The poor convergence of THM_SCM_coupling_pump.i

[Rforoeverad]

$\quad$ Hello all,
$\quad$ when I run modules/combined/test/tests/subchannel_thm_coupling/THM_SCM_coupling_pump.i, the following results show that the picard iteration does not converge. Is there any way to make it converge?
$\quad$ Any suggestions will be greatly appreciated.

subchannel0:
subchannel0: Postprocessor Values:
subchannel0: +----------------+----------------+------------------------+------------------------+--------------------------+------------------------+--------------------------------+
subchannel0: | time           | Total_power    | report_mass_flux_inlet | report_pressure_outlet | report_temperature_inlet | total_pressure_drop_SC | total_pressure_drop_SC_limited |
subchannel0: +----------------+----------------+------------------------+------------------------+--------------------------+------------------------+--------------------------------+
subchannel0: |   0.000000e+00 |   1.000000e+04 |           2.233982e+03 |           2.001823e+05 |             5.827747e+02 |           1.718278e+04 |                   1.718278e+04 |
subchannel0: +----------------+----------------+------------------------+------------------------+--------------------------+------------------------+--------------------------------+
subchannel0:
subchannel0: Executing subchannel solver
subchannel0: Solving Outer Iteration : 1
subchannel0: Solving Block: 0 From first level: 1 to last level: 25
subchannel0: T_block_error: 0.00427816
subchannel0: T_block_error: 2.98829e-05
subchannel0: P_error :0.404535
subchannel0: Solving Outer Iteration : 2
subchannel0: Solving Block: 0 From first level: 1 to last level: 25
subchannel0: T_block_error: 5.20666e-06
subchannel0: P_error :0.0015574
subchannel0: Solving Outer Iteration : 3
subchannel0: Solving Block: 0 From first level: 1 to last level: 25
subchannel0: T_block_error: 6.72943e-07
subchannel0: P_error :0.000299514
subchannel0:  Solve Converged!
subchannel0:
subchannel0: Postprocessor Values:
subchannel0: +----------------+----------------+------------------------+------------------------+--------------------------+------------------------+--------------------------------+
subchannel0: | time           | Total_power    | report_mass_flux_inlet | report_pressure_outlet | report_temperature_inlet | total_pressure_drop_SC | total_pressure_drop_SC_limited |
subchannel0: +----------------+----------------+------------------------+------------------------+--------------------------+------------------------+--------------------------------+
subchannel0: |   0.000000e+00 |   1.000000e+04 |           2.233982e+03 |           2.001823e+05 |             5.827747e+02 |           1.718278e+04 |                   1.718278e+04 |
subchannel0: |   1.000000e+00 |   1.000000e+04 |           2.233982e+03 |           2.001823e+05 |             5.827747e+02 |           2.839462e+04 |                   2.839462e+04 |
subchannel0: +----------------+----------------+------------------------+------------------------+--------------------------+------------------------+--------------------------------+
subchannel0:
subchannel0: Finished executing subchannel solver
subchannel0: Commencing calculation of Pin surface temperature
subchannel0:
subchannel0: Transfers on FINAL To MultiApps:
subchannel0: ---------------------------------------------------------------
subchannel0: |        Name         |          Type          |  From  | To  |
subchannel0: ---------------------------------------------------------------
subchannel0: | subchannel_transfer | SCMSolutionTransfer    | Parent | viz |
subchannel0: | pin_transfer        | SCMPinSolutionTransfer | Parent | viz |
subchannel0: ---------------------------------------------------------------
subchannel0: Transfers on FINAL Are Finished
subchannel0:
subchannel0:
subchannel0: No Transfers on FINAL Between MultiApps
subchannel0:
subchannel0:
subchannel0: Executing MultiApps on FINAL
subchannel0_viz0:  Solve Converged!
subchannel0: Subapp subchannel0_viz0 solve converged!
subchannel0: Finished Executing MultiApps on FINAL
subchannel0:
subchannel0:
subchannel0: No Transfers on FINAL From MultiApps
subchannel0:
Fixed point residual norm after TIMESTEP_END MultiApps: 1.846639e-01

 0 Picard |R| = 2.727717e+02
 1 Picard |R| = 5.132060e-01
 2 Picard |R| = 2.218452e-01
 3 Picard |R| = 9.074382e-02
 4 Picard |R| = 2.438963e-01
 5 Picard |R| = 1.846639e-01

