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k-epsilon model doesn't shows vortex

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Old   February 26, 2013, 06:17
Default k-epsilon model doesn't shows vortex
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Md. Naimul Haque
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Dear all,

I am running bluff body simulation in OpenFoam with various turbulence models. I used both kwsst and les 2D simulation and found after 2.5 sec of simulation it starts forming vortex after the body. drag and lift values are also okay.

But with same mesh and BC, for k-epsiln and k-w turbulence model, I don't know why but it doesn't shows any vortex. I run the simulations for long time, it didn't produce any vortexes after the body.

Does any body have any idea why is it happening? Please help me, your suggesting will expedite my research and I will be grateful to you.

Sincerely,
Naju.
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Old   February 26, 2013, 10:21
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Lieven
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Hi Naju,

It makes perfectly sense that you don't find vortices in the wake of the body when using the k-e and k-w model. With RANS, the effect of such vortices on the flow is fully modelled and translated into an additional turbulent viscosity term in the momentum equation (= additional momentum dissipation). Because of this term, all vortices you would expect are (should be) damped out.
This might sound strange but this in fact the whole purpose of RANS: you model the fluctuating component of the flow, regardless the scale of the fluctuations, and solve for the reynolds averaged components. So the result you get is (an approximation of) the time-averaged flow field.

In LES on the other hand, only the vortices/fluctuations below grid scale are modelled. Therefore, the added viscosity in the momentum equation is relatively small and does not damp out all vortices, hence you find a transient solution.

So first, based on this explanation, and in my modest opinion, it is not fully correct that you find vortices in case of k-w sst.
And second, 2D-LES does not make sense. Turbulence is by definition a 3D effect so I wouldn't trust those results...

Cheers,


L
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Old   February 26, 2013, 21:55
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Md. Naimul Haque
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Dear Lieven,

Greetings!

Thank you very much for your suggestion. Actually, I am new user of OpenFOAM. Also my theoretical background is not so strong. I am studying all the basic theories. By this time I need your help. I read the code in OpenFOam of all these three models,

In kepsioln model eddy viscosity is modeled from cmu, k and, epsilon; in kw, eddy viscosity is modeled from, k and w; in kwsst, eddy viscosity is modeled from a1, k, and gradient(U).

But k, epsilon and w are calculated from mesh or somehow they are calculating, so I can't modify. Only way is to modify coefficient, cmu, a1, alphaK, alphaw, betaStar or sigmaEps etc.

Now, What is your suggestion, Do I need to change the mesh or coefficients to get the vortex? In case coefficient, which parameter should I modify?

Sorry to bother you.

Please, help.
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Old   March 9, 2013, 02:52
Default mesh problem
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Baek, Dong Hae
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i think you should make your mesh more fine.

i had same problem several time.
I used pimplefoam and RANS(k-epsilon model).
before using RANS, I tried running case using LES(smagorisky) and it show me vortex.
so I modified this case to RANS in same mesh.
but the case calculated by RANS didn't show me vortex.
then I made mesh more fine in same condition (density of mesh was almost 2times).
finally, RANS model show me vortex.
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Old   March 9, 2013, 14:41
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Lieven
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The fact that the vortices appear is most likely due to unphysical wiggles caused by the discretization scheme. So they are not an accurate representation of the real vortices but the result of numerical instability because of large gradients near your object. You can simply test this by using an upwind scheme for all parameters in fvSchemes. I'm pretty this will make your vortices disappear, regardless the grid resolution.

You should use higher order schemes which can cope with this effect, such as Gamma or SFCD (not claiming that these are the best options, nor that the effect would disappear completely). And again, if you are simulating a steady situation with RANS, e.g. the flow through a channel with objects present, your result should be steady as well.

Cheers,

Lieven
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