[hwangpo]

Hey guys,
I am stuck on the problem to compute the pressure distribution of buoyancy flow, a large reservoir with temperature stratification.

The CFX theory/modeling Guide says:
"For buoyancy calculation, a source term is added to the momentum equations as follows: S_m,buoy = (rho - rho_ref)*g.
When buoyancy model is activated, the pressure in the momentum equation excludes the hydrostatic gradient due to rho_ref. This pressure is related to the absolute pressure as follows:
P_abs = P + P_ref + rho_ref*g(r - r_ref) where r_ref is a reference location."

I learned that 'pressure' at the inlet boundary can be computed from equation like this
'integration dP =Integration (rho - rho_ref)*g dz'
where z is the depth direction.

See the picture as well.
The model is simplified as shown in the figure. The tank has a vertical temperature stratification(i.e. temperature is a function with regard to depth), and the density is a function of temperature, and the pressure is relevant with the density but I am not sure how to calculate.
So, my question is, how can I set the static pressure at the inlet boundary? Does it matter to the results?

I computed the pressure using the method in the figure, and the simulation is not converged. I do not figure out the problem. Any suggestions?


I will very appreciate your help. Thank you very much.

[ghorrocks]

If you are defining pressure then can't you use a single constant value over the entire inlet? As the documentation says that hydrostatic pressure component has been taken out of the pressure variable.

[hwangpo]

Originally Posted by ghorrocks

If you are defining pressure then can't you use a single constant value over the entire inlet? As the documentation says that hydrostatic pressure component has been taken out of the pressure variable.

Thank you for your reply. And I am confused if the static pressure, which can be set as a constant according to your suggestion, including the part due to density difference. Does this mean the pressure here is not incorporated in the equation and the CFX solver? Is there any reference or sth like that? I have read all the CFX help/guide and still not clear about this.

Thank you so much.

[hwangpo]

another question:
consider no thermal stratification, inlet pressure can be set to constant?
I did that before.
I want to confirm that.
Thank you!

[ghorrocks]

Does this mean the pressure here is not incorporated in the equation and the CFX solver?

I do not understand this question. Of course pressure is included in the equations. It is just that the hydrostatic component plays no part in determining the fluid flow in an incompressible simulation, so it might as well be removed.

Yes, the inlet can be set to a constant with no thermal stratification as well.

You should just try these options and find this all out for yourself.

[hwangpo]

I see.
Thank you so much.