[wrightguy]

Hi all,

I am having an issue when running a simple 2D axisymmetric model of annular flow. The flow if fully turbulent (Re approx. 7E5) and I am using the k-epsilon model with intensity (3%) and hydraulic diameter (0.009 m) as parameters and standard wall functions. The mesh is well refined and structured with a maximum aspect ratio of 3.

I am employing a mass flow inlet BC and 0 Pa pressure outlet. Both walls are stationary with prescribed temperature.

When I run the model, all parameters converge nicely except for k and epsilon. They are simply constant. Also, I get the issue that the turbulence viscosity ratio is limited to 1E5 (default limit set by Fluent).

I have done some searching online yet the only advice is to refine mesh and correct the turbulence parameters. I believe my mesh and BCs are fine so I'm kind of stumped here. Any help would be greatly appreciated...


Attached below are snaps of the mesh and residuals:

mesh.PNG

residual.PNG

[CeesH]

1) your mesh is very crude. I'd advise to refine it. Use at least 20 cells in the height direction.

2) how do you have 2 walls? You are using the axisymmetric model right - that means the center should be a symmetry, not a wall. If you have walls on both sides, use 2D-planar

[wrightguy]

Originally Posted by CeesH

1) your mesh is very crude. I'd advise to refine it. Use at least 20 cells in the height direction.

2) how do you have 2 walls? You are using the axisymmetric model right - that means the center should be a symmetry, not a wall. If you have walls on both sides, use 2D-planar

1) I refined it further and the issue is still not resolved. I ran a similar model a few months ago and it wasn't much more defined than this. The first layer thickness at the walls comes from a yplus value which I calculated as 0.3 mm.

"2) This is an annular flow (between two tubes) meaning there has to be two walls. The line of symmetry is the x-axis, which is not visible from the screenshots.


I have spoke with someone who advised me to use a k-omega model, as it's more appropriate than k-epsilon due to this being a "two layer model". In the k-omega model, the yplus value should <1 meaning I will need to refine more at both walls.

[CeesH]

y+ is a dimensionless variable, so it can't be 0.3mm; how did you calculate that?

Anyway, the residuals should converge for k-e too, if it works for k-omega. Did switching to k-omega solve the problem?
I also am not sure if it's allowed to set up an annular flow problem like this - I can remember from when I was a FLUENT TA that for, axisym pipe flow simulations, small offset from the symmetry axis would lead to errors when they started solving, but I don't know if in that case the supposed axis was actually set to wall. Did you try running it as planar just to check whether it would converge?

[AbhishekShingalaCFD]

Viscosity limit error can be resolved by increasing maximum ratio (turb. viscosity/viscosity) by 1 or 2 order. simultaneously one can also try to reduce under relaxation factor of the relevant solver up to 0.01 for few iterations until properties of interest become stable again.