High strain rate at the walls, keeps changing with finer mesh

1995robert · August 19, 2019, 02:28

Dear,

Currently I am modelling a viscous fluid that flows between two disks. I do this in a 2D, axissymmetric way. The upper wall is the upper disk and the lower wall is the lower disk.

The upper disk is rotating at around 300 RPM. The simulation works well now and the pressure and shear stresses look as expected. The only problem that I have is that the strain rate at the wall is very high and keeps changing, when I refine the mesh (using finer inflation layers).

Also when I plot the strain rate from the upper disk downward to the lower disk, it can be seen that there are peaks in the strain rate, when it reaches the wall.

Does anyone knows why this is happening and how I can get a stable simulation?

Kind regards,
Robert

MIDHUN@CFD · September 2, 2019, 12:52

do u simply meshing or check for wall y plus and quality? pls dont cross max aspect ratio of 600

1995robert · September 2, 2019, 15:15

Thank you very much for your reply!

I have figured it out! The problem was that I model slip. I use shear wall in Fluent, which normally do not really have a velocity themselves, but I do use moving walls in a pre-simulation. This causes these walls to have some kind of velocity. The material that I model has a certain velocity, but this is only a percentage of the initial wall velocity. The difference between the velocity of the wall and the material that is slipping causes the high strain-rates. This is physically not really correct as the material is slipping and therefore does not really shear at the wall.

The solution is to just not take these first layers into account.

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September 2, 2019, 12:52		#2
MIDHUN@CFD Member MIDHUN Join Date: Nov 2012 Location: calicut Posts: 46 Rep Power: 13	do u simply meshing or check for wall y plus and quality? pls dont cross max aspect ratio of 600

September 2, 2019, 15:15		#3
1995robert New Member Robert Join Date: Jan 2019 Posts: 3 Rep Power: 7	Thank you very much for your reply! I have figured it out! The problem was that I model slip. I use shear wall in Fluent, which normally do not really have a velocity themselves, but I do use moving walls in a pre-simulation. This causes these walls to have some kind of velocity. The material that I model has a certain velocity, but this is only a percentage of the initial wall velocity. The difference between the velocity of the wall and the material that is slipping causes the high strain-rates. This is physically not really correct as the material is slipping and therefore does not really shear at the wall. The solution is to just not take these first layers into account.