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Use a turbulent solver with an uncertain turbulence flow

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Old   November 27, 2018, 16:37
Default Use a turbulent solver with an uncertain turbulence flow
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Nicola D'Ettole
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Hello,
I'm new in opefoam. I want to determine the velocity field in an internal flow, but I need to know the speed to calculate the number of Re.
The problem is that velocity changes at different points in the cavity, so the flow can be classified as turbulent (Re> 4000), transactional or laminar.
If I use a turbolent solver such as simpleFam and the real flow is laminar / transactional, are the results reliable? are they more accurate with turbulent models ?

Last edited by ing.nicola.dettole; November 28, 2018 at 04:54.
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Old   November 28, 2018, 04:25
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Robert
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Quote:
Originally Posted by RobertHB View Post
[...] Using a certain solver, says nothing about whether you simulate turbulence or not, and more about if you are able to resolve the spacial and temporal scales of your flow.
simpleFoam is a steady-state solver, which is perfectly fine for flows which do not change over time, e.g. a laminar one, or large vortexes that achieve a steady motion. If you want to resolve the turbulences of your flow dependent on time, you might want to use a transient solver like pisoFoam or pimpleFoam. Solvers are chosen in the system/fvSchemes dictionary.

Each of the three solvers can use a turbulence model to calculate key variables like the turbulent kinetic enery k or the turbulence dissipation epsilon. This is where turbulence models come into play. Turbulence models are chose in the constant/turbulenceProperties dictionary and they require you to solver for different variables dependent on the model used.

You can certainly run a simpleFoam simulation with a RANS turbulence model. No harm will come from solving a laminar flow with a turbulence model. But if you allready know that your flow will be laminar, why do so? You will solve for variables you don't need and will waste time doing so.

[...] But if your flow is turbulent and you do not use a turbulence model you will loose important information of your flow and, most likely, not see any turbulence.

If you are unsure of whether or not your flow is laminar or turbulent, use a turbulence model to begin with. Want a "quick" solution to approach the simulation of the transition of trubulent to laminar flow?

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Originally Posted by RobertHB View Post
The quickest solution: simpleFoam + RANS turbulence model. To get a first idea of your flow conditions, maybe its even enough to resolve some turbulence, but don't count on it.
Then use a pimpleFoam + RANS turbulence model. Map the velocity field, pressure and turbulence variables of your simpleFoam simulation to get a head start. Use an adjustable timestep to keep the maximum Courant number below 4.0. Give it alot of simulation runtime. Write timesteps regularily to check how and if your simulation changes from the simpleFoam solution.

*Quoted from: Turbulent and laminar conditions in one case, since you are dealing with a similar question.
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Old   November 28, 2018, 04:53
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Nicola D'Ettole
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I will try quick approach.I have to do some proofs and tests i hope this help me to performe minimal number of tests.

I think that in the my range of inlet flow the problem is laminar for a flow of 1 m^3/h and turbulent for a flow of 10m^/h.

Thank you.
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