Glitches around cylinder boundary layer

[Eric]

Greetings!

I met a problem when simulating 2D cylinder case at Re 4000. The resolution is 100 while the cylinder diameter is 1. BGK collision operator is applied. As shown in the vorticity contour,
[img]https://imgur.com/nEtRQDy[/img]

the boundary layer around the cylinder is correctly presented, with negative vorticity on the top and positive vorticity on the bottom. However, there exist some vorticity strips just off the boundary layer, resulting in these unreasonable glitches.

I also tested the same case with MRT collision operator but got same problem. I didn’t find any literature addressing this problem and would like to ask you for help.

Looking forward to hearing from you.

Best regards,
Eric

[Marc]

Hey Eric,

first guess: 100 is not a really fine resolution for a Reynoldys number of 4000.
I need some additional information to evaluate possible error sources:
– boundary condition at the cylinder
– used turbulence modell
– max Mach number

Best Marc

[Eric]

Hi Marc,

– The boundary condition around the cylinder is bouzidi boundary condition.
– Both BGK and MRT solver are applied and glitches are found in both of them.
– The charLatticeVelocity is set equal to dx = 0.01, which mean the max Mach number should be 0.01/Cs = 0.0173 if I am not mistaken.

I am now testing it with resolution 200 but I am worried that if I wanna to simulate higher Reynolds number, increasing resolution will increase computational cost to a great degree.

Best regards,
Eric

[Marc]

Hey Eric,

try to use instead of the bouzidi boundary condition the bounce back condition. Furthermore you can use a LES turbulence model SmagorinskyBGK or SmagorinskyMRT to avoid very high resolutions.

Best Marc

[Eric]

Hi Marc,

Thanks for your reply.

However, the bounce-back condition doesn’t seems to work. Are these strips around the cylinder reasonable in simulation? I mean, how do these strips and glitches arise in the vorticity contour?

I tried SmagorinskyBGK and SmagorinskyMRT as well. It seems these glitches can be tuned somewhat by adjusting Cs but they cannot be fully diminished. Also, at very high Re (50,000 in this case), there are upstream vortex formed in front of the cylinder, as can be show in the following plot:
[img]https://imgur.com/a/WSpVwEs[/img]

Best regards,
Eric

[Marc]

Hey Eric,

bounce back have to work in your simulation. If bounce back doesn’t work, your material number configuration is maybe not correct. There is no physical reason for this kind of stripes. The reason for the uptstream vortices could be a reflection of the pressure waves from the cylinder at the inlet. You can try to increase the space between your inlet and your cylinder and to change the boundary condition to velocity bounce back at the inlet.

Best Marc

[Author]

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