[ghorrocks]

FAQ: http://www.cfd-online.com/Wiki/Ansys...gence_criteria

[hwangpo]

Thanks for your reply

[Vishnu_bharathi]

Originally Posted by ghorrocks

Yes, the solid time scale factor is only applicable to steady state runs. In transient runs you have to model it with the physical time step the same in both regions.

Shoushou:
I mean set a solid time scale factor in the range of 100-1000.

You do not need to change the time step during a run. You can start it with this high factor.

For CHT simulations I recommend using the residuals AND imbalances as convergence tolerances. Do a sensitivity study to check your settings are OK and then use that setting on all your runs.

Setting time step size: Just try it out and see if it works. It is goes slow then make it bigger, If it diverges then make it smaller.

I write in this thread as it is more related in what I am doing. I am doing a transient heat transfer simulation. I have the simulation run as charging and discharging cycles. regarding solver convergence: I checked residuals and Imblances in fluid and solid region. The residual graph looks good until 350(iterations) and after that it started increasing as shown.. Why is this?

The global imblances are with in 1% . I checked the yplus value near the fluid and solid interface and it is 30+ for 70% of the area and 80+ for 30% of the area. I use SST model.

What can be the reason for this residual graph shoot up after 350 iterations?

[JuPa]

Without knowing what you're simulating it's hard to give a proper answer.

It could be due to a poor mesh, poor choice of model, both, or something else completely.

To me it might look like the simulation is resolving some evolving flow physics (maybe vorticies which grow as a function of time?).

Start a new thread and give a detailed description of what you're doing.

Also I'm sure Ghorrocks will be around to tell you to read the FAQ - so do that too.

[ghorrocks]

Sadly, I don't think an FAQ covers this one

I agree with Mr CFD's comments: First check for numerical problems (mesh, models), and if they look OK check for transient flow starting up, such as vortex shedding or gross flow instabilities.

[Vishnu_bharathi]

Originally Posted by JuPa

Without knowing what you're simulating it's hard to give a proper answer.

It could be due to a poor mesh, poor choice of model, both, or something else completely.

To me it might look like the simulation is resolving some evolving flow physics (maybe vorticies which grow as a function of time?).

Start a new thread and give a detailed description of what you're doing.

Also I'm sure Ghorrocks will be around to tell you to read the FAQ - so do that too.

Thank you. I m starting a new thread

[Vishnu_bharathi]

Originally Posted by ghorrocks

Sadly, I don't think an FAQ covers this one

I agree with Mr CFD's comments: First check for numerical problems (mesh, models), and if they look OK check for transient flow starting up, such as vortex shedding or gross flow instabilities.

okay,Ghorrocks. I ll try to create new thread with more details on this regard.

[gikamc]

Hello! Thank you all for this useful thread, it helps a lot in CHT related topics.



I am trying to simulate a multistage axial turbine throw CFX to account later for the temperature field on turbine blades by thermal analysis.



To succeed more acqurate results i implemented in my model Conjugate Heat Transfer with a loose couple (different mesh between fluid region (TurboGrid) and solid region (Ansys Meshing) through CFX.



When i applied CFX to only the second rotor stage (to check if it works fine with my settings (interfaces etc.)) it results with good convergence both from residuals side and global imbalance as well.



However, when i tried to add mesh for another stage then RMS-T Energy for the previous stage is diverging like in the attached image.


Timescale for fluid domain is set as 1/omega through an expression (as suggested by CFX-manual)



I used different Timescales for the solid domains like 1000, 100, 10 and 1s but it tends to give similar results in all cases.


I would be grateful if you could make me aware of what is the possible cause of that.



Again, Thank you!


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