Discrete Transfer - Maximum Number of Iterations

mcclud · June 27, 2016, 17:17

Hello all,

I am running into trouble with the use of the Discrete Transfer radiation model in CFX 13.0.

To be brief, I have a helium fluid domain within a Stainless Steel structure and then an air fluid domain outside of the steel structure. The air is encapsulated by another stainless steel structure that I apply a temperature boundary condition on. There is a previously calculated heat deposition within the stainless steel domain. I am modelling natural convection in both the fluid domains and surface to surface thermal radiation.

My goal of the simulation is a temperature distribution of the stainless steel structure.

I came upon an issue where the momentum and continuity equations of the fluid domains would solve in the regular ~400 CPU seconds but the 1st iteration would never complete because the energy equation simply would not solve. After some sifting through the documentation I changed the "Ray Tracing Control" for the Discrete Transfer model. I changed the iteration conv criteria to 0.05 and the maximum number of iterations to 100. This led to the output file as below. The simulation runs but you'll notice that the 1st iteration takes (226500 - 728) CPU seconds while the subsequent iterations take ~400-500 CPU seconds. The simulation goes on to solve and converge nicely but the error message has left me uneasy about what is going on in that 1st iteration.

I have searched the forums for the RTRACK subroutine and the error code with no luck.

Does anyone have an idea of what this error means? I am confused as to why the number of radiation iterations for the air and helium domain are pretty much the same between the outer iterations but the computation time differs by 3 orders of magnitude.

Any ideas would be greatly appreciated.

Cheers!

Code:

======================================================================
 OUTER LOOP ITERATION =  560 (    1) CPU SECONDS = 2.186E+05 (7.281E+02)
 ----------------------------------------------------------------------
 |       Equation       | Rate | RMS Res | Max Res |  Linear Solution |
 +----------------------+------+---------+---------+------------------+
 | U-Mom-Air            | 0.00 | 1.7E-03 | 8.7E-02 |       3.7E-02  OK|
 | V-Mom-Air            | 0.00 | 7.3E-03 | 1.1E-01 |       2.2E-02  OK|
 | W-Mom-Air            | 0.00 | 1.6E-03 | 1.0E-01 |       3.8E-02  OK|
 | P-Mass-Air           | 0.00 | 1.6E-04 | 9.7E-03 |  5.1  3.6E-02  OK|
 +----------------------+------+---------+---------+------------------+
 | U-Mom-Helium         | 0.00 | 8.0E-03 | 3.0E-01 |       1.4E-02  OK|
 | V-Mom-Helium         | 0.00 | 7.6E-03 | 2.5E-01 |       1.6E-02  OK|
 | W-Mom-Helium         | 0.00 | 7.8E-03 | 2.8E-01 |       1.4E-02  OK|
 | P-Mass-Helium        | 0.00 | 4.4E-04 | 1.4E-02 |  5.0  3.2E-03  OK|
 +----------------------+------+---------+---------+------------------+

 Parallel run: Received message from slave
 -----------------------------------------
 Slave partition :        2
 Slave routine   : RTRACK
 Master location : RCVBUF,MSGTAG=1081
 Message label   : 666000007
 Message follows below - : 
 
 +--------------------------------------------------------------------+
 | ERROR #666000007 has occurred in subroutine RTRACK.                |
 | Message:                                                           |
 |                                                                    |
 | Error finding neighbour.                                           |
 | Indicates programming error or an error in geometry specification. |
 |                                                                    |
 | Current location :                                                 |
 |   Zone              : Air$1                                        |
 |   Surface           : AirAir Side 1$42549                          |
 |   Location  (X,Y,Z) : -0.80895E-01  0.19050E-01  0.33086E-02       |
 |   Direction (U,V,W) : -0.42889E+00  0.83077E+00  0.35479E+00       |
 |                                                                    |
 | Started at location :                                              |
 |   Zone              : Air$1                                        |
 |   Surface           : FAir Side 1$110379                        |
 |                                                                    |
 +--------------------------------------------------------------------+
 | I-Radiation-Air      | #Its | Vol Chg | Sur Chg | %Lost  %Imbal    |
 |   Gray               |    8 | 0.0E+00 | 3.7E-02 |  0.00   19.13    |
 +----------------------+------+---------+---------+------------------+
 | I-Radiation-Helium   | #Its | Vol Chg | Sur Chg | %Lost  %Imbal    |
 |   Gray               |    9 | 0.0E+00 | 4.8E-02 |  0.00   23.10    |
 +----------------------+------+---------+---------+------------------+
 | H-Energy-Air         | 0.00 | 2.6E-02 | 2.7E-01 |       1.4E-02  OK|
 | T-Energy             | 0.00 | 7.1E-03 | 6.5E+00 |       1.4E-02  OK|
 | H-Energy-Helium      | 0.00 | 4.5E-03 | 4.9E-01 |  5.6  1.4E-02  OK|
 +----------------------+------+---------+---------+------------------+

