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Tank emptying problem using Ansys Fluent

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Old   November 12, 2014, 17:54
Default Tank emptying problem using Ansys Fluent
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Tanjina Afrin
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Hello,

I am modeling a reservoir-pipe system, where with time water from reservoir will drain out through pipe. I am using Ansys Fluent. I think it's pretty common simulation.

But I have one confusion. For top of the reservoir, what BC should I use ? Wall BC ( I assigned one zone above the top of the reservoir ar patched that zone with air) , or should I use pressure inlet and allow air to enter ?

Any suggestion will be highly appreciated.

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Old   November 13, 2014, 05:14
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What does it happen when you are drinking something with a straw, you have the straw full of liquid, you plug one extremity of the straw with a finger and you rise it from the glass?

PS: finger is the wall, straw is the reservoir
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Old   November 13, 2014, 08:04
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I'm working on an emptying tank, too, and while it seems obvious at first it's a actually quite a problem to simulate. you have to either use a two-phase model with water and air and possibly evaporation or you can use a wall with slip for the top reservoir boundary, if the vapor above the water is of no interest to you. however, you will have to move this wall as the water flows out with a dynamic mesh. depneding on geometry and mesh you'd need to code more or less complicated UDFs.
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Old   November 13, 2014, 10:58
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Thanks guys for replying.

@ Pilakin,

Yup, I am using VOF model. Is it that much complicated that I have to use dynamic mesh and UDF !!

I tried with both BC as I said in my previous thread .... using the no slip wall BC (I used one zone of air which is in between the top wall and water surface to make atmospheric pressure on water surface), I found that water level is not falling with time, instead of that air is entering from outlet and it goes to upstream !!

And with pressure inlet with air entering, I found water level is dropping with time. It seems reasonable to me, but one thing makes me confused. I am simulating another model at the same time with perforated pipe and water comes to the pipe from top. In that model, I used wall BC at the top and assigned a air zone between top wall and water surface. In this model, water is dropping and at the same time air is entering from outlet.... I can't explain this two phenomena

@ghost82, I am getting properly what you are saying ....could you please explain ?

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Old   November 13, 2014, 11:21
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Hi,
What I meant is that if you put a wall above the water level with air trapped between the free surface and this top wall, gravity will push down the water but if you are not letting air to enter it will be created vacuum and water will not drain!





So your top surface is a pressure inlet, so to let air to enter your domain from the top.
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Old   November 13, 2014, 11:38
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Thanks Ghost. I can understand clearly. Yup thats what I saw that water level was not dropping. But why is was dropping for my other model? Do you have any idea ? I am attaching a contour of my second model at 14s. around the pipe , zone was gravel zone, so water-air interface is not that much clear.
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File Type: jpg 14.24s.jpg (49.6 KB, 11 views)
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Old   November 13, 2014, 11:41
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Sorry, but I cannot understand clearly the contour plot: can you elaborate a bit more and specify bc?Can you specify where water exits/air enters?
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Old   November 13, 2014, 12:05
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I a attaching a new image. hope now you'll get a clear image.
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Old   November 13, 2014, 12:11
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Sorry, I didn't attach image with previous thread.
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File Type: jpg 0s_wth lebel_resize.jpg (27.1 KB, 9 views)
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Old   November 13, 2014, 12:17
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The setup doesn't seem correct to me.
Correct setup should be pressure inlet for the top horizontal surface even in this case.
Are you sure these are converged solutions?
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Old   November 13, 2014, 12:21
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Yea, it is converged solution.. it's running for 300s ( up to today-it took more the one month to get these 300sec data)

I was trying to run a model which is exactly this one but length of pipe is smaller with pressure inlet, but two time that model collapsed! so I am getting more confused.....
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Old   November 18, 2014, 11:54
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Hello Ghost82,

If I assign symmetry BC instead of pressure inlet I find the same phenomena as using wall BC i.e. water level is not dropping, air enters from outlet.... what can be the possible explanation? Symmetry BC can make the constant water level ?
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Old   November 18, 2014, 12:07
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Yes, symmetry at top means air cannot enter the domain from the top, so vacuum is created and water cannot drop.
You can think symmetry boundary condition as a zero shear slip wall.

Directly from the user guide:




ANSYS FLUENT assumes a zero flux of all quantities across a symmetry boundary. There is no convective flux across a symmetry plane: the normal velocity component at the symmetry plane is therefore zero. There is no diffusion flux across a symmetry plane: the normal gradients of all flow variables are therefore zero at the symmetry plane. The symmetry boundary condition can therefore be summarized as follows:
  • zero normal velocity at a symmetry plane
  • zero normal gradients of all variables at a symmetry plane
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