Fix NonHydrostaticTimeIntegrator2D for projection of solutions with a…

…daptivity

The advance method of a timeintegrator should always advance from the
solved-for-fields, which are the same fields that will contain the
solution at the end of the timestep _after_ calling advance(). This is
because this is the only field(s) that the "user" of a time integrator
has control over, and the user may want to adjust/overwrite/project (for
adaptivity) at the end of a timestep and expect the timestepper to
advance from the adjusted values in the next timestep.

Thus, if a time-integrator stores some copy of a "solution_old" it
should be set at the _beginning_ of advance() to the current solution,
rather than at the end.

thetis/coupled_timeintegrator_2d.py

@@ -199,11 +199,12 @@ def initialize(self, solution2d):
         self.timesteppers.swe2d.initialize(self.fields.solution_2d)
         if self.nh_options.update_free_surface:
             self.timesteppers.fs2d.initialize(self.fields.elev_2d)
-            self.elev_old.assign(self.fields.elev_2d)
 
     @PETSc.Log.EventDecorator("thetis.NonHydrostaticTimeIntegrator2D.advance")
     def advance(self, t, update_forcings=None):
         """Advances equations for one time step."""
+        if self.nh_options.update_free_surface:
+            self.elev_old.assign(self.fields.elev_2d)
         if self.serial_advancing:
             # --- advance in serial ---
             self.timesteppers.swe2d.advance(t, update_forcings=update_forcings)
@@ -213,7 +214,6 @@ def advance(self, t, update_forcings=None):
             if self.nh_options.update_free_surface:
                 self.fields.elev_2d.assign(self.elev_old)
                 self.timesteppers.fs2d.advance(t, update_forcings=update_forcings)
-                self.elev_old.assign(self.fields.elev_2d)
             # update old solution
             if self.options.swe_timestepper_type == 'SSPIMEX':
                 self.timesteppers.swe2d.erk.solution_old.assign(self.fields.solution_2d)
@@ -234,4 +234,3 @@ def advance(self, t, update_forcings=None):
                     elif last_stage:
                         self.fields.elev_2d.assign(self.elev_old)
                         self.timesteppers.fs2d.advance(t, update_forcings=update_forcings)
-                        self.elev_old.assign(self.fields.elev_2d)