# Alpha in IDDES

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 December 3, 2010, 12:10 Alpha in IDDES #1 New Member   James Join Date: Jan 2010 Posts: 6 Rep Power: 7 Hello, The way alpha is calculated in IDDES seems slightly wrong. It should be - alpha = 0.25 -dw/hmax where hmax is a local cell maximum length e.g. max(dx,dy,dz). In the source however I think it is calculating the max cell length in the entire mesh and hence not using a local max. Currently its - tmp SpalartAllmarasIDDES::alpha() const { return max ( 0.25 - y_/dimensionedScalar("hMax", dimLength, max(cmptMax(delta()))), scalar(-5) ); } Am I right in thinking this returns a mesh-wide maximum? what exactly does the cmptMax() do? How can I change it to find the local max, or use the deltamax calculated in IDDESDelta.c ? Thanks James

July 3, 2012, 05:44
#2
Senior Member

Anne Gerdes
Join Date: Aug 2010
Location: Hamburg
Posts: 152
Rep Power: 6
Hey James,
Quote:
 Originally Posted by tookey_1989 Hello, The way alpha is calculated in IDDES seems slightly wrong. It should be - alpha = 0.25 -dw/hmax where hmax is a local cell maximum length e.g. max(dx,dy,dz). In the source however I think it is calculating the max cell length in the entire mesh and hence not using a local max. Currently its - tmp SpalartAllmarasIDDES::alpha() const { return max ( 0.25 - y_/dimensionedScalar("hMax", dimLength, max(cmptMax(delta()))), scalar(-5) ); } Am I right in thinking this returns a mesh-wide maximum?
Yes, you are right. The implementation is wrong, as it returns a global and not a local maximum. It should be fixed in OF-2.1.1, see also
Anyone using IDDES

Quote:
 Originally Posted by tookey_1989 How can I change it to find the local max, or use the deltamax calculated in IDDESDelta.c ?
See the newest version of OpenFOAM, currently OF-2.1.1

Regards
Anne

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