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Old   February 4, 2015, 14:46
Default Problem with two phase model
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Aaron Manzali
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Hello all,

I am having some trouble regarding a simple two phase model simulating air entrainment in a dam. I have three boundaries and have set two as pressure conditions. One as an inlet with zero gauge pressure and the other as an outlet with zero gauge pressure as well.

When I start to run the model the air flows in the complete opposite direction to be expected. The water also backflows from the outlet even though there is a huge column of water in the simplified penstock.

Attached are two pictures with the initial conditions first. Red shows a volume fraction of water of unity and blue a volume fraction of zero.

The second picture shows the resulting velocity field.

The water inlet is to the left and in the attached pictures the imposed velocity is zero. My expectations are that the water drains out of the penstock until the elevation of the free surface matches with the elevation of the outlet boundary.

Any help would be appreciated.

thank you
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File Type: png initial conditions.png (3.7 KB, 17 views)
File Type: png velocity field.png (8.4 KB, 15 views)
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Old   February 5, 2015, 05:24
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Hi
Some suggestions
Do you set a reference pressure? If yes, are you sure about its position?
Du you set a value for density in operating condition? If no, enter density of air
Do you sure about backflow conditions in your outlet BC?
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Old   February 5, 2015, 10:55
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The reference pressure is set to atmospheric - 101325
The reference density is that of air
The outlet is a dummy outlet replacing a turbine/tailwater and the backflow is set to just water.


EDIT: If I switch the direction the gravity is pointing then the velocity vectors match up with what I expect, however the pressure is now calculated incorrectly
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Old   February 5, 2015, 11:33
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Are you sure you set a right place for position of reference pressure?
I had same issue, I changed this position and the issue solved! and when I set zero for ref. pressure and use this value in BCs the problem fully solved!
I suggest you if you don't know a right place with ref. pressure, switch it off!
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Old   February 5, 2015, 14:33
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I read that the reference position does not matter unless you do not have pressure boundaries. Where should I set the reference position?
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Old   February 5, 2015, 15:59
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attached is a screen shot of the total pressure contours with red being atmospheric and blue being low pressure.

mystifying problem...

would appreciate any help
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File Type: png pressure.PNG (5.7 KB, 9 views)
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Old   February 6, 2015, 05:03
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you should set a position for ref. pressure that you're sure it has atmospheric pressure (usually air inlet or outlet)
if you don't know a good position with this specification, don't use ref. pressure.

where is the position of ref. pressure now? show it in attached pic! and give me some explain about your BCs
as it's obvious in the pic, the pressure distribution is invalid!
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Old   February 6, 2015, 14:08
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The reference pressure is set at the air inlet simulating the gate slot at the top, exactly at one of the corners.

The boundary conditions are 2 m/s for the water inlet at the left. A gauge pressure of zero in the air inlet and a gauge pressure of zero at the dummy outlet simulating the tailwater at the bottom right. The boundary conditions are on the top faces of the two pressure inlets.

reference pressure is set to 101000 Pa
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Old   February 9, 2015, 01:43
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Quote:
Originally Posted by amanzali View Post
The reference pressure is set at the air inlet simulating the gate slot at the top, exactly at one of the corners.

The boundary conditions are 2 m/s for the water inlet at the left. A gauge pressure of zero in the air inlet and a gauge pressure of zero at the dummy outlet simulating the tailwater at the bottom right. The boundary conditions are on the top faces of the two pressure inlets.

reference pressure is set to 101000 Pa
Are you sure about gauge pressure at the dummy outlet? I think it's wrong! maybe you should use P=(ru)*(g)*(h) equation in outlet...
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Old   February 19, 2015, 16:58
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I have changed it so that the gauge pressure at the pressure outlet is rho*g*h higher than the gauge pressure at the air pressure inlet. I still get the same issue.
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