Unusual Asymmetry in results

OMJT · January 17, 2013, 12:50

Hi everyone,

I have a slight problem. I am using Fluent14 to attempt to model high Re Cylinder Flow in 2D and see if it gives remotely accurate results. There is however something very odd happening with the Lift Coefficient.

The lift coefficient oscillates as it should do with a reasonable period however it does not oscillate around 0 despite the mesh being symmetrical and the flow in being uniform. The mesh is structured and uses 2 Ogrids around each other to mesh the cylinder itself. Around this is H grid. The Cylinder is relatively very small compared the flow domain so edge conditions should not have any effect on the Lift Coefficient.

Any help with this would be much a appreciated.

Thanks,

Matt

cfd seeker · January 17, 2013, 14:03

You are performing unstaedy simulation? the amplitude of oscillations is small or large?

OMJT · January 19, 2013, 20:55

Sorry for the late response. I am not sure what you mean by are the oscillations large or small? Relative to what?

It is flow around a cylinder at Re ~ 10^7. So there should be vortices formed around the same diameter as the cylinder. The vortices produced in the simulation however were smaller and the lift force was of relatively small amplitude. This is a turbulent viscocity limit issue. The real oddity is the non-zero mean of the lift force on the cylinder. Could this be to do with the amplitude of the osciallation?

Thanks,

Matt

January 17, 2013, 12:50	Unusual Asymmetry in results	#1
OMJT New Member Join Date: Jan 2013 Posts: 10 Rep Power: 13	Hi everyone, I have a slight problem. I am using Fluent14 to attempt to model high Re Cylinder Flow in 2D and see if it gives remotely accurate results. There is however something very odd happening with the Lift Coefficient. The lift coefficient oscillates as it should do with a reasonable period however it does not oscillate around 0 despite the mesh being symmetrical and the flow in being uniform. The mesh is structured and uses 2 Ogrids around each other to mesh the cylinder itself. Around this is H grid. The Cylinder is relatively very small compared the flow domain so edge conditions should not have any effect on the Lift Coefficient. Any help with this would be much a appreciated. Thanks, Matt

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January 17, 2013, 14:03		#2
cfd seeker Senior Member Join Date: Mar 2011 Location: Germany Posts: 552 Rep Power: 20	You are performing unstaedy simulation? the amplitude of oscillations is small or large?

January 19, 2013, 20:55		#3
OMJT New Member Join Date: Jan 2013 Posts: 10 Rep Power: 13	Sorry for the late response. I am not sure what you mean by are the oscillations large or small? Relative to what? It is flow around a cylinder at Re ~ 10^7. So there should be vortices formed around the same diameter as the cylinder. The vortices produced in the simulation however were smaller and the lift force was of relatively small amplitude. This is a turbulent viscocity limit issue. The real oddity is the non-zero mean of the lift force on the cylinder. Could this be to do with the amplitude of the osciallation? Thanks, Matt