[MissCFD]

Hello,

I need to understand many things:

- In which case, we will use the option "Buoyant" ? I read that for a non-buoyant flow, the hydrostatic pressure doesn't exist ... Where is the term "gz" in the momentum equations in CFX ?

- Does the term "rho*g*h" take into account in the term "P" of momentum equations in CFX ?

For example, if I simulate a vertical pipe flow with a diffuser in a steady state study and the fluid is incompressible water, is hydrostatic pressure included in absolute pressure ?
If not, why ?

Thank you very much !

[JuPa]

Hello,

"For example, if I simulate a vertical pipe flow with a diffuser in a steady state study and the fluid is incompressible water, is hydrostatic pressure included in absolute pressure?"

In a vertical pipe, without gravity switched on, hydrostatic pressure is not included in the absolute pressure.

"If not, why?"

Because hydrostatic head is a type of body force due to gravity. If you don't switch on gravity then you simple don't have a hydrostatic head.

P_absolute = P_dynamic + P_static + P_hydrostatic

Without gravity P_hydrostatic = 0, and then you are just left with

P_absolute = P_dynamic + P_static


Edit: However, try this and see if hydrostatic head is included without buoyancy:

1.Go into CFX Pre. In Default Domain do not turn on gravity (no gravity terms are on).

2. Go to solver control, go to advanced options, and select pressure level information.

3. Select cartesian coordinates and provide the coordinates where your reference pressure is.

4. Select your relative pressure level.

5. Run a simulation. Is hydrostatic pressure activated?

[Opaque]

Better not be included. If the gravity vector is not defined, there is no way to add the hydrostatic pressure.

Recall the body force due to gravity is g dot (r - r_ref).

[MissCFD]

Thank you to both for your reply

But, what is (r-ref) ? I thought that in N.S equations, the body force du to gravity intervened through the term rho*g ...

And why, in most current applications of engineering, we don't defined the gravity vector ?

Originally Posted by Opaque

Better not be included. If the gravity vector is not defined, there is no way to add the hydrostatic pressure.

Recall the body force due to gravity is g dot (r - r_ref).

[ghorrocks]

You only define a gravity vector when gravity is significant.

Examples of where it is insignificant include: single phase pipe flow, IC engine modelling, airfoil modelling (just to name a few). Pretty big areas of engineering do not require gravity.

[MissCFD]

- But in a vertical pipe flow for example, the driving force is gravity ...
Does that mean that when I impose a velocity inlet and a pressure outlet, the pressure difference between inlet and outlet include the term ''rho*g*h'' ?

- Does the hydrostatic pressure contribution is included in the Pressure when buoyant condition isn't actived ?

Originally Posted by ghorrocks

You only define a gravity vector when gravity is significant.

Examples of where it is insignificant include: single phase pipe flow, IC engine modelling, airfoil modelling (just to name a few). Pretty big areas of engineering do not require gravity.

[ghorrocks]

The driving force might be gravity but that driving force may also be expressed as a simple pressure drop. The question is whether gravity acts as a force inside the simulation domain or not.

If there is no buoyancy (ie no gravity) then there is no hydrostatic head (rho*g*h = 0).