $\varphi_i$ leads to the $i$th row +of the linear system. This then gives us +@f{align*}{ +M + \frac{\partial U(t)}{\partial t} + + +AU(t) + &= + F(t) + \\ + U(0) = U_0, +@f} +plus appropriate boundary conditions.