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simpleFoam: Non-uniform mesh near inlet & outlet causes incorrect velocity profile?

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Old   January 28, 2015, 05:17
Default simpleFoam: Non-uniform mesh near inlet & outlet causes incorrect velocity profile?
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Saumitra Joshi
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Hi all!

I'm running a very simple case of steady state, incompressible, laminar pipe flow using simpleFoam. I created the mesh using snappyHexMesh. The following three types of meshes were generated:

1) Fully uniform hex-mesh (uniform spacing between cells in flow direction)
2) Non-uniform hex mesh without layers (finer cells near inlet and outlet, no layers)
3) Non-uniform hex mesh with layers (finer cells near inlet and outlet, layers present) - image below.



Geometry: Straight cylindrical pipe
Diameter: 1 cm, Length: 1 m

Kinematic viscosity: 1e-6 m2/s

Boundary Conditions:
Velocity:
-> Inlet: Uniform Fixed Value ux = 0, uy = 0.1 m/s, uz = 0
-> Outlet: Zero Gradient
-> Wall: Uniform fixed value
Pressure:
-> Inlet, Wall: Zero Gradient
-> Outlet: Fixed Value = 0

I'm using the GAMG solver for pressure (Abs tolerance: 1e-8, Rel tolerance: 1e-3, other settings default), PBiCG solver for velocity (Abs tolerance: 1e-8, Rel tolerance: 1e-3, other settings default). Residual for pressure and velocity is 1e-6.

The Issue

For case 1, I get very nice results. (Parabolic profile, Ucent = 2*Uavg, correct pressure drop).



However for case 2 and 3, although the pressure drop is fine, the velocity profile along the center-line looks something like this:



Firstly, the velocity reaches only about 0.19 m/s instead of 0.2. Also, there is a weird kink towards the end. I checked if this is because of interpolation issues, but that is not the case.

I'm baffled. Is this because of the non-uniform mesh near inlet and outlet? If yes, why does it occur?
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