This tutorial presents an example of using a “multi-implicit” spectral deferred corrections (MISDC) integrator to solve a simple scalar advection-diffusion-reaction equation in two dimensions. Both diffusion and reaction terms are treated implicitly but solved for independently in an operator splitting fashion. The advection is treated explicitly. The relative strengths of the three terms can be adjusted by changing the coefficients a,d, and r in inputs_2d.
The advection operator is a 4th-order centered difference in flux form. The diffusion operator is a 2nd order discretization of the Laplacian, and the implicit diffusion solve is done using multigrid. The “reaction” term here is just a simple linear damping hence the implicit solve is trivial. See the routines in functions_2d.f90 for the code that evaluates the rhs terms.
The simple form of the equation allows for an exact solution of the PDE in a periodic geometry. There is a flag called “plot_err” in main.cpp, which if set equal 1 will cause the code to output the error in the solution for plotting. If the advection term is omitted (a=0), then an exact solution to the method of lines ODE is computed and used to compute the error. Hence the error in this case will scale in dt with the order of the time integrator.
This code can also be run as an IMEX advection-diffusion example simply by setting Npieces=2 in main.cpp. This should also be equivalent to setting r=0.