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Channel395 with SpalartAllmarasIDDES u tau is too low

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Old   July 30, 2012, 09:25
Default Channel395 with SpalartAllmarasIDDES u tau is too low
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Anne Gerdes
Join Date: Aug 2010
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Dear Foamers,

I started to compute an IDDES (Improved Delayed Detached Eddy Simulation) with OpenFOAM-1.7.0
I already fixed the bugs which were in this version.

The setting I took is identical to the channel395 tutorial of OpenFOAM.
I only changed the turbulence settings, as I computed with SpalartAllmarasIDDES instead of oneEqEddy.
For delta I took delta IDDESDelta; Unfortunately I set deltacoefficient to 1, but is has to be set to 2 for a hexamesh.
I computed yPlus with yPlusLES, is this right for a DES?

The result was an y+ of 0.78. With nu=2e-5 and y(1)=0.0048 I computed an u tau of 3.25 e-3
This results in a Re_tau of 162.5 which is far too low.
In other threads I saw an u tau of 0.0079 which results in the proper Re tau of 395.

What might be the reason for this?
I started averaging after 900 s which is related to 30 flow through times. Then I computed another 20.000 time steps which results in 4000 s with a delta t of 0.2.

Is yPlusLES the correct method for computing yPlus when using an LES?
Might the delta coefficient be the reason for this low u tau?
I also wonder how u_tau can be 0.0079, with y=0.0048 and nu=2 e-5, because this results in a y+ of 1.8 >1....

Does someone have experiences with this case?
I would appreciate any help or comments.

Kind Regards
Anne
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