# How to keep the water level constant at inlet

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 August 13, 2013, 10:45 How to keep the water level constant at inlet #1 Senior Member   Tanjina Afrin Join Date: May 2013 Location: South Carolina Posts: 168 Rep Power: 6 Sponsored Links Hello all, For keeping the water height constant at inlet, which BC should I use when I don't know the mass flow rate ( for mass flow rate inlet) and velocity ( velocity inlet) ? I used pressure inlet, but from static pressure contour , I found negative pressure at inlet portion. I am using multiphase ( VOF) model . It will be great help if anyone can suggest me anything. Thanks in advance.

 August 14, 2013, 07:53 #2 Member   Stephan Langenberg Join Date: Sep 2011 Location: Germany Posts: 73 Rep Power: 7 You want to establish a constant water level? I think you have to possibilities (based on your model and settings). The easy one is working with profiles for the inlet (non constant velocity and non constant mass inlet). -> If you can calculate your massflow. The other way is using UDF for your inlet. You may set a pointer for (pressure, velocity, VolumeFraction) on a Cellzone or face and regulate your inlet velocity by it. I imagine something like a sensor for the volume fraction. If the Volume fraction in one cell is higher than X (because of raising water level) the pointer activates a clipping function in the inlet and reduces or stopps the velocity inlet. What I don't know without using the manual is how to point pressure or an other value in a seperate cellzone, but I've read it ones.

August 14, 2013, 11:15
#3
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Tanjina Afrin
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Quote:
 Originally Posted by Jim87 You want to establish a constant water level? I think you have to possibilities (based on your model and settings). The easy one is working with profiles for the inlet (non constant velocity and non constant mass inlet). -> If you can calculate your massflow. The other way is using UDF for your inlet. You may set a pointer for (pressure, velocity, VolumeFraction) on a Cellzone or face and regulate your inlet velocity by it. I imagine something like a sensor for the volume fraction. If the Volume fraction in one cell is higher than X (because of raising water level) the pointer activates a clipping function in the inlet and reduces or stopps the velocity inlet. What I don't know without using the manual is how to point pressure or an other value in a seperate cellzone, but I've read it ones.
Hi Jim,

Thank you very much for your reply. you know any way that I can calculate mass flow rate ? I am working with 2D and 3D both.

Unfortunately, I couldn't understand your UDF part. can you explain a little bit ?

 August 14, 2013, 11:52 #4 Member   Stephan Langenberg Join Date: Sep 2011 Location: Germany Posts: 73 Rep Power: 7 I don't know your model, so I can't say if you can calculate your needed mass flow inlet. If it is a easy model you can calculate with analytic and use fluid mechanics... The UDF is a function written to adapt the flow inlet to a a criteria you choose. In an experiment you could set an sensor in a position X, when the water level arrives the sensor he would give a signal and regulate the inlet. An UDF has the possibility to do the same in your numeric system. The sensor could be a pointer and activate a function (maybe stop velocity inlet) if a physical value (temperatur, volume fraction etc.) reachs a special magnitude. Hope my Position is clearer now.

August 14, 2013, 12:20
#5
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Tanjina Afrin
Join Date: May 2013
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Quote:
 Originally Posted by Jim87 I don't know your model, so I can't say if you can calculate your needed mass flow inlet. If it is a easy model you can calculate with analytic and use fluid mechanics... The UDF is a function written to adapt the flow inlet to a a criteria you choose. In an experiment you could set an sensor in a position X, when the water level arrives the sensor he would give a signal and regulate the inlet. An UDF has the possibility to do the same in your numeric system. The sensor could be a pointer and activate a function (maybe stop velocity inlet) if a physical value (temperatur, volume fraction etc.) reachs a special magnitude. Hope my Position is clearer now.
HI JIm,

Thank you very much . My model contains two part. One is filled with gravel and water will pass through this porous zone to a pipe. I want to keep water height constant at this porous region. I don't have any discharge or velocity data. So is it possible to calculate mass flow rate here?

And about UDF, now I am reading UDF manual fluent. Hopefully, after reading some example , I will be little bit familiar with it.

August 14, 2013, 13:33
#6
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Quote:
 Originally Posted by Tanjina Hello all, For keeping the water height constant at inlet, which BC should I use when I don't know the mass flow rate ( for mass flow rate inlet) and velocity ( velocity inlet) ? I used pressure inlet, but from static pressure contour , I found negative pressure at inlet portion. I am using multiphase ( VOF) model . It will be great help if anyone can suggest me anything. Thanks in advance.

To set a constant height at inlet area, you may try to split the domain to two. set the volume fraction of the sub-domain which includes inlet BC as fixed value (=1).

But first you have to make sure that such a setting is physically correct.

August 19, 2013, 22:12
#7
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Tanjina Afrin
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Quote:
 Originally Posted by John To set a constant height at inlet area, you may try to split the domain to two. set the volume fraction of the sub-domain which includes inlet BC as fixed value (=1). But first you have to make sure that such a setting is physically correct.
Hi John, I am sorry that I just missed the notification that someone has answered my query. I am not sure how to split the domain to two, but as a normal VOF model, Inlet had three phase, one is water, one is air and third one is mixture. I allowed to water volume fraction =1. From the volume fraction of water contour shows that water level is constant, but from static pressure contour I found that there are negative pressure just below the inlet with a magnitude of -84 pascal. So I got worried. Do you have any idea about that ?

 August 30, 2013, 07:41 #8 New Member   anonymous Join Date: Jan 2011 Posts: 23 Rep Power: 8 Have you used specified operating density option with the density of lighter phase.

August 30, 2013, 09:49
#9
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Tanjina Afrin
Join Date: May 2013
Location: South Carolina
Posts: 168
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Quote:
 Originally Posted by vig Have you used specified operating density option with the density of lighter phase.
Yup, I did.

My problem is somehow solved now..... what I did wrong there is I plotted the contour with static pressure. After plotting the total pressure, pressure is zero at inlet, which means that water level remains constant.

 January 21, 2016, 11:18 #10 New Member   Alireza Bahmanpour Join Date: Jul 2014 Posts: 10 Rep Power: 5 Hi Tanjina, Could you please clarify more how you solved your problem? I also need to set my inlet flow level to be constant. Thanks, Ali

 January 21, 2016, 11:29 #11 Senior Member   Tanjina Afrin Join Date: May 2013 Location: South Carolina Posts: 168 Rep Power: 6 Hello AliRezab, I had two type of settings. For one case, I had side inlet (water entered into the domain from side) and I needed to top water level of the domain be constant. For that case, I used symmetry boundary condition, which means I forced the domain's water surface to be constant. And for another case, I had top inlet (water entered into the domain from top) and again I needed top water level be constant. That case, I used pressure inlet with zero pressure. Reference pressure was atmospheric pressure. Hope it helps. Regards, Tanjina

 Tags inlet bc, vof

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