6.6 Extending the basic solver

The core functionality of the code, i.e., that part of the code that implements the time stepping, assembles matrices, solves linear and nonlinear systems, etc., is in the aspect::Simulator class (see the doxygen documentation of this class). Since the implementation of this class has more than 3,000 lines of code, it is split into several files that are all located in the source/simulator directory. Specifically, functionality is split into the following files:

• source/simulator/core.cc: This file contains the functions that drive the overall algorithm (in particular Simulator::run) through the main time stepping loop and the functions immediately called by Simulator::run.
• source/simulator/assembly.cc: This is where all the functions are located that are related to assembling linear systems.
• source/simulator/solver.cc: This file provides everything that has to do with solving and preconditioning the linear systems.
• source/simulator/initial_conditions.cc: The functions in this file deal with setting initial conditions for all variables.
• source/simulator/checkpoint_restart.cc: The location of functionality related to saving the current state of the program to a set of files and restoring it from these files again.
• source/simulator/helper_functions.cc: This file contains a set of functions that do the odd thing in support of the rest of the simulator class.
• source/simulator/parameters.cc: This is where we define and read run-time parameters that pertain to the top-level functionality of the program.

Obviously, if you want to extend this core functionality, it is useful to first understand the numerical methods this class implements. To this end, take a look at the paper that describes these methods, see [KHB12]. Further, there are two predecessor programs whose extensive documentation is at a much higher level than the one typically found inside ASPECT itself, since they are meant to teach the basic components of convection simulators as part of the deal.II tutorial:

• The step-31 program at https://www.dealii.org/developer/doxygen/deal.II/step_31.html: This program is the first version of a convection solver. It does not run in parallel, but it introduces many of the concepts relating to the time discretization, the linear solvers, etc.
• The step-32 program at https://www.dealii.org/developer/doxygen/deal.II/step_32.html: This is a parallel version of the step-31 program that already solves on a spherical shell geometry. The focus of the documentation in this program is on the techniques necessary to make the program run in parallel, as well as some of the consequences of making things run with realistic geometries, material models, etc.

Neither of these two programs is nearly as modular as ASPECT, but that was also not the goal in creating them. They will, however, serve as good introductions to the general approach for solving thermal convection problems.