[MissCFD]

Hello,

I am new with Fluent and I don’t understand something about the hydrostatic contribution.

I simulate a simple water flow in a circular pipe with a velocity at inlet and a static pressure at outlet.

I read that in order to display the hydrostatic contribution in the static pressure, we have to activate the gravity and set the specified operating density option to zero if we assume a fluid density of 1000 kg/m^3.

So, by deactivating the gravity, I thought that the static pressure difference between the two models (gravity activated, and gravity not activated) will be equal to the hydrostatic contribution but it’s not the case in my simulations …

Please, can somebody explain it to me or tell me if I made a mistake somewhere ?

[LuckyTran]

Probably you make an unclear mistake somewhere. Instead of deactivating gravity, try just setting it to 0.

With gravity, are you able to successfully see the hydrostatic pressure?

Originally Posted by MissCFD

will be equal to the hydrostatic contribution but it’s not the case in my simulations …

What do you mean? Hydrostatic contribution to the flow or what? If you want to get the hydrostatic part of the pressure, you could just quickly calculate rho*g*h from a custom field function.

[MissCFD]

Yeah I can see the hydrostatic pressure but the reference for the ''h'' is not good.
Fluent seems to take the top of the pipe for h=0 and not the center where the coordinates are (0,0,0). Is it normal ? How can we set the reference height ?

[LuckyTran]

Well it depends how you set the reference pressure.

[MissCFD]

Originally Posted by LuckyTran

Well it depends how you set the reference pressure.

Do you talk about the operating pressure or the reference pressure?

To give you a better understanding of my problem, here is a picture of the pipe.
The coordinates (0,0,0) are at the center of the inlet face.
The radius of the circle is 2m.

Boundary conditions: velocity of 0.5m/s at inlet and static pressure = 0Pa at outlet

The operating pressure is 101325 + 998*9.81*5 = 150 200 Pa at (0,0,0)
Gravity is activated (0,-9.81,0) and operating density is set to 0.

I don’t understand why when I display the static pressure at inlet, I have -7600Pa at the top of the pipe (y=2m) and 30000Pa at the bottom of the pipe (y=-2m).

I expected to have something around -15000/-20000Pa at the top and +15000/20000Pa at the bottom (a smaller pressure than 998*9.81*2=20 000Pa because of the velocity of the fluid).
But it seems that Fluent takes the top of the pipe for h=0, why is it like this ?

[LuckyTran]

Oops, operating pressure not reference pressure. It's easy to get confused because you set the operating pressure at a reference pressure location.

Are you assuming that the static pressure at 0,0,0 is going to be 0 Pa? Because you set the outlet static pressure as 0 Pa, which means the inlet would need to be greater static pressure. When you have a pressure boundary, you cannot specify the operating pressure to be at a specific location. The reference pressure location (0,0,0) is ignored.


What is your equation of state for density?

[MissCFD]

Originally Posted by LuckyTran

Oops, operating pressure not reference pressure. It's easy to get confused because you set the operating pressure at a reference pressure location.

Are you assuming that the static pressure at 0,0,0 is going to be 0 Pa? Because you set the outlet static pressure as 0 Pa, which means the inlet would need to be greater static pressure. When you have a pressure boundary, you cannot specify the operating pressure to be at a specific location. The reference pressure location (0,0,0) is ignored.


What is your equation of state for density?

I impose a velocity at inlet and a static pressure at outlet. So I don't assume the pressure at (0,0,0) is going to be 0 Pa.

I simulate an incompressible flow with the pressure-based solver .

Do you understand why I am unable to have the good hydrostatic pressure ?
What is the reference coordinate for the ''h'' in the hydrostatic contribution (rho*g*h) ? Where is this parameter set ?

[LuckyTran]

There is no reference h, I mentioned rho*g*h simply to demonstrate a means to back-calculate the hydrostatic part. There is only a (rho-rho_ref)*g in the momentum equation.


What is your equation of state for density? Incompressible flow is not an option in Fluent.

[MissCFD]

Originally Posted by LuckyTran

There is no reference h, I mentioned rho*g*h simply to demonstrate a means to back-calculate the hydrostatic part. There is only a (rho-rho_ref)*g in the momentum equation.


What is your equation of state for density? Incompressible flow is not an option in Fluent.

Density constant

[LuckyTran]

At this point I would recommend to just set the operating presssure to 0 so you can view the absolute pressure.


I can't follow which way the pipe is oriented and what elevation your outlet is at to say what the pressure at the inlet would be. Just know that the pressure level is established based on where you set the pressure boundary.


Setting the operating density to 0 and constant pressure at the outlet adds a bit of a complication because you say the absolute pressure is constant despite having a hydrostatic contribution.