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 )

Moderator: oliveroxtoby

thschrader
Posts: 1403
Joined: Sat May 20, 2017 12:06 pm
Location: Germany

Re: Transient or steady state CFD problem

Postby thschrader » Sun Dec 23, 2018 12:10 am

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.
thschrader
Posts: 1403
Joined: Sat May 20, 2017 12:06 pm
Location: Germany

Re: Transient or steady state CFD problem

Postby thschrader » Sun Dec 23, 2018 10:09 am

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.
bad.JPG
bad.JPG (16.65 KiB) Viewed 502 times
good.JPG
good.JPG (13.59 KiB) Viewed 502 times
User avatar
oliveroxtoby
Posts: 279
Joined: Fri Dec 23, 2016 9:43 am
Location: South Africa

Re: Transient or steady state CFD problem

Postby oliveroxtoby » Sun Dec 23, 2018 11:19 am

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.
julieng
Posts: 40
Joined: Sun Nov 25, 2018 8:57 pm

Re: Transient or steady state CFD problem

Postby julieng » 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
thschrader
Posts: 1403
Joined: Sat May 20, 2017 12:06 pm
Location: Germany

Re: Transient or steady state CFD problem

Postby thschrader » Tue Dec 25, 2018 11:14 am

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.
User avatar
oliveroxtoby
Posts: 279
Joined: Fri Dec 23, 2016 9:43 am
Location: South Africa

Re: Transient or steady state CFD problem

Postby oliveroxtoby » Tue Dec 25, 2018 6:30 pm

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.
julieng
Posts: 40
Joined: Sun Nov 25, 2018 8:57 pm

Re: Transient or steady state CFD problem

Postby julieng » Fri Dec 28, 2018 3:41 pm

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

Best regards
User avatar
oliveroxtoby
Posts: 279
Joined: Fri Dec 23, 2016 9:43 am
Location: South Africa

Re: Transient or steady state CFD problem

Postby oliveroxtoby » Sat Dec 29, 2018 12:14 pm

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.
julieng
Posts: 40
Joined: Sun Nov 25, 2018 8:57 pm

Re: Transient or steady state CFD problem

Postby julieng » Tue Jan 01, 2019 8:18 pm

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
thschrader
Posts: 1403
Joined: Sat May 20, 2017 12:06 pm
Location: Germany

Re: Transient or steady state CFD problem

Postby thschrader » Fri Jan 04, 2019 10:27 pm

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.