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Sadhna May 22, 2005 01:19

Gravity flow
 
Hi,

I am a new user in Fluent

I have a porous cylinder through which a fluid trickles down by gravity (the inlet velocity is known)

Both the flow inlet and outlet are open to the atmosphere (i.e. pressure here should be 1 atm). The sides of the cylinder are walls.

I declare a velocity inlet for the flow inlet, porous fluid zone for the cylinder and pressure outlet for the outlet.After simulation,the pressure countours dont show 1 atm static pressure at the inlet.But at the exit you see 1 atm (because of the pressure-outlet B.C., I guess)

How can I specify the inlet velocity and pressure both of them at the flow inlet? can this be done? Also,How should I ensure flow is by gravity (Will switching on the Gravity option in Operating Parameters panel suffice?)

Please help,

Sadhna


Honza May 23, 2005 08:20

Re: Gravity flow
 
Hi, all the pressures you specify are RELATIVE pressures with respet to 'operating pressure' in Oper. cond. panel (default is 1 atm). Set 0 Pa at outlet. To display absolute press. scroll down in Pressures ... subpanel to Absolute pressure. You cant specify pressure at both inlet & outlet AND velocity - it would be overspecified. Gravity option on is sufficient. Regards Honza

sadhna May 23, 2005 11:03

Re: Gravity flow
 
Hi Honza,

Thanks for your kind attention

I have done the following

inlet-velocity inlet (I specified a given velocity)

outlet-pressure outlet - 0 Pa (gauge pressure)

Plz note that the inlet is at a vertical height of 0.6m above the outlet and the coordinates of the origin are at the inlet (so that y coordinate is negative into the geometry)

For the cylindrical region containing porous media, I enabled the Porous model and supplied the necessary resistance coefficients

In operating panel, I switched on the gravity option and specified operating pressure as 101325 Pa (1 atm)

And now for the surprise!!!

In the absolute pressure contours I see that the pressure at the outlet is 1 atm (as expected) but the pressure at the inlet is more!!! (pressure should increase with depth, am I right???)

What do you think has gone wrong in my case?

waiting for suggestions,

Sadhna

Honza May 24, 2005 09:00

Re: Gravity flow
 
Hi, the hydrostatic pressure is not displayed. What you can see is pressure drop caused by flow resistance. Honza

Sadhna May 24, 2005 10:19

Re: Gravity flow
 
Hi Honza,

Can you explain in detail what you mean?

Do you mean to say that pressure at the top is more than that at the bottom due to flow resistance present in between the two?

Is this pressure calculated by something like Hagen Poisuelle equation (that gives pressure drop for packed beds such as the one in my case)?

Please elaborate,

Thank you,

Sadhna

Rajesh G May 30, 2005 05:06

Re: Gravity flow
 
I agree with Honsa and what you can do is to specify the operating pressure as zero (not 1 atm!)along with the zero gauge pressure condition at the outlet.

Rajesh

Sadhna May 30, 2005 08:15

Re: Gravity flow
 
Hi Rajesh,

What do you mean by setting the operating pressure as 0 atm (btw,is this guage pressure???).How does that fit in the context?Did Honza mean that?

Regards, Sadhna

Rajesh G May 30, 2005 10:19

Re: Gravity flow
 
If you have given the operating pressure as 1 atm that means that you have given the absolute pressure as 2atm as the operating pressure generally will be gauge pressure . If you are giving the operating pressure as 0, your absolute pressure will be 1 atm , i.e the atmospheric pressure. That's what Honsa also told that you might've gone wrong with the relative pressure concept used in the Fluent

Rajesh G May 30, 2005 10:48

Re: Gravity flow
 
What I mean to say is normally if you want fluent to start the calculations from the initial conditions, i.e especially in your case, its atmospheric pressure at both inlet and outlet, the operating pressure should be atmospheric, that's, the gauge pressure is zero (hope u know pressure gauges read zero when the presure measured is atmospheric.... as you are wondering whether there can be zero gauge pressure!). So what I think, the "culprit" in your case, as you are also thinking, is the inlet pressure, being high because you have given the operating pressure as 1 atm (gauge) that means 1atm above atmospheric.

Balaji May 30, 2005 12:41

Re: Gravity flow
 
hi,

The pressure at the outlet is atm as you have specified.But there may(should be because you are using porous media) be pressure drop inside the domain it will include frictional,accelerational and gravitational.So you calculate the all the pressuredrops in the domain and add this pressure with atm pressure and check whether this is the pressure which has been shown by Fluent.Since you have set the outlet pressure the inlet pressure will adjust according to the pressure drop.

Inlet pressure should be more than outlet pressure.If there is no pressure difference how the flow take place.

I feel I am right.You can check with this.

bye


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