# VOF modelling, water drainage from an elevated tank

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November 28, 2013, 07:48
VOF modelling, water drainage from an elevated tank
#1
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ASHRAF ALFANDI
Join Date: Oct 2012
Posts: 17
Rep Power: 6
Hi,
I am trying to simulate water draining from at a tank at 10 meter elevation through a pipe by the aid of gravity. As seen in the figure 1

I am using ANSYS FLUENT to do that. I am using VOF for modelling.
I am having difficulties in defining the inlet and outlet boundary condition.
Right now, I am using pressure-inlet and pressure-outlet for inlet and outlet boundary conditions,
but after running the code,the results is as shown in figure 2,

anyone have an idea about that, and how to resolve this issue?
Attached Images
 figure 1.png (21.1 KB, 34 views) figure 2.PNG (15.2 KB, 35 views) figure 2_1.PNG (27.8 KB, 27 views)

 November 28, 2013, 08:53 #2 New Member   Join Date: Nov 2011 Posts: 27 Rep Power: 7 i guess that since you have a gravity driven flow, you should make additional sets in fluent by setting the operational density to 0 and the reference density equal to the fluid that you are simualting, the gravitational influence will be considered in the equations

November 28, 2013, 10:01
how to change the operational density ?
#3
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ASHRAF ALFANDI
Join Date: Oct 2012
Posts: 17
Rep Power: 6
Quote:
 Originally Posted by Jabba by setting the operational density to 0 and the reference density equal to the fluid that you are simualting
Dear Jabba,
Thanks a lot for your replay.
you mean to change the density of the water to zero from the material edit window ?
if not, how to change the operational density ?

thanks....

 November 28, 2013, 16:44 #4 Senior Member   Join Date: Jan 2010 Location: Germany Posts: 268 Rep Power: 9 So, Gravity has to be enabled. Hence give always a refrence density: either the lighter phase or zero ( i usually give zero). You habe very difficult simulation with both pressure B.C: here you have to patch the pressure fied with the hydrostatic head or define proper profiles at the pressure intlet /outlet. After doing this then we can discuss whether VOF or MultiFluid-VOF is appropriate for the simulation Good Luck

 November 28, 2013, 16:48 #5 Senior Member   Join Date: Jan 2010 Location: Germany Posts: 268 Rep Power: 9 looking into the plots, we are experiencing high reversal flows coming from the outlet. This occurs because of the pressure B.C. Put zero density and define pressure profile for the inlet since there you have at the beginning water (tank regio).

November 29, 2013, 09:05
#6
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Join Date: Nov 2011
Posts: 27
Rep Power: 7
Quote:
 Originally Posted by ashraf88 Dear Jabba, Thanks a lot for your replay. you mean to change the density of the water to zero from the material edit window ? if not, how to change the operational density ? thanks....
hi, you should keep the water density in materials tab with usual values
the operating density should be changed at Boundary Conditions or Cell zone Conditions > Operating Conditions > Check Specified Operating Density and setting it to 0 or to the lighter phase density
and then you should also change the reference density at Reference Values tab to the value of the water density

through this way, the hidrostatic pressure will be considered in the calculation

don't forget to set the gravity magnitude and direction properly

regards

 Tags free surface flow, open channel flow, vof modeling

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