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drops are moving against gravity with my mesh

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Old   July 29, 2011, 12:17
Default drops are moving against gravity with my mesh
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/edit: since my first description was to confusing, i now updated the first post. now with pictures and even with downloadable case file. thank you all for your support!

3 drops are initialised at rest at different positions over a structured surface in a 3D case without gravity



>> depending on initialising-position: 1 is rising (right drop), 1 is at rest (middle drop), 1 is falling (left drop)



details:

- 3D case, cubic domain, 1 bottom patch with structures on it, 5 ambient patches
- boundary conditions: see below
- drops = phase1 = water, surounding = phase2 = air
- interFoam solver
- gravity g is set to 0
- drops initialised at rest with 0.8mm radius in 2mm height
- cells per radius: 6
- only hexaedrons
- mesh created in icem cfd, imported via fluent3DMeshToFoam
- checkMesh: see below

casefile download:
http://dl.dropbox.com/u/29343553/ope...sHappen.tar.gz

BCs for U:
Quote:
BOTTOM
{
type fixedValue;
value uniform (0 0 0);
}

AMBIENT
{
type zeroGradient;
}
BCs for p_rgh:
Quote:
BOTTOM
{
type buoyantPressure;
value uniform 0;
}

AMBIENT
{
type fixedValue;
value uniform 0;
}
BCs for alpha1:
Quote:
BOTTOM
{
type zeroGradient;
}

AMBIENT
{
type zeroGradient;
}
checkMesh:
Quote:
Mesh stats
points: 105633
faces: 302080
internal faces: 287744
cells: 98304
boundary patches: 2
point zones: 0
face zones: 1
cell zones: 1

Overall number of cells of each type:
hexahedra: 98304
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
AMBIENT 11264 11393 ok (non-closed singly connected)
BOTTOM 3072 3201 ok (non-closed singly connected)

Checking geometry...
Overall domain bounding box (-7.7249405e-19 -7.7249405e-19 -7.7249405e-19) (0.012 0.0040000002 0.0040000002)
Mesh (non-empty, non-wedge) directions (1 1 1)
Mesh (non-empty) directions (1 1 1)
Boundary openness (3.05749e-16 5.4128779e-15 -5.4168725e-15) OK.
Max cell openness = 2.8894061e-16 OK.
Max aspect ratio = 2.0677983 OK.
Minumum face area = 1.3916016e-08. Maximum face area = 2.2097108e-08. Face area magnitudes OK.
Min volume = 1.7547609e-12. Max volume = 1.9531253e-12. Total volume = 1.8789846e-07. Cell volumes OK.
Mesh non-orthogonality Max: 44.548899 average: 14.58467
Non-orthogonality check OK.
Face pyramids OK.
Max skewness = 0.97916705 OK.

Mesh OK.

Last edited by of_user_; August 1, 2011 at 15:14.
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Old   July 29, 2011, 12:59
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happens also with more correctors:

Quote:
momentumPredictor no;
nCorrectors 3;
nNonOrthogonalCorrectors 2;
nAlphaCorr 2;
nAlphaSubCycles 3;
cAlpha 2;
courant number limitation i use:

Quote:
maxCo 0.2;
maxAlphaCo 0.2;
fvSchemes i use for this case:

Quote:
ddtSchemes
{
default Euler;
}

gradSchemes
{
default Gauss linear;
}

divSchemes
{
div(rho*phi,U) Gauss limitedLinearV 1;
div(phi,alpha) Gauss vanLeer;
div(phirb,alpha) Gauss interfaceCompression;
}

laplacianSchemes
{
default Gauss linear corrected;
}

interpolationSchemes
{
default linear;
}

snGradSchemes
{
default corrected;
}

fluxRequired
{
default no;
p_rgh;
pcorr;
alpha1;
}
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Old   July 31, 2011, 05:28
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anyone any idea why this might happen?
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Old   July 31, 2011, 14:56
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i guess ur problem is in boundary conditions, check ur boundary conditions again!
and share it in openFOAM maybe some one can help you
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Old   July 31, 2011, 15:07
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Quote:
Originally Posted by nimasam View Post
i guess ur problem is in boundary conditions, check ur boundary conditions again!
and share it in openFOAM maybe some one can help you
thank you for your help. i tested the case using this BCs for alpha:

Quote:
BOTTOM
{
type constantAlphaContactAngle;
theta0 90;
limit gradient;
value uniform 0;
}

