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-   -   How to run a steady state case in OpenFOAM (http://www.cfd-online.com/Forums/openfoam/104837-how-run-steady-state-case-openfoam.html)

Vignesh V July 17, 2012 01:06

How to run a steady state case in OpenFOAM
 
can anyone tell me how to run a steady state case in openFOAM.

I'm a new user of openFoam.

Bajji July 17, 2012 01:59

More Details.
 
Hi,
People will be able to help you if you provide a bit more details and be specific in your question.

Vignesh V July 17, 2012 04:07

Staedy State case
 
I'm trying to find a steady state flow ventilation of a parking basement.

But we need to specify the timestep and start and end time in controldict. This shows a results in unsteady state.

can anyone help in running a steady state solution.

sharonyue December 23, 2012 10:55

Quote:

Originally Posted by Vignesh V (Post 371807)
I'm trying to find a steady state flow ventilation of a parking basement.

But we need to specify the timestep and start and end time in controldict. This shows a results in unsteady state.

can anyone help in running a steady state solution.

It depends on which solver u use. for example. simple is steady,interFoam is unsteady.

Lieven December 23, 2012 11:38

Most obvious choice would be to take a steady state solver such as simpleFoam. But you can also use a transient solver like pisoFoam or pimpleFoam in combination with false time stepping (global or local).

Don't forget that it is the turbulence modelling which determines whether a case is steady or not (so not the choice of solver):
1. LES = by definition unsteady
2. RANS = depends on the boundary conditions
a. constant boundary conditions = steady flow
b. time-dependent boundary conditions = unsteady flow
So only with RANS + constant BC you will be able to obtain a steady solution.

You will obtain exactly the same solution with a steady or a transient solvers, but my experience is that a transient solver (piso/pimpleFoam) with local time stepping (localEuler) results in the fastest convergence...

Kind regards,


L

sharonyue December 23, 2012 11:48

Quote:

Originally Posted by Lieven (Post 398787)
Most obvious choice would be to take a steady state solver such as simpleFoam. But you can also use a transient solver like pisoFoam or pimpleFoam in combination with false time stepping (global or local).

Don't forget that it is the turbulence modelling which determines whether a case is steady or not (so not the choice of solver):
1. LES = by definition unsteady
2. RANS = depends on the boundary conditions
a. constant boundary conditions = steady flow
b. time-dependent boundary conditions = unsteady flow
So only with RANS + constant BC you will be able to obtain a steady solution.

You will obtain exactly the same solution with a steady or a transient solvers, but my experience is that a transient solver (piso/pimpleFoam) with local time stepping (localEuler) results in the fastest convergence...

Kind regards,


L

Thanks a lot,dear Lieven.

but if I switch of the tubulence via SimpleFoam. What does this mean... It should be steady state. but I set it is laminer..

and I find in my case that the velocity fields changes depends on time even many iterations. I think this is not normal in steady state. I dont know where Im wrong about the concept. any hints please? thank in advance.~

wouter December 23, 2012 16:54

hello,
look at this forum item:
http://www.cfd-online.com/Forums/ope...-unsteady.html

maybe that answers your question.
best
Wouter

Lieven December 24, 2012 06:40

SimpleFoam doesn't include the time derivative in the momentum equation so what you see is not a time dependence simply because the time variable is not treated. It's simply the solver trying to find a solution to the problem. You can monitor the residuals and if they go down smoothly (without peaks) you are converging towards a steady solution.
Have a look at http://www.cfd-online.com/Forums/ope...tml#post398119 to see how to do this.

I'm pretty sure however that it will be very difficult to obtain a steady solution when you treat the flow as laminar because you won't have a lot of momentum dissipation. So even a small fluctuation can prevent you from reaching a steady solution.
I would therefore recommend you to turn on the turbulence and use a RANS model. This way the modeled turbulence appears as an additional diffusive flux in the momentum equation. The result is that the time scale of any resulting unsteadyness is drastically increased and in most cases a steady solution can be obtained this way.

Regards,


L

sharonyue December 24, 2012 11:52

Quote:

Originally Posted by Lieven (Post 398886)
I'm pretty sure however that it will be very difficult to obtain a steady solution when you treat the flow as laminar because you won't have a lot of momentum dissipation. So even a small fluctuation can prevent you from reaching a steady solution.


Regards,


L

Thanks Lieven, sorry about this thing. now I am more confused. AFAIK, icoFoam is an unsteady solver for laminar flow of Newtonian fluids. but according to what you have said. that seems a little contradiction?

Lieven December 26, 2012 06:29

Hi Sharonyue,

I don't really see the contradiction. icoFoam is a transient solver. The contradiction would be if icoFoam were a steady state solver for laminar flow of Newtonian fluids...
I hope this makes it a bit more clear.

Kind regards,

L

sharonyue December 26, 2012 06:31

Quote:

Originally Posted by Lieven (Post 399058)
Hi Sharonyue,

I don't really see the contradiction. icoFoam is a transient solver. The contradiction would be if icoFoam were a steady state solver for laminar flow of Newtonian fluids...
I hope this makes it a bit more clear.

Kind regards,

L

Woo,its my fault....I must be dizzy that time.~ Thanks so much!


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