From: bangerth Date: Mon, 6 Feb 2012 21:54:46 +0000 (+0000) Subject: Document solve(), the central function of this program. X-Git-Url: https://gitweb.dealii.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=571f9eea72829d14d48a927b461f4ac9a83903d7;p=dealii-svn.git Document solve(), the central function of this program. git-svn-id: https://svn.dealii.org/trunk@24997 0785d39b-7218-0410-832d-ea1e28bc413d --- diff --git a/deal.II/examples/step-43/step-43.cc b/deal.II/examples/step-43/step-43.cc index 374c308acf..958c703b3a 100644 --- a/deal.II/examples/step-43/step-43.cc +++ b/deal.II/examples/step-43/step-43.cc @@ -355,8 +355,8 @@ namespace Step43 const double saturation_level; const double saturation_refinement_threshold; - double old_macro_time_step; double current_macro_time_step; + double old_macro_time_step; double time_step; double old_time_step; @@ -680,6 +680,9 @@ namespace Step43 saturation_level (2), saturation_refinement_threshold (0.5), + current_macro_time_step (0), + old_macro_time_step (0), + time_step (0), old_time_step (0), viscosity (0.2), @@ -1687,54 +1690,31 @@ namespace Step43 // @sect3{TwoPhaseFlowProblem::solve} - // This function is to implement the operator - // splitting algorithm. At the beginning of - // the implementation, we decide whther to - // solve the pressure-velocity part by - // running an a posteriori criterion, which - // will be described in the following - // function. If we get the bool variable true - // from that function, we will solve the - // pressure-velocity part for updated - // velocity. Then, we use GMRES with the - // Schur complement preconditioner to solve - // this linear system, as is described in the - // Introduction. After solving the velocity - // and pressure, we need to keep the - // solutions for linear extrapolations in the - // future. It is noted that we always solve - // the pressure-velocity part in the first - // three micro time steps to ensure accuracy - // at the beginning of computation, and to - // provide starting data to linearly - // extrapolate previously computed velocities - // to the current time step. - // - // On the other hand, if we get a false - // variable from the criterion, we will - // directly use linear extrapolation to - // compute the updated velocity for the - // solution of saturation later. - // - // Next, like step-21, this program need to - // compute the present time step. - // - // Next, we need to use two bool variables - // solve_for_pressure_and_velocity and - // previous_solve_for_pressure_and_velocity to - // decide whether we stop or continue - // cumulating the micro time steps for linear - // extropolations in the next iteration. With - // the reason, we need one variable - // current_macro_time_step for keeping the - // present aggregated micro time steps and - // anther one old_macro_time_step for - // retaining the previous micro time steps. + // This function implements the + // operator splitting algorithm, + // i.e. in each time step it either + // re-computes the solution of the + // Darcy system or extrapolates + // velocity/pressure from previous + // time steps, then determines the + // size of the time step, and then + // updates the saturation + // variable. The implementation + // largely follows similar code in + // step-31. // - // Finally, we start to calculate the - // saturation part with the use of the - // incomplete Cholesky decomposition for - // preconditioning. + // At the beginning of the + // function, we decide whether + // to solve the pressure-velocity + // part by evaluating the + // posteriori criterion, which will + // be implemented in the following + // function. If necessary, we will + // solve the pressure-velocity part + // using the GMRES solver with the + // Schur complement preconditioner + // as is described in the + // introduction. template void TwoPhaseFlowProblem::solve () { @@ -1792,7 +1772,18 @@ namespace Step43 // time step, then we need to // simply extrapolate the // previous two Darcy solutions - // to the current time: + // to the same time as we would + // have computed the + // velocity/pressure at. Note + // that the algorithm here only + // works if we have at least two + // previously computed Darcy + // solutions from which we can + // extrapolate to the current + // time, and this is ensured by + // requiring re-computation of + // the Darcy solution for the + // first 3 time steps. else { darcy_solution = last_computed_darcy_solution; @@ -1810,35 +1801,60 @@ namespace Step43 } - - // compute optimal time step... + // With the so computed velocity + // vector, compute the optimal + // time step based on the CFL + // criterion discussed in the + // introduction... old_time_step = time_step; time_step = porosity * GridTools::minimal_cell_diameter(triangulation) / get_maximal_velocity_times_dF_dS() / 12; - //TODO: need to figure out how - //this is supposed to work. i - //think the inner if can only - //happen in time step==1 + // ...and then also update the + // length of the macro time steps + // we use while we're dealing + // with time step sizes. In + // particular, this involves: (i) + // If we have just recomputed the + // Darcy solution, then the + // length of the previous macro + // time step is now fixed and the + // length of the current macro + // time step is, up to now, + // simply the length of the + // current (micro) time + // step. (ii) If we have not + // recomputed the Darcy solution, + // then the length of the current + // macro time step has just grown + // by time_step. if (solve_for_pressure_and_velocity == true) { -// if (previous_solve_for_pressure_and_velocity == true) - old_macro_time_step = time_step; -// else - old_macro_time_step = current_macro_time_step; - - current_macro_time_step = 0; + old_macro_time_step = current_macro_time_step; + current_macro_time_step = time_step; } else current_macro_time_step += time_step; - std::cout << " Solving saturation transport equation..." << std::endl; + // The last step in this function + // is to recompute the saturation + // solution based on the velocity + // field we've just + // obtained. This naturally + // happens in every time step, + // and we don't skip any of these + // computations. At the end of + // computing the saturation, we + // project back into the allowed + // interval $[0,1]$ to make sure + // our solution remains physical. + { + std::cout << " Solving saturation transport equation..." << std::endl; - assemble_saturation_system (); + assemble_saturation_system (); - { SolverControl solver_control (saturation_matrix.m(), 1e-16*saturation_rhs.l2_norm()); SolverCG cg (solver_control); @@ -1849,18 +1865,14 @@ namespace Step43 cg.solve (saturation_matrix, saturation_solution, saturation_rhs, preconditioner); - saturation_constraints.distribute (saturation_solution); - project_back_saturation (); std::cout << " " << solver_control.last_step() << " CG iterations for saturation." << std::endl; - } - } @@ -2511,6 +2523,9 @@ namespace Step43 old_saturation_solution); timestep_number = 0; + time_step = old_time_step = 0; + current_macro_time_step = old_macro_time_step = 0; + double time = 0; do