Transient or steady state CFD problem

A subforum specific to the development of the OpenFoam-based workbenches ( Cfd https://github.com/qingfengxia/Cfd and CfdOF https://github.com/jaheyns/CfdOF )

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thschrader
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Re: Transient or steady state CFD problem

Post by thschrader »

julieng wrote: Sat Dec 22, 2018 11:22 pm Thschrader, do you tried my 2D laminar case?
Sorry, I forgot to mention.
Some hints:
with the given parameters v=5 mm/s inlet speed and 100 mm cylinder diameter
you get a vortex frequency of only 0,01 Hz (see definition of Strouhal-Number).
That means: to see one vortex, the simulation time must be at minimum 100 sec.
I increased the speed a little to get 1 Hz, but as I said above, pimplefoam crashed.
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Re: Transient or steady state CFD problem

Post by thschrader »

Same problem as in
https://forum.freecadweb.org/viewtopic.php?f=37&t=32963
After writing the case, edit the fvSchemes-dict an replace
"steadyState" by "Euler".
@julieng: your file runs without crash, but due to the very low inlet speed,
you see no movement.
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Re: Transient or steady state CFD problem

Post by oliveroxtoby »

thschrader wrote: Sat Dec 22, 2018 9:56 pm pimplefoam crashes.
I rebuilt this tutorial
http://www.tfd.chalmers.se/~hani/kurser ... Report.pdf
but both 2D/3D cases dont work. After 30 iterations, pimplefoam stops with error.
tutorial2D.FCStd
tutorial3D.FCStd
Sorry about that. Fixed the bug.
Please provide all the information requested in this post before reporting problems with CfdOF.
julieng
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Re: Transient or steady state CFD problem

Post by julieng »

Ok. My question was to know if the residual error for the pressure was ok because during all the transient simulation p always larger than 0.1

Best regards
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Re: Transient or steady state CFD problem

Post by thschrader »

julieng wrote: Sun Dec 23, 2018 5:10 pm Ok. My question was to know if the residual error for the pressure was ok because during all the transient simulation p always larger than 0.1
On my simulations too. I am not sure how to interpret the residuals
when running a transient case (if the flow seems plausible, for me the solution is "good")
you can activate the report-view to see what the solver is doing (view/panels...).
Maybe this helps.
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Re: Transient or steady state CFD problem

Post by oliveroxtoby »

julieng wrote: Sun Dec 23, 2018 5:10 pm Ok. My question was to know if the residual error for the pressure was ok because during all the transient simulation p always larger than 0.1

Best regards
If the residuals are not reducing by a few orders of magnitude, this is probably an indication that the problem being modelled has a lot of unsteady behaviour and a time-resolved simulation (i.e., transient) may be more appropriate.
julieng wrote: Sun Dec 23, 2018 5:10 pm I don't understand when I can compute turbulent cfd problems in steady or in transient simulations ? It is impossible to know before the simulation if the flow will be steady or not.
It depends on whether you are trying to determine the time-accurate evolution of the flow field over time (transient simulation) or the steady state flow field that eventually emerges (in which case a transient simulation will get you the answer quicker). But the latter depends on whether the setup actually admits a reasonable approximation to a steady solution.
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julieng
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Re: Transient or steady state CFD problem

Post by julieng »

Hello, the residual error doesn t decrease during a transient simulation

Best regards
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Re: Transient or steady state CFD problem

Post by oliveroxtoby »

julieng wrote: Fri Dec 28, 2018 3:41 pm Hello, the residual error doesn t decrease during a transient simulation

Best regards
It won't do unless the flow reaches a steady state.
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julieng
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Re: Transient or steady state CFD problem

Post by julieng »

Hello

Can you have a look on the model please (the model is attached in a previous named flow pass a cylinder Re 05). The model cannot be more simple but I have always a very high value of the error for pressure even for very low inlet velocity.

Best regards

Happy new year
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Re: Transient or steady state CFD problem

Post by thschrader »

julieng wrote: Tue Jan 01, 2019 8:18 pm Can you have a look on the model please (the model is attached in a previous named flow pass a cylinder Re 05).
The flow past a cylinder is always instable.
I have a question to you:
Can you compute the resulting force on the body (your cylinder)?
It is only a cylinder, should be easy.
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