# Passive Scalar Out of Bounds Channel

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August 22, 2017, 17:22
Passive Scalar Out of Bounds Channel
#1
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Hello to all,
I'm solving the flow in a very simple geometry: the well known Tjunction channel. It is a microfluids device so the dimensions are of the order of 10^(-6). The flow is completely laminar due to the very low Re number (Re=100). The channel is characterized by two inlets and one outlet. I linked an image to be more clear.
My flow is single phase, water at ambient temperature. Now, after the calculation of the velocity and pressure field with simpleFoam, i run a simulation to calculate the passive scalar (DT=3*10^(-10)). At one inlet i imposed a T=0 while at the other T=1 and then i run scalarTransportFoam. In this manner i can observe how the two flows mix in the outlet channel. The big problem is that my scalar goes out of bound, this means i find value larger 1 and it shouldn't happen. I tried to fix the problem using different schemes for the divergence as suggested in other threads but nothing, the problem is always there.
Is there someone who have experience in this type of problems?

I link also the two cases.
Thanks for help.

Best Regards,
Roberto
Attached Images
 Schermata del 2017-08-22 23-06-09.png (55.0 KB, 12 views)
Attached Files
 Direct.tar.gz (2.2 KB, 1 views) Scalar.tar.gz (1.8 KB, 3 views)

 August 23, 2017, 02:45 #2 Senior Member     Anton Kidess Join Date: May 2009 Location: Germany Posts: 1,274 Rep Power: 23 Check your mesh. Tobi likes this. __________________ *On twitter @akidTwit *Spend as much time formulating your questions as you expect people to spend on their answer.

August 23, 2017, 04:24
#3
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Quote:
 Originally Posted by akidess Check your mesh.
What do you mean? Sorry, i don't understand.
Here there is my output after running checkMesh

Code:
```/*---------------------------------------------------------------------------*\
| =========                 |                                                 |
| \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox           |
|  \\    /   O peration     | Version:  v1612+                                |
|   \\  /    A nd           | Web:      www.OpenFOAM.com                      |
|    \\/     M anipulation  |                                                 |
\*---------------------------------------------------------------------------*/
Build  : v1612+
Exec   : checkMesh
Date   : Aug 23 2017
Time   : 10:19:27
Host   : "Nitro"
PID    : 4231
Case   : /home/roby/Scrivania/0.4/Direct
nProcs : 1
sigFpe : Enabling floating point exception trapping (FOAM_SIGFPE).
fileModificationChecking : Monitoring run-time modified files using timeStampMaster (fileModificationSkew 10)
allowSystemOperations : Allowing user-supplied system call operations

// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //
Create time

Create polyMesh for time = 0

Time = 0

Mesh stats
points:           306516
faces:            863850
internal faces:   810150
cells:            279000
faces per cell:   6
boundary patches: 5
point zones:      0
face zones:       0
cell zones:       0

Overall number of cells of each type:
hexahedra:     279000
prisms:        0
wedges:        0
pyramids:      0
tet wedges:    0
tetrahedra:    0
polyhedra:     0

Checking topology...
Boundary definition OK.
Point usage OK.
Upper triangular ordering OK.
Face vertices OK.
Number of regions: 1 (OK).

Checking patch topology for multiply connected surfaces...
Patch               Faces    Points   Surface topology
walls               45900    46330    ok (non-closed singly connected)
inlet1              300      336      ok (non-closed singly connected)
inlet2              300      336      ok (non-closed singly connected)
movable             6600     6951     ok (non-closed singly connected)
outlet              600      651      ok (non-closed singly connected)

Checking geometry...
Overall domain bounding box (-0.00015 -0.00165 0) (0.003 0.00165 0.0002)
Mesh has 3 geometric (non-empty/wedge) directions (1 1 1)
Mesh has 3 solution (non-empty) directions (1 1 1)
Boundary openness (-6.91724e-18 1.96638e-18 2.57098e-17) OK.
Max cell openness = 1.29247e-16 OK.
Max aspect ratio = 1 OK.
Minimum face area = 1e-10. Maximum face area = 1e-10.  Face area magnitudes OK.
Min volume = 1e-15. Max volume = 1e-15.  Total volume = 2.79e-10.  Cell volumes OK.
Mesh non-orthogonality Max: 0 average: 0
Non-orthogonality check OK.
Face pyramids OK.
Max skewness = 3.4169e-13 OK.
Coupled point location match (average 0) OK.

Mesh OK.

End```
It seems ok to me. Is there something i forgot?

Best Regards,
Roberto
Attached Files
 blockMeshDict.tar.gz (906 Bytes, 1 views)

 August 23, 2017, 04:41 #4 Member   Robert Join Date: May 2015 Location: Bremen, GER Posts: 65 Rep Power: 4 Hello Roberto, did you check your fvSchemes settings? Using Gauss limitedLinear 1; for the divergence scheme of the passive scalar usually works fine for me. Last edited by RobertHB; August 23, 2017 at 05:57.

August 23, 2017, 08:51
#5
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Join Date: Mar 2016
Location: Bergamo
Posts: 124
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Quote:
 Originally Posted by RobertHB Hello Roberto, did you check your fvSchemes settings? Using Gauss limitedLinear 1; for the divergence scheme of the passive scalar usually works fine for me.
Hello, Robert.
I tried the "Gauss limitedLinear01 1" scheme for the divergence but nothing, the solution continues to be out of bounds

 August 25, 2017, 03:31 #6 Senior Member     Join Date: Mar 2016 Location: Bergamo Posts: 124 Rep Power: 3 Little update I noticed few days ago something strange: of you not copy the phi file in the 0 folder scalarTransportFoam produces a phi file which is different from that produced by simpleFoam. This is very strange. Using the phi file produced by simpleFoam the solution improves but not enough. Why this difference? Best Regards, Roberto

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