## 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

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

### Transient or steady state CFD problem

Dear all,

I continue my CFD learning.

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.

What should I do; try first a steady state case (play with the mesh) and if it does not converge switch to transient case ?

My next step is to compare cfdOF simulations (velocity, pressure, Drag force) with another software; and with literature experimental cases.
To acheive this, I am looking for the simplest 2D case where I can find experiments data. What can I choose, can you tell me a good example for doing this? Maybe you can share your experience on this point also.
I see the Openfoam guide and tutorials which are very helpfull also.

Best regards
Posts: 1417
Joined: Sat May 20, 2017 12:06 pm
Location: Germany

### Re: Transient or steady state CFD problem

I would choose a cylinder.
-easy to design
-2D-case possible
-different phenoma to investigate (von Karman Vortex, rapid falling drag-coeff at Re=100000 ...)
-try different meshers
Should be enough homework over christmas
What other cfd-software do you use for comparison of the results?
julieng
Posts: 40
Joined: Sun Nov 25, 2018 8:57 pm

### Re: Transient or steady state CFD problem

Yes i have a lot of homework, my boss want answers.
I compare with elmer in 2d cases

Best regards
Posts: 1417
Joined: Sat May 20, 2017 12:06 pm
Location: Germany

### Re: Transient or steady state CFD problem

yess i have a lot of homework, my boss want answers.
I compare with elmer in 2d cases

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

### Re: Transient or steady state CFD problem

Hello,

I work nights and days , give me a little bit more time please...

More seriously I try a simple 2D case of flow past a cylinder. I don't know if my the residual is normal (residual of p higher than 0.1 for all time steps)
The Reynolds number in very low 0.5.
Mesh is max 10 mm, velocity max is 5 mm/s, timestep is 0.5s Courant number should be 0.25.
See my controlDict file attached.

I don't understand if I can constraint automatically the timestep of the solver in using the line "maxCo" in the controlDict file.
Maybe if I don't specify any timestep the solver choose himself the timestep to respect maxCo?

Best regards
Attachments
controlDict.txt
julieng
Posts: 40
Joined: Sun Nov 25, 2018 8:57 pm

### Re: Transient or steady state CFD problem

I found this from oliveroxtoby:

"There are a few different ways of using PIMPLE. With a fixed time step size, it is wise to solve to a particular residual tolerance. One can use a fixed number of iterations if one keeps a limit on the max Courant number. The latter is the mode we use at the moment in CfdOF, as it is not really dependent on appropriate user input of time step size. The first option can give you faster run times, however, and should perhaps be considered."

It means that cfdOF fixe a number of iterations a timestep, no matter if the solution is converged or not and the solver continue the calculation for the next timestep? So the only important value is the maxCo?

If I leave timstep = 0, I have:

"19.9: Simulation exited with error"

Best regards
Posts: 1417
Joined: Sat May 20, 2017 12:06 pm
Location: Germany

### Re: Transient or steady state CFD problem

julieng wrote:
Sat Dec 22, 2018 7:52 am
I work nights and days , give me a little bit more time please...

OK. sorry for my impatience...
it is easier to understand your problem when you can post your FC-file.
julieng
Posts: 40
Joined: Sun Nov 25, 2018 8:57 pm

### Re: Transient or steady state CFD problem

Yes you are right

File attached

Best regards
Attachments
flow_past_cylinder_Re05.FCStd
file
Posts: 1417
Joined: Sat May 20, 2017 12:06 pm
Location: Germany

### Re: Transient or steady state CFD problem

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