OUTSIDE
{
type inletOutlet;
inletValue uniform 0;
value uniform 0;
}
this BCs for p:

Quote:
BOTTOM
{
type buoyantPressure;
value uniform 0;
}

OUTSIDE
{
type totalPressure;
p0 uniform 0;
U U;
phi phi;
rho rho;
psi none;
gamma 1;
value uniform 0;
}
and this BCs for U:

Quote:
BOTTOM
{
type buoyantPressure;
value uniform 0;
}

OUTSIDE
{
type totalPressure;
p0 uniform 0;
U U;
phi phi;
rho rho;
psi none;
gamma 1;
value uniform 0;
}
without any symmetry planes or other fancy stuff to eliminate possible mistakes and make sure it is mesh related. the surface uses BC BOTTOM and all other faces are OUTSIDE... i testet the same conditions in a mesh without the pyramid structures and it worked...

what do you mean by share it in openFOAM? posting on their site?

regards,
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Old   August 1, 2011, 03:42
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What is the topology of OUTSIDE (upload a picture of your domain)? If there is any corner in the patch, applying totalPressure is unphysical. Also, you could try zeroGradient on alpha instead of constantAngle 90 and see if that helps at all.

- Anton
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Old   August 1, 2011, 04:42
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the domain is a cubic box. all patches except the bottom patch are plain, using patchtype wall and their BCs are defined by OUTSIDE. the bottom is not plain. there are the pyramid structures. the patchtype is wall and the BCs are defined as BOTTOM.

i defined all outside patches as one part in icem and i specify just one BC for all those ambient patches.
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Old   August 1, 2011, 04:49
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So you have five sides of a box in OUTSIDE, which means you are trying to apply an isopressure boundary condition (totalPressure with p0=const) to a surface which contains corners and edges (i.e. discontinuities).
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Old   August 1, 2011, 04:53
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yes, i do.

i will check what happens when i use 5 different patches for it and report my results.

but thank you so far.
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Old   August 1, 2011, 05:02
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It doesn't matter if you use one or five patches. The important point is to change your boundary conditions.
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Old   August 1, 2011, 05:10
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so instead i should formulate the BC like this?

{
type fixedValue;
value uniform 0;
}

or zero gradient?
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Old   August 1, 2011, 06:52
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No! fixedValue again is specifying a constant pressure, which cannot physically exist on a discontinuous surface. You will need to specify a gradient instead. The interFoam tutorials use buoyantPressure, or you can use zeroGradient depending on your definition of pressure.
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Old   August 1, 2011, 09:55
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okay..

now i used zeroGradient for all BCs, except U at the bottom wall (this is fixed to 0) and apha1 at the ambient patches (this is inletOutlet).

but my drop is still rising against gravity...


/edit:
maybe i will upload a clean and simple version of my case soon, so that you can get an impression of what is happening, since it is hard to explain...
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Old   August 1, 2011, 10:22
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hi friend

for alpha:
zeroGradient

for U zergoradient for all patch except the bottom put it fixedValue zero

for p put all patch bouyant pressure except for out-let and put fixed value zero

ofcurse i suppose ur geometry is a column!
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Old   August 1, 2011, 10:43
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i am sorry, it seems my case description was quite bad since all of you are unsure about my setup

here again, hopefully i can express myself more clearly this time:

- i do use a cube as my domain.
- the bottom patch is a solid surface (it is not plain but structured).
- the other 5 patches are ambient.
- there is no inlet or outlet in this case except from the ambient patches.

- i tried using zeroGradient for all BCs to fix my problems. with exception of:
- U at the bottom wall (this is fixed to 0)
- apha1 at the ambient patches (this is inletOutlet).

the droplet is still accelerated against gravity.

/edit:

Quote:
hi friend

for alpha:
zeroGradient

for U zergoradient for all patch except the bottom put it fixedValue zero

for p put all patch bouyant pressure except for out-let and put fixed value zero

ofcurse i suppose ur geometry is a column!
even with this setup, there is no change...

Last edited by of_user_; August 1, 2011 at 11:01.
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Old   August 1, 2011, 14:39
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hello of_user,

Maybe an obvious question, did you use -9.81 for the gravity?

best
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Old   August 1, 2011, 15:11
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i updated post 1 to make my case clearer and i provide a downloadable casefile for anyone how is interested in testing
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