# Turbulence modeling

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 July 2, 2011, 03:27 Turbulence modeling #1 Senior Member   omid Join Date: Apr 2011 Posts: 102 Rep Power: 8 Hi all I'm simulating flow around sphere at Re=1E05 with DES(Detached Eddy Simulation) turbulence model. the Y plus=0.2 and time step size=0.008. The mean drag coefficient is 0.27 and there is large error with experiment data that say Cd=0.42. whats wrong in my simulation? Best regards. omid

 July 2, 2011, 05:31 #2 Super Moderator   Sijal Join Date: Mar 2009 Location: Islamabad Posts: 4,358 Blog Entries: 6 Rep Power: 45 use the gamma theta model

 July 2, 2011, 06:51 #3 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 14,091 Rep Power: 109 Could be dozens of things, not just the turbulence model. Read this FAQ: http://www.cfd-online.com/Wiki/Ansys..._inaccurate.3F

July 4, 2011, 02:37
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 Originally Posted by omidiut Hi all Re=1E05 with DES(Detached Eddy Simulation) turbulence model. the Y plus=0.2 and time step size=0.008. Best regards. omid
Hi.
Try to solve it with the timestep of order of 1e-4... it may work..
mesh density along the chord and in downsteam also are important.

good luck

July 4, 2011, 09:53
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omid
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Quote:
 Originally Posted by mohammad Hi. Try to solve it with the timestep of order of 1e-4... it may work.. mesh density along the chord and in downsteam also are important. good luck

Hi
Thanks
but its very small! i use timestep value equal 0.008 but the Drag be constant, i think with smaller timestep this problem repeat, whats your idea?

 July 4, 2011, 09:59 #6 Senior Member   omid Join Date: Apr 2011 Posts: 102 Rep Power: 8 Thanks all I find something about grid resolution and surface grid spacing in ANSYS CFX- solver theory guide p.81 : DES simulation. is this problem because of grid induced separation?

July 4, 2011, 09:59
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 Originally Posted by omidiut Hi Thanks but its very small! i use timestep value equal 0.008 but the Drag be constant, i think with smaller timestep this problem repeat, whats your idea?
Hi, about the DES timestep I'm sure that it must be small.+ number of nodes in chordwise and Y+ value are important..you can see teh CFX 11 help manual about DES .

regards

 November 28, 2016, 11:37 #8 New Member   Join Date: Jun 2016 Posts: 4 Rep Power: 3 I am having a similar issue. did you ever get to the bottom of this?

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