mesh convergence

jiko · November 24, 2010, 09:29

Hi, I am trying to simulate an oscillating GMO in still fluid. so far I 've been unable to get a mesh independence solution. I did modify the turbulence options to RNG with length scale of 7% of the turbulent region and got slightly better results but still not perfect.
any suggestion would be greatly appreciated. (do I need to switch on something in numerics)

Regards
jko

JBurnham · November 24, 2010, 14:24

Jiko - the solution is going to be mesh-dependent because the near-wall treatment of turbulent boundary effects (velocity and shear) are mesh-dependent. The cells nearest the wall or inclusive of wall apply a logarithmic-law-of-the-wall profile to iteratively determine the velocity profile. If the cells near the wall are too large, then the near wall velocity and shear will be underpredicted, and vice-versa. The FLOW-3D website has a hints&tips article and a tech-note that describe the phenomena and discuss how to account for it:

http://www.flow3d.com/resources/news...esolution.html

http://www.flow3d.com/pdfs/tn/FloSci-TN86.pdf

jiko · November 29, 2010, 06:26

Hi Jeff, Thanks allot for the tips and the related links.
there is one more thing that I would like to be commented on.
I do get a good mesh independece for a set of two meshes (say with sizes 0.005 and 0.007) now if I further reduce the cell size to 0.003 the solution, instead of a better agreement, differs (from the previous results of 0.006 and 0.008) by much greater extent.
thanks for your time.

jiko · November 29, 2010, 06:27

correction

Quote:

Originally Posted by jiko

Hi Jeff, Thanks allot for the tips and the related links.
there is one more thing that I would like to be commented on.
I do get a good mesh independece for a set of two meshes (say with sizes 0.005 and 0.007) now if I further reduce the cell size to 0.003 the solution, instead of a better agreement, differs (from the previous results of 0.005 and 0.007) by much greater extent.
thanks for your time.

MuxaB · November 30, 2010, 00:37

Hi,

What output are you using to gauge the mesh dependence?

jiko · November 30, 2010, 05:54

Hi Michael, good question! I am plotting the total x force (pressure+shear), as for now I am interested only in the x force. and GMO is moving with prescribed sinosoidal velocity in the x direction.
thanks for the consideration.
looking forward to your reply.

November 24, 2010, 09:29	mesh convergence	#1
jiko Member jiko Join Date: Sep 2010 Posts: 31 Rep Power: 16	Hi, I am trying to simulate an oscillating GMO in still fluid. so far I 've been unable to get a mesh independence solution. I did modify the turbulence options to RNG with length scale of 7% of the turbulent region and got slightly better results but still not perfect. any suggestion would be greatly appreciated. (do I need to switch on something in numerics) Regards jko

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November 24, 2010, 14:24		#2
JBurnham Senior Member Jeff Burnham Join Date: Apr 2010 Posts: 204 Rep Power: 17	Jiko - the solution is going to be mesh-dependent because the near-wall treatment of turbulent boundary effects (velocity and shear) are mesh-dependent. The cells nearest the wall or inclusive of wall apply a logarithmic-law-of-the-wall profile to iteratively determine the velocity profile. If the cells near the wall are too large, then the near wall velocity and shear will be underpredicted, and vice-versa. The FLOW-3D website has a hints&tips article and a tech-note that describe the phenomena and discuss how to account for it: http://www.flow3d.com/resources/news...esolution.html http://www.flow3d.com/pdfs/tn/FloSci-TN86.pdf

November 29, 2010, 06:26		#3
jiko Member jiko Join Date: Sep 2010 Posts: 31 Rep Power: 16	Hi Jeff, Thanks allot for the tips and the related links. there is one more thing that I would like to be commented on. I do get a good mesh independece for a set of two meshes (say with sizes 0.005 and 0.007) now if I further reduce the cell size to 0.003 the solution, instead of a better agreement, differs (from the previous results of 0.006 and 0.008) by much greater extent. thanks for your time.

November 30, 2010, 00:37		#5
MuxaB Senior Member michael barkhudarov Join Date: Mar 2009 Location: Sante Fe, New Mexico, USA Posts: 337 Rep Power: 18	Hi, What output are you using to gauge the mesh dependence?

November 30, 2010, 05:54		#6
jiko Member jiko Join Date: Sep 2010 Posts: 31 Rep Power: 16	Hi Michael, good question! I am plotting the total x force (pressure+shear), as for now I am interested only in the x force. and GMO is moving with prescribed sinosoidal velocity in the x direction. thanks for the consideration. looking forward to your reply.