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Supersonic flow crashing in CFX Solver

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Old   March 7, 2016, 21:12
Default Supersonic flow crashing in CFX Solver
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Faizan
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Hi all,

I am modeling a supersonic flow through convergent-divergent nozzle with throat dia 2.6mm. I am using a physical timescale of 1e-6[s] and sometimes 1e-7[s]. In both situations the CFX solver crashing by causing an huge increase in mach no value of greater than 8 or 9 after 50 iterations with 1e-6[s] physical timescale and after 200 iterations in case of 1e-7[s] timescale. I plotted the velocity profile of failed solver file and found that the highest no of mach no was found in divergent section of the nozzle. Means in convergent section and upto the throat the solver was stable. I have changed the mesh nodes by making it more dense but still CFX solver is crashing. Please suggest what should I do to make the supersonic flow up and running. I used total pressure and total temperature inlet conditions at nozzle inlet and the values are 550 deg centigrade and 1.4MPa.

Any help and support will be much appreciated.

Thanks in advance

Faizan
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Old   March 8, 2016, 04:22
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Maybe your divergent section has quite large angle and expand too fast, so it may cause a big separation vortex? If yes, you should simulate this case as a transient? Did you perform hand calculation of this nozzle before?
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Old   March 8, 2016, 04:41
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Hi Tomson,

Thanks for your response. The nozzle which I used in the simulation is an exact replica of actual nozzle used in the lab equipment. So I am very much sure that nozzle geometry is absolutely correct. The flow I am modeling is steady state flow.

Please suggest.

Thanks-faizan
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Old   March 8, 2016, 04:44
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Could you past a few photos and print CEL from solver and show? It will shows me in more details with what you are stuggling.
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Old   March 8, 2016, 04:54
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Hi Tomson,

I have just uploaded the pics of mach no, velocity and mesh from a nbackup of crashed solver file.

The message in solver before crashing is given below:

================================================== ====================
OUTER LOOP ITERATION = 233 ( 17) CPU SECONDS = 4.458E+05 (2.782E+04)
----------------------------------------------------------------------
| Equation | Rate | RMS Res | Max Res | Linear Solution |
+----------------------+------+---------+---------+------------------+
| U-Mom | 0.83 | 6.0E-04 | 1.4E-01 | 1.9E-03 OK|
| V-Mom | 0.88 | 6.0E-04 | 8.8E-02 | 1.7E-03 OK|
| W-Mom | 1.04 | 1.2E-03 | 5.1E-01 | 1.6E-03 OK|
| P-Mass | 1.02 | 4.8E-04 | 3.8E-01 | 15.1 5.0E-01 ok|
+----------------------+------+---------+---------+------------------+
| H-Energy | 0.96 | 6.3E-04 | 8.2E-01 | 1.1E-02 OK|
| T-Energy | 0.95 | 2.2E-04 | 7.0E-03 | 6.6 1.1E-02 OK|
+----------------------+------+---------+---------+------------------+
| K-TurbKE | 2.76 | 5.0E-04 | 1.7E-01 | 6.7 1.5E-03 OK|
| E-Diss.K | 1.75 | 4.9E-04 | 1.7E-01 | 12.4 7.9E-05 OK|
+----------------------+------+---------+---------+------------------+
| HELIUM HPGas Materia | 0.00 | 0.0E+00 | 0.0E+00 | 6.7 0.0E+00 OK|
| NITROGEN HPGas Mater | 0.86 | 1.2E-03 | 5.0E-01 | 6.7 1.6E-02 OK|
+----------------------+------+---------+---------+------------------+
----------------------------------------------------------------------
| Equation | Rate | RMS Res | Max Res | Linear Solution |
+----------------------+------+---------+---------+------------------+
| P-Mass |99.99 | 1.0E-01 | 8.2E-02 | 15.1 5.3E-02 OK|
+----------------------+------+---------+---------+------------------+
+--------------------------------------------------------------------+
| Notice: The maximum Mach number is 4.589E+01. |
+--------------------------------------------------------------------+

================================================== ====================
OUTER LOOP ITERATION = 234 ( 18) CPU SECONDS = 4.475E+05 (2.944E+04)
----------------------------------------------------------------------
| Equation | Rate | RMS Res | Max Res | Linear Solution |
+----------------------+------+---------+---------+------------------+

+--------------------------------------------------------------------+
| ERROR #004100018 has occurred in subroutine FINMES. |
| Message: |
| Fatal overflow in linear solver. |
+--------------------------------------------------------------------+

+--------------------------------------------------------------------+
| An error has occurred in cfx5solve: |
| |
| The ANSYS CFX solver exited with return code 1. No results file |
| has been created. |
+--------------------------------------------------------------------+

End of solution stage.

+--------------------------------------------------------------------+
| The following transient and backup files written by the ANSYS CFX |
| solver have been saved in the directory |
| M:\Multicomponent_Models\T550P14_MC_PIV_N2_MESH2_B _070316\T550P14- |
| _MC_PIV_N2_MESH2_B_070316_003: |
| |
| 232_full.bak |
+--------------------------------------------------------------------+


+--------------------------------------------------------------------+
| The following user files have been saved in the directory |
| M:\Multicomponent_Models\T550P14_MC_PIV_N2_MESH2_B _070316\T550P14- |
| _MC_PIV_N2_MESH2_B_070316_003: |
| |
| PT1.trk, solver.setup, mon |
+--------------------------------------------------------------------+


This run of the ANSYS CFX Solver has finished.


Please suggest.

thanks
Attached Images
File Type: jpg Mach No.jpg (30.5 KB, 25 views)
File Type: jpg Mesh.jpg (97.6 KB, 23 views)
File Type: jpg Velocity1.jpg (44.9 KB, 20 views)
File Type: jpg Vel2.jpg (34.2 KB, 21 views)
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Old   March 8, 2016, 05:09
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Ok, I saw that your grid looks good. You've got the problem with "Overflow". I had this problem few times. If it appeared, I change my turbulence model to SST and in the most cases it works.
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Old   March 8, 2016, 16:22
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It looks you have multiple fluids in the domain. Try to run a simple air case at first to see if the crash came from fluid material modelings. Building a nice initialization with multiple fluids would not be that easy.
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Old   March 8, 2016, 21:28
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Hi turbo,

Thanks for your suggestion. Actually I have already done the case with nitrogen only and this is my new project to deal with multicomponent fluid (Nitrogen and Air). The CFX set up is OK. I reckon the mesh I am using is not good for supersonic flow. Can you assist me to overcome this problem.

Thanks

faizan
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Old   March 9, 2016, 08:13
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If the nitrogen case was with a different mesh, you will need to run it again to check if the crash was mesh-related or material-related. Whenever you have unknown errors, the best way is to narrow down the cause for debugging.
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