 ======================================================================
 |                        Timescale Information                       |
 ----------------------------------------------------------------------
 |       Equation       |         Type           |      Timescale     |
 +----------------------+------------------------+--------------------+
 | U-Mom-Air            | Auto Timescale         |     1.33348E-01    |
 | V-Mom-Air            | Auto Timescale         |     1.33348E-01    |
 | W-Mom-Air            | Auto Timescale         |     1.33348E-01    |
 | P-Mass-Air           | Auto Timescale         |     1.33348E-01    |
 +----------------------+------------------------+--------------------+
 | U-Mom-Helium         | Auto Timescale         |     1.19554E-01    |
 | V-Mom-Helium         | Auto Timescale         |     1.19554E-01    |
 | W-Mom-Helium         | Auto Timescale         |     1.19554E-01    |
 | P-Mass-Helium        | Auto Timescale         |     1.19554E-01    |
 +----------------------+------------------------+--------------------+
 | H-Energy-Air         | Auto Timescale         |     1.79094E-01    |
 | T-Energy-F SS        | Auto Timescale         |     3.16945E+03    |
 | H-Energy-Helium      | Auto Timescale         |     1.79094E-01    |
 +----------------------+------------------------+--------------------+

 ======================================================================
 OUTER LOOP ITERATION =  561 (    2) CPU SECONDS = 4.443E+05 (2.265E+05)
 ----------------------------------------------------------------------
 |       Equation       | Rate | RMS Res | Max Res |  Linear Solution |
 +----------------------+------+---------+---------+------------------+
 | U-Mom-Air            | 0.57 | 9.4E-04 | 6.9E-02 |       6.7E-02  OK|
 | V-Mom-Air            | 0.56 | 4.1E-03 | 6.3E-02 |       2.6E-02  OK|
 | W-Mom-Air            | 0.56 | 9.1E-04 | 8.1E-02 |       6.2E-02  OK|
 | P-Mass-Air           | 0.29 | 4.6E-05 | 3.2E-03 |  5.1  8.7E-02  OK|
 +----------------------+------+---------+---------+------------------+
 | U-Mom-Helium         | 0.32 | 2.6E-03 | 1.1E-01 |       2.6E-02  OK|
 | V-Mom-Helium         | 0.35 | 2.7E-03 | 1.0E-01 |       3.5E-02  OK|
 | W-Mom-Helium         | 0.32 | 2.5E-03 | 1.2E-01 |       2.7E-02  OK|
 | P-Mass-Helium        | 0.26 | 1.1E-04 | 2.5E-03 |  5.0  5.1E-03  OK|
 +----------------------+------+---------+---------+------------------+
 | I-Radiation-Air      | #Its | Vol Chg | Sur Chg | %Lost  %Imbal    |
 |   Gray               |    8 | 0.0E+00 | 3.7E-02 |  0.00   19.12    |
 +----------------------+------+---------+---------+------------------+
 | I-Radiation-Helium   | #Its | Vol Chg | Sur Chg | %Lost  %Imbal    |
 |   Gray               |   10 | 0.0E+00 | 4.2E-02 |  0.00   19.23    |
 +----------------------+------+---------+---------+------------------+
 | H-Energy-Air         | 0.28 | 7.4E-03 | 1.4E-01 |       5.1E-02  OK|
 | T-Energy             | 0.15 | 1.1E-03 | 6.1E-01 |       5.1E-02  OK|
 | H-Energy-Helium      | 0.65 | 2.9E-03 | 3.9E-01 |  5.6  5.1E-02  OK|
 +----------------------+------+---------+---------+------------------+