Fixed point convergence reason: REACH_MAX_ITS

Postprocessor Values:
+----------------+----------------+----------------+----------------+----------------+----------------+----------------+----------------+------------------+----------------+----------------+----------------+----------------+----------------+----------------+
| time           | Dh             | Kloss          | T_out          | av_rho         | av_rhouA       | core_T_out     | core_delta_p   | core_delta_p_tgt | core_f         | core_p_in      | core_p_out     | core_power     | hx_pri_T_out   | hx_sec_T_in    |
+----------------+----------------+----------------+----------------+----------------+----------------+----------------+----------------+------------------+----------------+----------------+----------------+----------------+----------------+----------------+
:                :                :                :                :                :                :                :                :                  :                :                :                :                :                :                :
|   1.000000e+01 |   7.978846e-02 |   1.349571e+03 |   5.821337e+02 |   7.775988e+02 |   9.466487e-01 |   5.896577e+02 |   2.743435e+04 |     2.839462e+04 |   1.076802e+02 |   2.276166e+05 |   2.001823e+05 |   1.000000e+04 |   5.812705e+02 |   3.000162e+02 |
+----------------+----------------+----------------+----------------+----------------+----------------+----------------+----------------+------------------+----------------+----------------+----------------+----------------+----------------+----------------+
+----------------+----------------+----------------------+----------------+-------------------+----------------+----------------+------------------+
| time           | hx_sec_T_out   | inlet_mass_flow_rate | inlet_mass_flux| inlet_temperature | m_dot_sec      | outlet_pressure| power_to_coolant |
+----------------+----------------+----------------------+----------------+-------------------+----------------+----------------+------------------+
:                :                :                      :                :                   :                :                :                  :
|   1.000000e+01 |   3.000220e+02 |         1.112702e+00 |   2.233982e+03 |      5.827747e+02 |   1.000000e+00 |   2.001823e+05 |     1.000000e+04 |
+----------------+----------------+----------------------+----------------+-------------------+----------------+----------------+------------------+

Finished Executing                                                                       [  9.80 s] [  286 MB]

[Rforoeverad]

$\quad$ Could you kindly help me diagnose this problem? @GiudGiud @kyriv1980

[GiudGiud]

Try to tighten tolerances in subchannel. The parameters are in the [subchannel] block

[kyriv-lab]

Also, try plotting the following per iteration: total_pressure_drop_SC, total_pressure_drop_SC_limited, core_delta_p, Kloss. They should all converge since THM is running a pseudo transient.

I am not sure if: Fixed point residual norm, is an accurate metric in this specific type of coupling (@GiudGiud ?)

PS.

Something to note and possibly improve in this example is that ideally power should be transfered from THM to SCM and not the other way around.

[GiudGiud]

THM is the parent so it's fine here.
There are always better choice

[Rforoeverad]

$\quad$ Hello, @GiudGiud @kyriv1980 I find that after I make the following modifications:

- A_core = 0.005 #dummy
+ A_core = ${SC_core} #dummy

$\quad$ The results seem to be more reasonable. Is this modification correct?

