How to Create New Strategy
The SolvingStrategy is the object demanded to implement the “order of the calls” to the
different solution phases. All the system matrices and vectors will be stored in the strategy, which allows to deal with multiple LHS and RHS. Trivial examples of these strategies are the linear strategy and the Newton Raphson strategy.
SolvingStrategy is derived from Process and use the same structure as shown in figure 8.9.
Deriving SolvingStrategy from Process lets users to combine them with some other processes using composition in order to create a more complex Process. The strategy pattern used in this structure lets users to implement a new Strategy and add it to Kratos easily which increases the extendability of Kratos. Also lets them selecting an strategy and use it instead of another one in order to change the solving algorithm, which increases the flexibility of Kratos.
Composite pattern is used to let users combining different strategies in one. For example a
fractional step strategy can be implemented by combining different strategies used for each step in one composite strategy. Like for Process, the interface for changing the children of the composite strategy is considered to be too sophisticated and is removed from the Strategy. So a composite structure can be constructed by giving all its components at the constructing time and then it can be used but without changing its sub algorithms.
The interface of SolvingStrategy reflects the general steps in usual finite element algorithms
like prediction, solving, convergence control and calculating results. This design results in the following interface:
Predict A method to predict the solution. If it is not called, a trivial predictor is used and the
values of the solution step of interest are assumed equal to the old values.
Solve This method implements the solving procedure. This means building the equation system
by assembling local components, solving them using a given linear solver and updating the results.
IsConverged It is a post-solution convergence check. It can be used for example in coupled
problems to see if the solution is converged or not.
CalculateOutputData Calculates non trivial results like stresses in structural analysis.
Hierarchical position of the Solving Strategy in KRATOS
Solving strategy is the class, which utilizes such classes as builder_and_solver and scheme as its slaves. Those two terms will be described in the subsequent sections. Roughly speaking, solving strategy provides the overall sequence of calls, necessary for the solution of the problem. It calls at the necessary point the builder_and_solver (see the article on builder_and_solver for the detailed description), which assembles the global matrices (by gathering the elemental contributions) and (possibly) solves the resulting system. The flexibility at this step is provided in a way, that one can "plug-in" many of the provided solvers or write his own one, and then pass it as an argument to the strategy.
'Scheme on the other hand is responsible for the time integration. It is also one of the arguments of the solving strategy. As we see, the abstract and unified structure of the solving strategy enables one really to customize the application by choosing and plugging in different building blocks.
Let us have a brief look at an example of "residual_based_newton-raphson_strategy":
ResidualBasedNewtonRaphsonStrategy( ModelPart& model_part, typename TSchemeType::Pointer pScheme, typename TLinearSolver::Pointer pNewLinearSolver, typename TConvergenceCriteriaType::Pointer pNewConvergenceCriteria, int MaxIterations = 30, bool CalculateReactions = false, bool ReformDofSetAtEachStep = false, bool MoveMeshFlag = false )
first argument is the model_part, which is the union of elements thought of as to belong to one entity - what we call a model part. E.g. this could be structure_model_part and fluid_model_part in an FSI application.
pScheme - defines the time integration scheme. E.g. Newmark. Linear solver - the solver which will be (in this case) used for the solution of the linear system arising at every iteration of Newton-Raphson. It could be e.g. a Conjugate Gradient solver. Convergence criterion - is the criterion for the Newton-Raphson(in this case) procedure to be converged. It could be a norm of the residual or something else - like the energy norm. MaxIterations is a cut of criterion for the Newton-Raphson (in this case) - if the convergence is not achieved within the allowed number of iterations, the solution terminates and the value of variable of interest achieved at the last iteration is taken as the result, though a message appears that the solution did not converge.
Last two flags are important when choosing between Eulerian and Lagrangian frameworks - if we erase or add nodes or elements during the solution of the problem, we need to set the ReformDofSetAtEachStep to true, and if use non-Eulerian approach, the mesh is also moved - so set the MoveMeshFlag to true in this case.