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Boundary Conditions Problem

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Old   February 11, 2013, 09:32
Question Unresolved problem and Queries
  #21
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Himanshu Sharma
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Quote:
Originally Posted by asoltoon View Post
Hello again,
you can reduce time-step, it doesn't affect the solution but will reduce the Co number and can help your solution to converge. you should determine your time-step such as your Co number doesn't exceed 1 in the solution. 1-reduce and reduce it until the Co number in the terminal shows the accepted value.
as I understood, your mesh is tetrahedral. unstructured meshes if they are not generated well, will have some cells that are very smaller than the biggest cell in your mesh. it will also make your solution dependent to these small cells, so that you should reduce your time step as much as the courant will be max 1 for these cells. If the geometry is not so complicated, such as your case, structured grids are more suggested. they often don't face to these problems. 2-make a good structured grid and run again.
if your problem didn't solved I will be glad to help you if I could.

Regards,
Ali
Hi,
From other post on the forums (http://www.cfd-online.com/Forums/ope...rintstack.html) i came across checking the mesh by "checkMesh" i did that thing and found few things about my mesh bold ones.
Code:
Overall number of cells of each type:
    hexahedra:     223866
    prisms:        0
    wedges:        0
    pyramids:      0
    tet wedges:    0
    tetrahedra:    0
    polyhedra:     0

Checking topology...
    Boundary definition OK.
    Cell to face addressing 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                  
    wall                21803    21858    ok (non-closed singly connected)  
    inlet               278      300      ok (non-closed singly connected)  
    outlet              493      528      ok (non-closed singly connected)  

Checking geometry...
    Overall domain bounding box (-0.0199212 -4.33681e-19 -0.0008) (-0.00992118 0.01 0.0108)
    Mesh (non-empty, non-wedge) directions (1 1 1)
    Mesh (non-empty) directions (1 1 1)
    Boundary openness (-2.29843e-15 2.69037e-17 -4.91973e-18) OK.
    Max cell openness = 3.26926e-16 OK.
    Max aspect ratio = 21.2719 OK.
    Minumum face area = 1.27927e-10. Maximum face area = 1.16148e-07.  Face area magnitudes OK.
    Min volume = 1.28223e-14. Max volume = 2.27969e-11.  Total volume = 1.00065e-06.  Cell volumes OK.
    Mesh non-orthogonality Max: 43.6834 average: 7.57081
    Non-orthogonality check OK.
    Face pyramids OK.
    Max skewness = 1.12467 OK.
    Coupled point location match (average 0) OK.
you can see my cells are very small is it the reason of my Courant number getting raised very high?? i have another doubt that as the flow inside develops in ,since my outlet sections is way small comparative to the wall,are my boundary conditions right to make the flow patterns in side the box ?? what i want as a result is after providing a pressure outlet condition i should get a velocity at the outlet patch since there is no other outlet present in the domain.Is my approach right in setting the boundary conditions since this is the common approach for seeing boundary conditions in the commercial softwares. Do open foam work in same way? i.e. if we provide zeroGradient condition in the 0/p at the outlet then the solver will extrapolate values from inside for the velocity.These are some basic doubts regarding the software if you can throw some light on it.
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Old   February 12, 2013, 09:08
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  #22
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Ali Khalifesoltani
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Hi,

According to CFL condition, your dt should be less than 1.2e-14 sec, so you should make a coarser mesh. I am not an expert on openFoam but as much as I know if you don't overconstrain the B.Cs there should be no problem in the boundary conditions you defined.
I ran your case and worked a little bit on it but no success was observed. I thought that the problem is probably for your Initial Pressure Condition in the domain and it will cause some math. error in the solution but there was no success. I think the first step to solve your problem is to obey the CFL condition(by decreasing dt or making a coarser mesh) and then investigate on the B.Cs.

Regards.
Ali
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Old   February 13, 2013, 00:43
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Himanshu Sharma
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Quote:
Originally Posted by asoltoon View Post
Hi,

According to CFL condition, your dt should be less than 1.2e-14 sec, so you should make a coarser mesh. I am not an expert on openFoam but as much as I know if you don't overconstrain the B.Cs there should be no problem in the boundary conditions you defined.
I ran your case and worked a little bit on it but no success was observed. I thought that the problem is probably for your Initial Pressure Condition in the domain and it will cause some math. error in the solution but there was no success. I think the first step to solve your problem is to obey the CFL condition(by decreasing dt or making a coarser mesh) and then investigate on the B.Cs.

Regards.
Ali
Hi,
Thanks for Reply
I also think that it necessary for defining an interior pressure conditions in side the box since i don't think wit the given velocity inlet the solver is developing the interior pressure field. but then if i specify some interior pressure inside the domain then it will start affecting my inlet boundary conditions hence i am totally in a mess how to make this problem as this is on of my projects in the university.And i will now try to coarse my mesh and then try to implement the boundary condition how it might work.if you find some ways than please do share.

Regards
Himanshu sharma
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