Issue: Accessing MainApp time _t in a Steady SubApp (via FullSolveMultiApp) returns 0.0 in Custom Material

[bo-qian]

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Question

Description:
I am running a coupled simulation where a Transient MainApp drives a Steady SubApp (Quasi-static approach). My goal is to calculate a time-dependent viscosity in the SubApp. Since the physical time is controlled by the MainApp, I expect the SubApp to inherit the current time step value when called.

However, I found that inside my custom C++ Material in the SubApp, the member variable _t (current time) always evaluates to 0.0, even when the MainApp has advanced to t > 0 (e.g., t=0.2).

Configuration:

1. MainApp (Transient):
   Uses FullSolveMultiApp to call the SubApp at INITIAL and TIMESTEP_END.

[Executioner]
  type = Transient
  dt = 0.2
[]

[MultiApps]
  [./Stokes]
    type = FullSolveMultiApp
    input_files = 'stokes_sub.i'
    execute_on = 'INITIAL TIMESTEP_END'
  [../]
[]

2. SubApp (Steady):
   Uses Steady executioner to solve the Stokes equation at the current state.

[Executioner]
  type = Steady
[]

[Materials]
  [./viscosity]
    type = LinearHeatingViscosityMaterial
    # This material calculates viscosity based on time (Temperature = T0 + rate * t)
  [../]
[]

3. C++ Custom Material (LinearHeatingViscosityMaterial):
   In computeQpProperties(), I tried to access _t to calculate temperature and viscosity. I added a print statement to debug:

void
LinearHeatingViscosityMaterial::computeQpProperties()
{
   // ... logic ...

   // Debug output
   if (processor_id() == 0 && _current_elem->id() == 0 && _qp == 0)
   {
       Moose::out << "SubApp Material Time: " << _t << std::endl;
   }
}

Observation:

• MainApp Time Step 0 (t=0): Output SubApp Material Time: 0 (Correct)
• MainApp Time Step 1 (t=0.2): Output SubApp Material Time: 0 (Incorrect, expected 0.2)
• MainApp Time Step 2 (t=0.4): Output SubApp Material Time: 0 (Incorrect, expected 0.4)

Questions:

1. Does FullSolveMultiApp automatically update the _t variable for a Steady SubApp? Or does the Steady executioner reset _t to 0.0 at the beginning of each solve?
2. If Steady resets the time, what is the recommended way to pass the MainApp's physical time to the SubApp so my Material can read it? Do I need to use a MultiAppPostprocessorTransfer to pass time as a parameter?

Any insights would be appreciated!

[GiudGiud]

Hello

No the subapp handles its own starting time. So it resets every time because a Steady solve goes from T=0s to t=1s

Using a postprocessor (a Receiver) to keep track of the time in the parent application is a workable solution.