Objects of multiapp and transfer system

[lvkas521424]

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Question

I'm currently trying to use an external program's solver as a multi-app to interact with moose, so I have a large number of arrays within my multi-app‘s code(.C and .h), and in the process of trying it out, I'm puzzled by the execution of the transfer:

1. The name of the data transferred by transfer is determined by the source_variable and variable of the input card, so how can I execute transfer so that the transfer to recognize which arrays of the multiiapp to be transferred (it's too many arraysin my .C code interface ), wethear set the array with the same name in the multiiapp code with the input card's source_variable?; similarly, when a multiiapp receives data from an external transfer, how to determine which data to transfer this data to?

2. for a multi-app system, it seems that there can only be one data, if I define two variables, all their sizes are 10 * 10 * 10 , but in the multi-app system actually defined a 10 * 10 * 10 * 2 data?
   I apologize for what may be a clumsy question, thanks.

example : I can't transfer my neutronicsmultiapp's power_density to parents-app's powerdensity

This is input file

[MultiApps]
  [neutronics]
    type = NeutronicsMultiApp
    app_type = MooseprojectsApp
    input_files = 'scapn_input.i'
    mesh_dims = '4 4 4'           
    power_var_name = power_density  
    temperature_var_name = temperature  
    execute_on = 'NEUTRONIC PRENEUTRONIC CORNEUTRONIC'     
  []
[]

[Transfers]
  [from_neutronics]
    type = ReactorTransfer
    from_multi_app = neutronics
    source_variable = power_density
    variable = powerdensity
    execute_on = 'FROM_NEUTRONIC'       
  []
[]

Here's my explicit scheduling for multi-app and transfer

  std::vector<std::string> neutronics_apps = {_neutronics_app_name};
   
  _fe_problem.execMultiApps(LevelSet::EXEC_NEUTRONIC, 
                          neutronics_apps.empty() ? nullptr : &neutronics_apps);

  _fe_problem.execTransfers(LevelSet::EXEC_FROM_NEUTRONIC);

Here's part of the code for my external multi-app. b1_excute is my external Fortran interface for reading temperature_data.data(), output power_data.data()

  b1_execute(mesh_dims_copy.data(), power_data.data(), temperature_data.data(), field_size);
  
  auto & power_density_var = app.getVariable(0, "power_density");
  auto & power_density_sys = power_density_var.sys();
  auto & power_density_solution = power_density_sys.solution();
  
  for (unsigned int i = 0; i < field_size; ++i)
    power_density_solution.set(i, power_data[i]);
  
  power_density_sys.update();

  for (unsigned int i = 0; i < field_size; ++i)
  {
    power_density[i] = power_density_solution(i);
    std::cout << "#" << "power_density[" << i << "] = " << power_density[i] << std::endl;
  }
}
bool
NeutronicsMultiApp::solveStep(Real dt, Real target_time, bool auto_advance)
{
  executeB1Solver();
  return true; 
} 

Unfortunately, when I try to output the powerdensity of the parents-app, it's always 0, while my multi-app does execute normally and passes the value to power_density.