 ======================================================================
 OUTER LOOP ITERATION =  562 (    3) CPU SECONDS = 4.448E+05 (2.270E+05)
 ----------------------------------------------------------------------
 |       Equation       | Rate | RMS Res | Max Res |  Linear Solution |
 +----------------------+------+---------+---------+------------------+
 | U-Mom-Air            | 0.66 | 6.2E-04 | 5.0E-02 |       1.7E-02  OK|
 | V-Mom-Air            | 0.64 | 2.6E-03 | 4.8E-02 |       6.6E-03  OK|
 | W-Mom-Air            | 0.65 | 5.9E-04 | 5.7E-02 |       1.8E-02  OK|
 | P-Mass-Air           | 0.44 | 2.0E-05 | 1.1E-03 |  9.2  5.4E-02  OK|
 +----------------------+------+---------+---------+------------------+
 | U-Mom-Helium         | 0.49 | 1.3E-03 | 5.6E-02 |       3.1E-02  OK|
 | V-Mom-Helium         | 0.56 | 1.5E-03 | 5.9E-02 |       4.7E-02  OK|
 | W-Mom-Helium         | 0.50 | 1.2E-03 | 6.1E-02 |       3.2E-02  OK|
 | P-Mass-Helium        | 0.39 | 4.4E-05 | 1.7E-03 |  5.0  9.1E-03  OK|
 +----------------------+------+---------+---------+------------------+
 | I-Radiation-Air      | #Its | Vol Chg | Sur Chg | %Lost  %Imbal    |
 |   Gray               |    8 | 0.0E+00 | 3.7E-02 |  0.00   19.12    |
 +----------------------+------+---------+---------+------------------+
 | I-Radiation-Helium   | #Its | Vol Chg | Sur Chg | %Lost  %Imbal    |
 |   Gray               |    9 | 0.0E+00 | 5.0E-02 |  0.00   22.73    |
 +----------------------+------+---------+---------+------------------+
 | H-Energy-Air         | 0.29 | 2.1E-03 | 4.9E-02 |       8.4E-02  OK|
 | T-Energy             | 0.44 | 4.8E-04 | 9.1E-02 |       8.4E-02  OK|
 | H-Energy-Helium      | 0.58 | 1.7E-03 | 2.4E-01 |  5.6  8.4E-02  OK|
 +----------------------+------+---------+---------+------------------+

ghorrocks · June 27, 2016, 20:45

I am not familiar with this error, you would have to talk to ANSYS support for detailed information about it. But I am suspicious that there is a problem with your mesh - so I would remesh your geometry and try again.

Opaque · June 28, 2016, 09:54

A few comments from your output file:

1 - The message indicates the tracking algorithm could not continue when traversing an element near the location shown. Using CFD-Post you can create a point in that location, as well as a line from that point in the direction shown and see if the geometry/mesh has a problem.

2 - Your imbalances are way too high for the calculation to be reliable. An imbalance of ~20% means there is 20% energy loss during the simulation for that radiation calculation.

3 - The first radiation iteration of the Discrete Transfer method is the most expensive of them since it is tracking through the domains, and storing the information. In subsequent iterations that information is read from disk with a considerable time saving.

4 - From the diagnostics, I assume you are solving a problem with two disconnected flow regions separated by an non-transparent interface, correct ? Similarly, I assume you may have turned off "constant domain physics"; therefore, you must now be extremely careful with your physics setup.

Hope the above helps,

mcclud · June 28, 2016, 17:27

Thank you Glenn and Opaque!

The mesh was the spot of bother. Simulation is running fine as I type.

Opaque: I have a question regarding your point 2. Each iteration reports the I-Radiation imbalances for the two fluid domains. But I can see that this is different than the I-Radiation Global Imbalances printed at the conclusion of the run. Why are these numbers not the same?

I can see from the radiation diagnostic file that the per iteration imbalances are the sum of the individual mesh surfaces while the global imbalances show the sum per boundary. My understanding is that this is a different way of doing the same thing. I must be missing something here.

Thank you again for your time.

Per Iteration Imbalances (at final iteration)

Code:

======================================================================
 OUTER LOOP ITERATION =  762 (  203) CPU SECONDS = 5.403E+05 (3.224E+05)
 ----------------------------------------------------------------------
 |       Equation       | Rate | RMS Res | Max Res |  Linear Solution |
 +----------------------+------+---------+---------+------------------+
 | U-Mom-Air            | 0.99 | 1.7E-05 | 1.2E-03 |       1.4E-02  OK|
 | V-Mom-Air            | 0.99 | 4.2E-05 | 3.5E-03 |       5.1E-03  OK|
 | W-Mom-Air            | 1.00 | 1.4E-05 | 8.0E-04 |       1.2E-02  OK|
 | P-Mass-Air           | 1.00 | 3.8E-07 | 3.7E-05 |  9.2  2.3E-02  OK|
 +----------------------+------+---------+---------+------------------+
 | U-Mom-Helium         | 1.00 | 5.3E-07 | 1.1E-05 |       2.4E-02  OK|
 | V-Mom-Helium         | 0.99 | 1.3E-06 | 3.1E-05 |       3.1E-02  OK|
 | W-Mom-Helium         | 1.01 | 5.2E-07 | 1.4E-05 |       2.4E-02  OK|
 | P-Mass-Helium        | 1.00 | 2.4E-08 | 1.1E-06 | 17.0  9.0E-02  OK|
 +----------------------+------+---------+---------+------------------+
 | I-Radiation-Air      | #Its | Vol Chg | Sur Chg | %Lost  %Imbal    |
 |   Gray               |    8 | 0.0E+00 | 3.7E-02 |  0.00   19.17    |
 +----------------------+------+---------+---------+------------------+
 | I-Radiation-Helium   | #Its | Vol Chg | Sur Chg | %Lost  %Imbal    |
 |   Gray               |    8 | 0.0E+00 | 4.7E-02 |  0.00    5.32    |
 +----------------------+------+---------+---------+------------------+
 | H-Energy-Air         | 0.99 | 6.7E-05 | 5.8E-03 |       1.9E-02  OK|
 | T-Energy             | 1.00 | 2.0E-06 | 2.6E-03 |       1.9E-02  OK|
 | H-Energy-Helium      | 0.98 | 3.9E-05 | 2.1E-03 |  5.6  1.9E-02  OK|
 +----------------------+------+---------+---------+------------------+

