Equation class

Optionally output named terms in BOUT++ evolution equations.

To use, create Equation members of your PhysicsModel, e.g.

Equation density_equation{n, "n", options, dump, rhs_counter};

rhs_counter should be another member of your PhysicsModel class that is incremented every time the rhs() method is called, so that the Equation object knows to reset the internal map of fields if necessary.

Note: The Equation object should be constructed after the variable that it evolves, otherwise the save_ddt option does not work. Therefore, for example, this example Equation density_equation declaration should be placed after the Field3D n declaration in the PhysicsModel class definition.

Then add named terms in the rhs() method, e.g.

density_equation["ExB advection"] = bracket(phi, n);
density_equation["Parallel advection"] = Vpar_Grad_par(V, n);

The terms can also be modified by index by using a local accessor, e.g.

auto local = density_equation.localAccessor();
for (auto i : foo) {
  local[i] = 3.45 * foo[i];
}
for (RangeIterator r = mesh->iterateBndryLowerY(); !r.isDone(); r++) {
  i = r.ind;
  for (int k = 0; k < mesh->LocalNz; k++) {
    local(i, mesh->ystart, k) = bar(i, mesh->ystart, k);
  }
}

If no options are set, this is equivalent (with negligible performance penalty) to adding directly to ddt(n) like

ddt(n) = bracket(phi, n);
ddt(n) += Vpar_Grad_par(V, n);

But options can be set:

restart_and_save_all_terms = true - must set restart = true, and nout = 0; saves each named term in the Equation object to a new dump file.
restart_and_append_all_terms = true - must set restart = true, append = true, and nout = 0; appends each named term in the Equation object to existing dump files.
save_all_terms = true - saves all named terms at every output timestep; likely to lead to very large output files.
save_ddt = true - saves the final time derivative at every output timestep.

johnomotani/BoutEquation

Equation class