Main app solve:
 0 Nonlinear |R| = 6.083455e-04
 1 Nonlinear |R| = 1.360194e-03
 2 Nonlinear |R| = 1.169984e-08
 Solve Converged!
subchannel0:
subchannel0: Postprocessor Values:
subchannel0: +----------------+----------------+------------------------+------------------------+--------------------------+------------------------+--------------------------------+
subchannel0: | time           | Total_power    | report_mass_flux_inlet | report_pressure_outlet | report_temperature_inlet | total_pressure_drop_SC | total_pressure_drop_SC_limited |
subchannel0: +----------------+----------------+------------------------+------------------------+--------------------------+------------------------+--------------------------------+
subchannel0: |   0.000000e+00 |   1.000000e+04 |           2.173429e+03 |           2.001566e+05 |             5.828370e+02 |           2.762243e+04 |                   2.762243e+04 |
subchannel0: +----------------+----------------+------------------------+------------------------+--------------------------+------------------------+--------------------------------+
subchannel0:
subchannel0: Executing subchannel solver
subchannel0: Solving Outer Iteration : 1
subchannel0: Solving Block: 0 From first level: 1 to last level: 25
subchannel0: T_block_error: 0.000126621
subchannel0: P_error :0.00319203
subchannel0: Solving Outer Iteration : 2
subchannel0: Solving Block: 0 From first level: 1 to last level: 25
subchannel0: T_block_error: 3.85145e-05
subchannel0: P_error :0.0035639
subchannel0: Solving Outer Iteration : 3
subchannel0: Solving Block: 0 From first level: 1 to last level: 25
subchannel0: T_block_error: 5.71656e-06
subchannel0: P_error :0.00185758
subchannel0: Solving Outer Iteration : 4
subchannel0: Solving Block: 0 From first level: 1 to last level: 25
subchannel0: T_block_error: 1.06839e-06
subchannel0: P_error :0.000306431
subchannel0:  Solve Converged!
subchannel0:
subchannel0: Postprocessor Values:
subchannel0: +----------------+----------------+------------------------+------------------------+--------------------------+------------------------+--------------------------------+
subchannel0: | time           | Total_power    | report_mass_flux_inlet | report_pressure_outlet | report_temperature_inlet | total_pressure_drop_SC | total_pressure_drop_SC_limited |
subchannel0: +----------------+----------------+------------------------+------------------------+--------------------------+------------------------+--------------------------------+
subchannel0: |   0.000000e+00 |   1.000000e+04 |           2.173429e+03 |           2.001566e+05 |             5.828370e+02 |           2.762243e+04 |                   2.762243e+04 |
subchannel0: |   1.000000e+00 |   1.000000e+04 |           2.173429e+03 |           2.001566e+05 |             5.828370e+02 |           2.742440e+04 |                   2.742440e+04 |
subchannel0: +----------------+----------------+------------------------+------------------------+--------------------------+------------------------+--------------------------------+
subchannel0:
subchannel0: Finished executing subchannel solver
subchannel0: Commencing calculation of Pin surface temperature
subchannel0:
subchannel0: Transfers on FINAL To MultiApps:
subchannel0: ---------------------------------------------------------------
subchannel0: |        Name         |          Type          |  From  | To  |
subchannel0: ---------------------------------------------------------------
subchannel0: | subchannel_transfer | SCMSolutionTransfer    | Parent | viz |
subchannel0: | pin_transfer        | SCMPinSolutionTransfer | Parent | viz |
subchannel0: ---------------------------------------------------------------
subchannel0: Transfers on FINAL Are Finished
subchannel0:
subchannel0:
subchannel0: No Transfers on FINAL Between MultiApps
subchannel0:
subchannel0:
subchannel0: Executing MultiApps on FINAL
subchannel0_viz0:  Solve Converged!
subchannel0: Subapp subchannel0_viz0 solve converged!
subchannel0: Finished Executing MultiApps on FINAL
subchannel0:
subchannel0:
subchannel0: No Transfers on FINAL From MultiApps
subchannel0:
Fixed point residual norm after TIMESTEP_END MultiApps: 1.698065e-06

 0 Picard |R| = 1.953362e+02
 1 Picard |R| = 6.517808e-04
 2 Picard |R| = 6.083455e-04
 3 Picard |R| = 1.698065e-06

Fixed point convergence reason: CONVERGED_RELATIVE

Postprocessor Values:
+----------------+----------------+----------------+----------------+----------------+----------------+----------------+----------------+------------------+----------------+----------------+----------------+----------------+----------------+----------------+
| time           | Dh             | Kloss          | T_out          | av_rho         | av_rhouA       | core_T_out     | core_delta_p   | core_delta_p_tgt | core_f         | core_p_in      | core_p_out     | core_power     | hx_pri_T_out   | hx_sec_T_in    |
+----------------+----------------+----------------+----------------+----------------+----------------+----------------+----------------+------------------+----------------+----------------+----------------+----------------+----------------+----------------+
:                :                :                :                :                :                :                :                :                  :                :                :                :                :                :                :
|   1.000000e+01 |   2.518283e-02 |   9.436187e+00 |   5.820963e+02 |   7.772105e+02 |   1.060066e+00 |   5.899500e+02 |   2.749706e+04 |     2.742440e+04 |   2.376299e-01 |   2.276536e+05 |   2.001566e+05 |   1.000000e+04 |   5.812185e+02 |   3.000179e+02 |
+----------------+----------------+----------------+----------------+----------------+----------------+----------------+----------------+------------------+----------------+----------------+----------------+----------------+----------------+----------------+
+----------------+----------------+----------------------+----------------+-------------------+----------------+----------------+------------------+
| time           | hx_sec_T_out   | inlet_mass_flow_rate | inlet_mass_flux| inlet_temperature | m_dot_sec      | outlet_pressure| power_to_coolant |
+----------------+----------------+----------------------+----------------+-------------------+----------------+----------------+------------------+
:                :                :                      :                :                   :                :                :                  :
|   1.000000e+01 |   3.000220e+02 |         1.082541e+00 |   2.173429e+03 |      5.828370e+02 |   1.000000e+00 |   2.001566e+05 |     1.000000e+04 |
+----------------+----------------+----------------------+----------------+-------------------+----------------+----------------+------------------+

Finished Executing                                                                       [ 20.76 s] [  287 MB]

[kyriv-lab]

A_core is the flow area of the core in THM, and SC_core is the flow area in SCM. The purpose is to reproduce the pressure drop that is calculated in SCM in THM. So theoretically, you can do that with any A_core value. But i think you are correct and it is more consistent for the two simulations to have the same flow area.