Fix negotiation of dt value in two-scale-heat-conduction/macro-dumux (#664)

* Fix negotiation of dt value in macro-dumux

* Add changelog entry

* Incorporate review feedback

Author: IshaanDesai

changelog-entries/664.md

@@ -0,0 +1 @@
+- Fix negotiation of dt value in two-scale-heat-conduction/macro-dumux [#664](https://github.com/precice/tutorials/pull/664)

two-scale-heat-conduction/macro-dumux/appl/main.cc

@@ -223,17 +223,22 @@ int main(int argc, char **argv)
   // initialize
   couplingParticipant.initialize();
 
-  // get some time loop parameters
+  // time loop parameters
   const auto tEnd      = getParam<Scalar>("TimeLoop.TEnd");
   double     preciceDt = couplingParticipant.getMaxTimeStepSize();
+  double     solverDt;
   double     dt;
-  if (getParam<bool>("Precice.RunWithCoupling") == true)
-    dt = preciceDt;
-  else
+
+  if (getParam<bool>("Precice.RunWithCoupling") == true) {
+    solverDt = getParam<Scalar>("TimeLoop.InitialDt");
+    dt       = std::min(preciceDt, solverDt);
+  } else {
     dt = getParam<Scalar>("TimeLoop.InitialDt");
+  }
 
   // instantiate time loop
   auto timeLoop = std::make_shared<TimeLoop<Scalar>>(0.0, dt, tEnd);
+  timeLoop->setMaxTimeStepSize(getParam<Scalar>("TimeLoop.MaxDt"));
 
   // the assembler with time loop for instationary problem
   using Assembler = FVAssembler<TypeTag, DiffMethod::numeric>;
@@ -296,23 +301,23 @@ int main(int argc, char **argv)
 
     // advance precice
     if (getParam<bool>("Precice.RunWithCoupling") == true) {
-      couplingParticipant.advance(dt);
+      couplingParticipant.advance(timeLoop->timeStepSize());
+
       preciceDt = couplingParticipant.getMaxTimeStepSize();
-      dt        = std::min(preciceDt, std::min(nonLinearSolver.suggestTimeStepSize(
-                                                   timeLoop->timeStepSize()),
-                                               getParam<Scalar>("TimeLoop.MaxDt")));
-      if (preciceDt != dt) {
-        std::cout << "preciceDt too large. We currently assume fixed timestep "
-                     "size but timesteps no longer correspond: preciceDt = "
-                  << preciceDt << " and dt =" << dt << std::endl;
-        // exit(1);
+      solverDt  = std::min(nonLinearSolver.suggestTimeStepSize(timeLoop->timeStepSize()), timeLoop->maxTimeStepSize());
+      dt        = std::min(preciceDt, solverDt);
+
+      if ((!fabs(preciceDt - dt)) < 1e-14) {
+        std::cout << "dt from preCICE is different than dt from DuMuX. "
+                     "preCICE dt = "
+                  << preciceDt << " and DuMuX dt =" << solverDt << std::endl;
       }
     } else
       dt = std::min(
           nonLinearSolver.suggestTimeStepSize(timeLoop->timeStepSize()),
           getParam<Scalar>("TimeLoop.MaxDt"));
 
-    std::cout << "dt: " << dt << std::endl;
+    std::cout << "Using dt: " << dt << std::endl;
 
     if (getParam<bool>("Precice.RunWithCoupling") == true) {
       if (couplingParticipant.requiresToReadCheckpoint()) {
@@ -350,7 +355,7 @@ int main(int argc, char **argv)
         n = 0;
       }
     }
-    // set new dt as suggested by the newton solver or by precice
+    // set new dt as suggested by the newton solver or by preCICE
     timeLoop->setTimeStepSize(dt);
 
     std::cout << "Time: " << timeLoop->time() << std::endl;