CFD Online Logo CFD Online URL
www.cfd-online.com
[Sponsors]
Home > Forums > Software User Forums > ANSYS > CFX

CFX CHT transient simulation: T-Energy (Solid Doamin) Imbalance

Register Blogs Members List Search Today's Posts Mark Forums Read

Reply
 
LinkBack Thread Tools Search this Thread Display Modes
Old   October 28, 2019, 09:31
Default CFX CHT transient simulation: T-Energy (Solid Doamin) Imbalance
  #1
New Member
 
Aleksei
Join Date: Jun 2018
Posts: 24
Rep Power: 6
alexksei is on a distinguished road
Hello everyone

It seems such type of question has been asked many times here, but I still can't find proper answer.

I have: fluid domain, solid domain inside fluid domain, the solid is hot, the fluid is cool. Natural convetion occurs. I run transient simulation, because steady-state solution for natural convection does not give proper residuals.
So, in transient simulations I have proper residuals: 10^-5 and 10^-6 for RMS-Mom and Heat Transfer. Imbalance for H-Energy is good (0%), but IMBALANCE IS LARGE up to 20% for T-Energy (heat conduction is solid domain).

For steady state they recommend increasing Solid Time Scale factor, but what should I do for transient?

Total time is 30s, time step is 0.1s
alexksei is offline   Reply With Quote

Old   October 28, 2019, 17:07
Default
  #2
Super Moderator
 
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 17,269
Rep Power: 136
ghorrocks is just really niceghorrocks is just really niceghorrocks is just really niceghorrocks is just really nice
You cannot use acceleration factors like solid time scale factor in transient runs. Your large imbalance means you are not converging well each time step, so I would recommend adding imbalances as a convergence criteria. This means you will do more iterations per time step.
__________________
Note: I do not answer CFD questions by PM. CFD questions should be posted on the forum.
ghorrocks is online now   Reply With Quote

Old   October 29, 2019, 03:11
Default
  #3
New Member
 
Aleksei
Join Date: Jun 2018
Posts: 24
Rep Power: 6
alexksei is on a distinguished road
Thank you, Glenn, for your reply.

If you mean setting convergence criteria in Solver Control, then I am already using Conservation Target of 1%, but, as I wrote, imbalance for T-Energy is up to 20%.

After I'd posted question here I've found TimeScale Factor Control in Domain Control for solid domain. Making it 1000 gave me 4% imbalance for T-Energy (instead of 20%). Should I consider increasing this parameter or you think for transient it is not appropriate?

I actually don't understand why such TimeScale control can be appled for transient, as timestep is specified explicitly in Analisys Type.
alexksei is offline   Reply With Quote

Old   October 29, 2019, 05:43
Default
  #4
Super Moderator
 
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 17,269
Rep Power: 136
ghorrocks is just really niceghorrocks is just really niceghorrocks is just really niceghorrocks is just really nice
If you set a conservation target of 1% but it is only getting to 20% then you must have some other factor stopping it. Have you set a maximum number of iterations? You will need to increase it - but a preferable path forwards is likely to be to reduce the time step so it converges easier. Either way, you need a good imbalance convergence, and if you are not getting it you need to fix the simulation so you achieve it.

I am not sure what the time scale factor does in this case, so read the documentation and do some testing to confirm it does not affect results before using it for important simulations.
__________________
Note: I do not answer CFD questions by PM. CFD questions should be posted on the forum.
ghorrocks is online now   Reply With Quote

Old   October 29, 2019, 09:29
Default
  #5
New Member
 
Aleksei
Join Date: Jun 2018
Posts: 24
Rep Power: 6
alexksei is on a distinguished road
Quote:
Originally Posted by ghorrocks View Post
If you set a conservation target of 1% but it is only getting to 20% then you must have some other factor stopping it. Have you set a maximum number of iterations? You will need to increase it - but a preferable path forwards is likely to be to reduce the time step so it converges easier. Either way, you need a good imbalance convergence, and if you are not getting it you need to fix the simulation so you achieve it.

I am not sure what the time scale factor does in this case, so read the documentation and do some testing to confirm it does not affect results before using it for important simulations.
I don't set maximum number of iterations. For transient analysis what I specify is the total simulation time and time step delta t.

Let's clarify one thing: for transient simulation the imbalance must be close to zero at every time step (as RMS should have converged values at each time step), is it correct? Or only the value of imbalance at the very last timestep shoud be considered?

As for the domain timescale factor for solid domain: I have found it in official CFX tutorial named "Time Transformation Method for a Transient Rotor-stator Case with Conjugate Heat Transfer", in which they consider rotor blades with conjugate heat transfer. This is what they wrote for blades:
"Increasing the timescale factor to 1.0e6 from the default 1 increases the physical time scale of this transient run inside the solid domain by a factor of 1.0e6"

I did not find anything else related to this in the documentation. Only for steady-state there is smth.

Last edited by alexksei; October 29, 2019 at 10:34.
alexksei is offline   Reply With Quote

Old   October 29, 2019, 09:56
Default
  #6
Senior Member
 
Join Date: Jun 2009
Posts: 1,644
Rep Power: 29
Opaque will become famous soon enough
If I am not mistaken, Glenn was referring to the maximum number of coefficient loops within a given timestep.

If you set a MaxCoefLoop to say 5, and your imbalance is set to 1%, the software will finish the timestep at 5 inner iterations regardless of the imbalance value at that iteration/loop. If the imbalance is critical for you, you must set a higher number for the maximum number of coefficient loops.

Just try some settings, and you can see how it behaves for a few timesteps. Once you understand how it works, and what you need of the solution, go ahead and do the full calculation.
Opaque is offline   Reply With Quote

Reply

Tags
cfx, cht, imbalance, solid

Thread Tools Search this Thread
Search this Thread:

Advanced Search
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are On
Refbacks are On


Similar Threads
Thread Thread Starter Forum Replies Last Post
Transient Simulation in CFX ashtonJ CFX 1 September 28, 2018 02:18
a transient cfx simulation suddenly stopped writing .out and then .bak while running mona.li CFX 1 March 5, 2018 05:15
Transient simulation CFX - BC setup Jared1986 CFX 8 June 15, 2017 06:52
Simulation FPEs - turbulence for transient and steady-state? DaveR OpenFOAM Running, Solving & CFD 5 March 5, 2017 16:06
ATTENTION! Reliability problems in CFX 5.7 Joseph CFX 14 April 20, 2010 16:45


All times are GMT -4. The time now is 05:56.