[ROY4]

hello
I'm modeling a bubble column but the pressure that I get from the contours does not seem correct, I have studied the tutorials and some papers. but they couldn't help me solve my problem. the problem is that the pressure increases with height, as we know it is not true because the pressure will increase with height decrease.
I MEAN AS WE GO DEEPER IN A FLUID, THE PRESSURE WILL INCREASE, RIGHT? BUT IN MY CASE AS I GO DEEPER THE PRESSURE DECREASES AS I GO UP TO SURFACE IT INCREASES...
is there any one here modeling an Eulerian bubble column or have modeled it so that can give me some information?
please if any one can help I'm in a serious need...
THANKS

[ghorrocks]

Read the CFX documentation on the pressure variable in buoyant simulations. The pressure variable is replaced by a modified pressure with the hydrostatic component removed. But if you do not give it the correct buoyancy reference density this correction will not be done correctly and you can have pressure increasing with height.

[ROY4]

Originally Posted by ghorrocks

Read the CFX documentation on the pressure variable in buoyant simulations. The pressure variable is replaced by a modified pressure with the hydrostatic component removed. But if you do not give it the correct buoyancy reference density this correction will not be done correctly and you can have pressure increasing with height.

dear ghorrocks
thanks for answering,
I have checked the documentation but still I do not get the reason of bubble column pressure increase, and the very interesting part is that most of researchers did not report the pressure contour at all and did no give any idea of how and why the pressure changes.
So you mean that even if the pressure is modified we should get this result that as we go deeper in a fluid the pressure must increase?
by the way, I checked the tutorial of CFX about airlift column and I got the same result and the pressure was increasing with height(LIKE MY CASE). And my results on gas hold up and axial liquid velocity are validated. If the pressure is wrong, why do I get the validated results?
And what should I specify as pressure to get the right pressure data?
THANKS

[ghorrocks]

In a buoyant simulation the pressure variable is replaced with a buoyancy corrected pressure, p(corr) = p + rho(ref).g.z; assuming z is the gravitational direction. If rho(ref) is set to the density of the incompressible fluid, then this will cause a column of fluid to have constant p(corr), even though the actual pressure will vary with z height.

If you set the rho(ref) too high or too low the correction will not cancel out the hydrostatic component. This will mean p(corr) will increase or decrease with height. So it is quite possible to have pressure increasing with height if the reference density is wrong.

[ROY4]

Originally Posted by ghorrocks

In a buoyant simulation the pressure variable is replaced with a buoyancy corrected pressure, p(corr) = p + rho(ref).g.z; assuming z is the gravitational direction. If rho(ref) is set to the density of the incompressible fluid, then this will cause a column of fluid to have constant p(corr), even though the actual pressure will vary with z height.

If you set the rho(ref) too high or too low the correction will not cancel out the hydrostatic component. This will mean p(corr) will increase or decrease with height. So it is quite possible to have pressure increasing with height if the reference density is wrong.

you said "If rho(ref) is set to the density of the incompressible fluid, then this will cause a column of fluid to have constant p(corr), even though the actual pressure will vary with z height" I activate buoyancy for both phases and set the water density as the reference density for both of them(997 kg/m^3 ), Now I have two pressures in CFX-Post which are pressure and total pressure. And they both increase as the height of the column increases. you said that actual pressure will vary with z, do you know how it varies?
The airlift column also uses the water density as a reference density,
I want to know if I change the reference density to the air density(1.187)(as it is in FLUENT), do you think that the pressure will give a true result?
and is it right to activate buoyancy for both phases and use water density as the reference density?