# Problem with two phase model

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February 4, 2015, 15:46
Problem with two phase model
#1
New Member

Aaron Manzali
Join Date: Feb 2015
Posts: 8
Rep Power: 10
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
Attached Images
 initial conditions.png (3.7 KB, 17 views) velocity field.png (8.4 KB, 15 views)

 February 5, 2015, 06:24 #2 Senior Member     Amin Join Date: Oct 2013 Location: Germany Posts: 396 Rep Power: 13 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?

 February 5, 2015, 11:55 #3 New Member   Aaron Manzali Join Date: Feb 2015 Posts: 8 Rep Power: 10 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

 February 5, 2015, 12:33 #4 Senior Member     Amin Join Date: Oct 2013 Location: Germany Posts: 396 Rep Power: 13 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!

 February 5, 2015, 15:33 #5 New Member   Aaron Manzali Join Date: Feb 2015 Posts: 8 Rep Power: 10 I read that the reference position does not matter unless you do not have pressure boundaries. Where should I set the reference position?

February 5, 2015, 16:59
#6
New Member

Aaron Manzali
Join Date: Feb 2015
Posts: 8
Rep Power: 10
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
Attached Images
 pressure.PNG (5.7 KB, 9 views)

 February 6, 2015, 06:03 #7 Senior Member     Amin Join Date: Oct 2013 Location: Germany Posts: 396 Rep Power: 13 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!

 February 6, 2015, 15:08 #8 New Member   Aaron Manzali Join Date: Feb 2015 Posts: 8 Rep Power: 10 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

February 9, 2015, 02:43
#9
Senior Member

Amin
Join Date: Oct 2013
Location: Germany
Posts: 396
Rep Power: 13
Quote:
 Originally Posted by amanzali 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...

 February 19, 2015, 17:58 #10 New Member   Aaron Manzali Join Date: Feb 2015 Posts: 8 Rep Power: 10 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.

 Tags multiphase, pressure boundaries