# Constant Residual of Turbulence Kinetic Energy and Dissipation Rate

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 September 8, 2017, 11:05 Constant Residual of Turbulence Kinetic Energy and Dissipation Rate #1 New Member   Keith Join Date: Sep 2017 Posts: 2 Rep Power: 0 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

 September 9, 2017, 08:17 #2 Senior Member   Cees Haringa Join Date: May 2013 Location: Delft Posts: 540 Rep Power: 10 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

September 9, 2017, 08:29
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Keith
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 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.

 September 9, 2017, 08:41 #4 Senior Member   Cees Haringa Join Date: May 2013 Location: Delft Posts: 540 Rep Power: 10 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?

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