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omidiut July 2, 2011 03:27

Turbulence modeling
 
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

Far July 2, 2011 05:31

use the gamma theta model

ghorrocks July 2, 2011 06:51

Could be dozens of things, not just the turbulence model. Read this FAQ:

http://www.cfd-online.com/Wiki/Ansys..._inaccurate.3F

mohammad July 4, 2011 02:37

Quote:

Originally Posted by omidiut (Post 314465)
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:)

omidiut July 4, 2011 09:53

Quote:

Originally Posted by mohammad (Post 314565)
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?

omidiut July 4, 2011 09:59

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?

mohammad July 4, 2011 09:59

Quote:

Originally Posted by omidiut (Post 314644)
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


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