Global Imbalances

Code:

+--------------------------------------------------------------------+
 |                            I-Radiation                             |
 +--------------------------------------------------------------------+
 Boundary         : F168 Bottom1                           -4.6278E-02
 Boundary         : F168 Cylinder                          -1.2365E+02
 Boundary         : F168Top                                -5.9195E+00
 Boundary         : F444Air Side 1                          1.1914E+02
 Boundary         : LidAir Side 1                           1.0456E+01
                                                           -----------
 Global Imbalance :                                        -1.6131E-02

 Global Imbalance, in %:                                     -0.0130 %

 +--------------------------------------------------------------------+
 |                            I-Radiation                             |
 +--------------------------------------------------------------------+
 Boundary         : AluminumHelium Side 2                   2.0644E+01
 Boundary         : F444Helium Side 2                      -2.0838E+01
 Boundary         : Helium Default                          1.3092E-03
 Boundary         : LidHelium Side 1                        2.3812E-01
                                                           -----------
 Global Imbalance :                                         4.5776E-02

 Global Imbalance, in %:                                      0.2197 %

Opaque · June 28, 2016, 17:45

I would not be comfortable with a 19% imbalance. Something is off somewhere.

How many number of rays are you using ? 8 ? Have you made the solution number of rays independent ? Try 16, and 24 and see if the imbalance goes down further..

Hope the above helps,

mcclud · June 28, 2016, 18:08

I am currently running with 12 rays whereas the previous simulation had 8 rays. Imbalances are down to 3%. I tightened the rad convergence criteria as well.

This discussion has certainly helped.

Thanks

June 28, 2016, 09:54		#3
Opaque Senior Member Join Date: Jun 2009 Posts: 1,880 Rep Power: 33	A few comments from your output file: 1 - The message indicates the tracking algorithm could not continue when traversing an element near the location shown. Using CFD-Post you can create a point in that location, as well as a line from that point in the direction shown and see if the geometry/mesh has a problem. 2 - Your imbalances are way too high for the calculation to be reliable. An imbalance of ~20% means there is 20% energy loss during the simulation for that radiation calculation. 3 - The first radiation iteration of the Discrete Transfer method is the most expensive of them since it is tracking through the domains, and storing the information. In subsequent iterations that information is read from disk with a considerable time saving. 4 - From the diagnostics, I assume you are solving a problem with two disconnected flow regions separated by an non-transparent interface, correct ? Similarly, I assume you may have turned off "constant domain physics"; therefore, you must now be extremely careful with your physics setup. Hope the above helps, latermary likes this.

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June 27, 2016, 20:45		#2
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,871 Rep Power: 144	I am not familiar with this error, you would have to talk to ANSYS support for detailed information about it. But I am suspicious that there is a problem with your mesh - so I would remesh your geometry and try again.

June 28, 2016, 17:45		#5
Opaque Senior Member Join Date: Jun 2009 Posts: 1,880 Rep Power: 33	I would not be comfortable with a 19% imbalance. Something is off somewhere. How many number of rays are you using ? 8 ? Have you made the solution number of rays independent ? Try 16, and 24 and see if the imbalance goes down further.. Hope the above helps,

June 28, 2016, 18:08		#6
mcclud New Member Darryl McClure Join Date: Dec 2011 Posts: 10 Rep Power: 15	I am currently running with 12 rays whereas the previous simulation had 8 rays. Imbalances are down to 3%. I tightened the rad convergence criteria as well. This discussion has certainly helped. Thanks