add sub-cycling capabilities to MultiApp closes #1880

r17959

framework/include/multiapps/TransientMultiApp.h

@@ -50,6 +50,8 @@ class TransientMultiApp :
 
 private:
   std::vector<Transient *> _transient_executioners;
+
+  bool _sub_cycling;
 };
 
 #endif // TRANSIENTMULTIAPP_H

framework/src/multiapps/TransientMultiApp.C

@@ -23,13 +23,17 @@ InputParameters validParams<TransientMultiApp>()
 {
   InputParameters params = validParams<MultiApp>();
   params += validParams<TransientInterface>();
+
+  params.addParam<bool>("sub_cycling", false, "Set to true to allow this MultiApp to take smaller timesteps than the rest of the simulation.  More than one timestep will be performed for each 'master' timestep");
+
   return params;
 }
 
 
 TransientMultiApp::TransientMultiApp(const std::string & name, InputParameters parameters):
     MultiApp(name, parameters),
-    TransientInterface(parameters, name, "multiapps")
+    TransientInterface(parameters, name, "multiapps"),
+    _sub_cycling(getParam<bool>("sub_cycling"))
 {
   if(!_has_an_app)
     return;
@@ -91,10 +95,44 @@ TransientMultiApp::solveStep()
   for(unsigned int i=0; i<_my_num_apps; i++)
   {
     Transient * ex = _transient_executioners[i];
-    ex->takeStep(_dt);
-    if(!ex->lastSolveConverged())
-      mooseWarning(_name<<_first_local_app+i<<" failed to converge!"<<std::endl);
-    ex->endStep();
+
+    if(_sub_cycling)
+    {
+      // Get the dt this app wants to take
+      Real dt = ex->computeConstrainedDT();
+
+      // Divide the "master" dt by that
+      Real partial_steps = _dt / dt;
+
+      unsigned int num_steps = 0;
+
+      if(partial_steps-std::floor(partial_steps) <= 2.0e-14)
+        num_steps = std::floor(partial_steps);
+      else
+        num_steps = std::ceil(partial_steps);
+
+      // Split the master dt up into the number of steps (so we can hit the time perfectly)
+      dt = _dt / (Real)num_steps;
+
+      // Now do all of the solves we need
+      for(unsigned int i=0; i<num_steps; i++)
+      {
+        ex->takeStep(dt);
+
+        if(!ex->lastSolveConverged())
+          mooseWarning("While sub_cycling "<<_name<<_first_local_app+i<<" failed to converge!"<<std::endl);
+
+        ex->endStep();
+      }
+    }
+    else
+    {
+      ex->takeStep(_dt);
+
+      if(!ex->lastSolveConverged())
+        mooseWarning(_name<<_first_local_app+i<<" failed to converge!"<<std::endl);
+      ex->endStep();
+    }
   }
 
   // Swap back
@@ -106,6 +144,9 @@ TransientMultiApp::solveStep()
 Real
 TransientMultiApp::computeDT()
 {
+  if(_sub_cycling) // Bow out of the timestep selection dance
+    return std::numeric_limits<Real>::max();
+
   Real smallest_dt = std::numeric_limits<Real>::max();
 
   if(_has_an_app)

test/tests/multiapps/sub_cycling/master.i

@@ -0,0 +1,64 @@
+[Mesh]
+  type = GeneratedMesh
+  dim = 2
+  nx = 10
+  ny = 10
+[]
+
+[Variables]
+  [./u]
+  [../]
+[]
+
+[Kernels]
+  [./diff]
+    type = Diffusion
+    variable = u
+  [../]
+  [./td]
+    type = TimeDerivative
+    variable = u
+  [../]
+[]
+
+[BCs]
+  [./left]
+    type = DirichletBC
+    variable = u
+    boundary = left
+    value = 0
+  [../]
+  [./right]
+    type = DirichletBC
+    variable = u
+    boundary = right
+    value = 1
+  [../]
+[]
+
+[Executioner]
+  type = Transient
+  num_steps = 4
+  dt = 0.1
+  petsc_options = '-snes_mf_operator -ksp_monitor'
+  petsc_options_iname = '-pc_type -pc_hypre_type'
+  petsc_options_value = 'hypre boomeramg'
+[]
+
+[Output]
+  output_initial = true
+  exodus = true
+  perf_log = true
+[]
+
+[MultiApps]
+  [./sub]
+    type = TransientMultiApp
+    app_type = MooseTestApp
+    execute_on = timestep
+    positions = '0 0 0'
+    input_files = sub.i
+    sub_cycling = true
+  [../]
+[]
+

test/tests/multiapps/sub_cycling/sub.i

@@ -0,0 +1,53 @@
+[Mesh]
+  type = GeneratedMesh
+  dim = 2
+  nx = 10
+  ny = 10
+[]
+
+[Variables]
+  [./u]
+  [../]
+[]
+
+[Kernels]
+  [./diff]
+    type = Diffusion
+    variable = u
+  [../]
+  [./td]
+    type = TimeDerivative
+    variable = u
+  [../]
+[]
+
+[BCs]
+  [./left]
+    type = DirichletBC
+    variable = u
+    boundary = left
+    value = 0
+  [../]
+  [./right]
+    type = DirichletBC
+    variable = u
+    boundary = right
+    value = 1
+  [../]
+[]
+
+[Executioner]
+  type = Transient
+  num_steps = 4
+  dt = 0.01
+  petsc_options = '-snes_mf_operator -ksp_monitor'
+  petsc_options_iname = '-pc_type -pc_hypre_type'
+  petsc_options_value = 'hypre boomeramg'
+[]
+
+[Output]
+  output_initial = true
+  exodus = true
+  perf_log = true
+[]
+

test/tests/multiapps/sub_cycling/tests

@@ -0,0 +1,7 @@
+[Tests]
+  [./test]
+    type = 'Exodiff'
+    input = 'master.i'
+    exodiff = 'master_out.e master_out_sub0.e'
+  [../]
+[]