# Falling Water Drop Simulation (VOF)

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March 22, 2017, 10:34
Falling Water Drop Simulation (VOF)
#1
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Maksim
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I simulate a water drop falling on the solid surface, but when the drop is near the bottom, it changes its form (see attachment)
I use surface tension and static contact angle = 90°

I use VOF explicite model with implicite body force
I tested this case with different solution methodes:
1)PISO-Green Gausse Node Based - Body Force W. - Second O. Momentum - Geo Reconstract
2)PISO-Green Gausse Node Based - Body Force W. - Second O. Momentum - Compressive
And also with PRESTO! pressure scheme

but the results are equal

Attached Images
 Screenshot_6.jpg (49.0 KB, 23 views) Screenshot_7.jpg (49.0 KB, 24 views)

 March 22, 2017, 11:48 #2 New Member   valerie Join Date: Nov 2015 Posts: 20 Rep Power: 3 Could you try a contact angle of 0 to see the effect on the result?

March 22, 2017, 12:05
#3
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Maksim
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I set angle = 0
but the result is same
Attached Images
 Screenshot_8.jpg (44.8 KB, 14 views)

 March 22, 2017, 13:11 #4 New Member   valerie Join Date: Nov 2015 Posts: 20 Rep Power: 3 Do you have the same result if you switch the wall adhesion formulation and/or a finer mesh near the wall? Sent from my ALE-L21 using CFD Online Forum mobile app

March 22, 2017, 13:29
#5
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Maksim
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I turned it off
The results are better, but i think it is still wrong

The mesh is fine
0.05 mm
that means 40 elements in diameter of drop
Attached Images
 Screenshot_9.jpg (27.0 KB, 13 views) Screenshot_10.jpg (25.5 KB, 13 views)

March 22, 2017, 14:08
#6
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Alex
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Quote:
 I simulate a water drop falling on the solid surface, but when the drop is near the bottom, it changes its form (see attachment)
Is this a problem? Are you sure that the deformation you see can not be a physical effect? What exactly did you expect and why?

Quote:
 The mesh is fine
No offense, but I heard that phrase a lot. And usually when it is stated like that the assumption is wrong. Your mesh might be fine enough to resolve the droplet itself, but it might be too coarse to resolve the scales of the impact.
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 March 22, 2017, 14:11 #7 New Member   valerie Join Date: Nov 2015 Posts: 20 Rep Power: 3 If the mesh cannot b e refi.end because of calculaation time, eulerian wall film can be coupled with vof. I agreed with flotus1. The mesh refinement should br tested. Sent from my ALE-L21 using CFD Online Forum mobile app

March 22, 2017, 14:13
#8
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Maksim
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Quote:
 Originally Posted by flotus1 Is this a problem? Are you sure that the deformation you see can not be a physical effect? What exactly did you expect and why?
Which the physical effect? Can you this explain from the physical point of view?

I think that circle must not be deformed before contact with the wall
it was falling 95% of the path with perfect shape, and near the wall it decided to change the shape?
I don't think so

 March 22, 2017, 14:31 #9 Senior Member     Alex Join Date: Jun 2012 Location: Germany Posts: 1,470 Rep Power: 25 Is your droplet falling in a vacuum? Concerning the cell size: only one way to find out. __________________ Please do not send me CFD-related questions via PM

March 22, 2017, 15:23
#10
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Maksim
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The mesh was refined by 66%
0.05 mm ->0.03 mm

Problem is the same (see att.)

i think that my mistake may be in setting of parametres or so on, because it changes shape always in the last row of cells

what can i change ??
Attached Images
 Screenshot_15.jpg (57.3 KB, 9 views) Screenshot_16.jpg (57.1 KB, 12 views)

 March 23, 2017, 03:28 #11 Senior Member     Alex Join Date: Jun 2012 Location: Germany Posts: 1,470 Rep Power: 25 This does not convince me that the mesh you are using is fine enough to have no qualitative effect on the solution. For example I would argue that the distance between the two contact points is ~12 cells in both simulations. If you can not afford a proper global mesh refinement study, it is sufficient to refine the mesh locally in the contact region. Concerning the physical validity of the phenomenon you observe: you still did not answer the question if your droplet is falling in a vacuum. If there is a fluid surrounding the droplet, a similar deformation can actually occur under certain circumstances. See for example http://pubs.rsc.org/en/content/artic.../SM/C4SM02474E and the references. Looking at the pressure and velocty contours outside the droplet might shed some light on this issue. __________________ Please do not send me CFD-related questions via PM

March 23, 2017, 07:17
#12
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Maksim
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Velocity in this paper is greater then 10 m/s
I have vel. less then 1 m/s

I tested mesh 0.01 mm, that means 200 elements in diameter of drop

And as i have already said, the problem is not in mesh, because the changes appear ONLY IN LAST ROW OF CELLS
in every case, that's why problem in case options, such schemes, URF and so on

can anybody help me with advice ?
Attached Images
 Screenshot_17.jpg (157.6 KB, 8 views) Screenshot_18.jpg (156.7 KB, 9 views)

 March 24, 2017, 05:04 #13 New Member   valerie Join Date: Nov 2015 Posts: 20 Rep Power: 3 Do you run the simulation in 2D? I tried in 3D and I do not observe the behavior you mentioned.

March 24, 2017, 05:06
#14
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Maksim
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Quote:
 Originally Posted by Tigrgrou Do you run the simulation in 2D? I tried in 3D and I do not observe the behavior you mentioned.
I tested only 2D

Sent from my Lenovo K50-t5 using CFD Online Forum mobile app

 March 24, 2017, 05:19 #15 New Member   valerie Join Date: Nov 2015 Posts: 20 Rep Power: 3 it could be the problem, specially if you use planar 2D solver

March 24, 2017, 05:20
#16
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Maksim
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Quote:
 Originally Posted by Tigrgrou it could be the problem, specially if you use planar 2D solver
And i can`t resolve it?

Sent from my Lenovo K50-t5 using CFD Online Forum mobile app

 March 24, 2017, 05:51 #17 New Member   valerie Join Date: Nov 2015 Posts: 20 Rep Power: 3 if you expect a solution that has physical meaning, your simulation has to be more representative of your real problem. If it a spherical bubble, you can try the 2D axisymetry solver or 3D solver.

April 6, 2017, 03:46
#18
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Maksim
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Quote:
 Originally Posted by Tigrgrou Do you run the simulation in 2D? I tried in 3D and I do not observe the behavior you mentioned.
i have built the same case but in 3D, but if i set in CFD post 3 contours and see XZ and YZ planes, i see the same effect
Attached Images
 Screenshot_8.jpg (36.9 KB, 7 views)

 April 9, 2017, 11:01 #19 New Member   Maksim Join Date: Jan 2016 Posts: 21 Rep Power: 3 up this topic

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