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-   -   How to keep the water level constant at inlet (http://www.cfd-online.com/Forums/fluent/122161-how-keep-water-level-constant-inlet.html)

Tanjina August 13, 2013 10:45

How to keep the water level constant at inlet
 
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.

Jim87 August 14, 2013 07:53

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.

Tanjina August 14, 2013 11:15

Quote:

Originally Posted by Jim87 (Post 445677)
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 ?

Thanks in advance. :)

Jim87 August 14, 2013 11:52

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.

Tanjina August 14, 2013 12:20

Quote:

Originally Posted by Jim87 (Post 445763)
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.


Thanks for your generous help. :)

John August 14, 2013 13:33

Quote:

Originally Posted by Tanjina (Post 445460)
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.

Tanjina August 19, 2013 22:12

Quote:

Originally Posted by John (Post 445790)
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 ?

vig August 30, 2013 07:41

Have you used specified operating density option with the density of lighter phase.

Tanjina August 30, 2013 09:49

Quote:

Originally Posted by vig (Post 448898)
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.


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