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Plotting of normal stresses from k-e model

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Old   July 13, 2015, 12:55
Default Plotting of normal stresses from k-e model
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i would like to ask as to how to plot the normal stresses for the k-epsilon model in fluent? I am assuming that the flow is isotropic and also that i will have to use the boussinesq relationship. However im not sure as to how to actually calculate them.
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Old   July 15, 2015, 06:31
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hi,as far i know stresses are modeled in k-e model , stresses are available in RSM
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Old   July 15, 2015, 11:39
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I'm also trying to do the same calculation but haven't come up with a solution yet.

Note that the stresses are not isotropic, only the turbulent viscosity is isotropic. It's incorrect to say the turbulence is isotropic (since u' is not the same as v' and w'). This is very important point.

I think you would have to compute the stresses from the boussinesq model from the shear and strain. But again I haven't done this part yet. Maybe in a few days or if someone else is able to post the solution.

Last edited by LuckyTran; August 3, 2015 at 17:38.
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Old   August 14, 2015, 06:11
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Quote:
Originally Posted by LuckyTran View Post
I'm also trying to do the same calculation but haven't come up with a solution yet.

Note that the stresses are not isotropic, only the turbulent viscosity is isotropic. It's incorrect to say the turbulence is isotropic (since u' is not the same as v' and w'). This is very important point.

I think you would have to compute the stresses from the boussinesq model from the shear and strain. But again I haven't done this part yet. Maybe in a few days or if someone else is able to post the solution.
Did you manage to find a solution? In the end I just calculated the u stress and assumed that the v and w were the same due to the fact that the standard k-e model has this assumption inherent within it i.e: from the boussinesq as shown, the former part is cancelled due to the assumption of isotropic nature of the flow.

I may have completely misunderstood everything and would be more than happy to be corrected.
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Old   August 14, 2015, 17:18
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Quote:
Originally Posted by Bubblyflow View Post
Did you manage to find a solution? In the end I just calculated the u stress and assumed that the v and w were the same due to the fact that the standard k-e model has this assumption inherent within it i.e: from the boussinesq as shown, the former part is cancelled due to the assumption of isotropic nature of the flow.

I may have completely misunderstood everything and would be more than happy to be corrected.
I just did it earlier this week. =)

I ended up using custom field functions. The eddy viscosity hypothesis assumes that the turbulent viscosity is isotropic. It does not assume that the flow is isotropic or that the uu,vv,ww stresses equal. This is a common misconception. The assumption is that the functional relation between the strain and reynolds stresses is isotropic, whereas both the strain and reynolds stresses can be anisotropic.

If you load the attached file, plz be careful as these are the proper stresses with density included. I usually compute only uu,vv,ww without the density.
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