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John222 August 10, 2012 14:38

Boundary Condition for a U-bend Duct

I need to run 2d simulation for a U-bend duct, with Re=10^5 and U=30.1m/s, reference pressure is 1.2 atmospheric.

What are the pressure boundary conditions at inlet and outlet?

If you know the answer please help as I have everything set and just need to the boundary conditions to continue.


Touré August 10, 2012 17:27

I assume that the velocity is given for the inlet. Use velocity inlet instead of pressure inlet.
If your outlet is connected to the atmosphere use the atmosphere pressure at the outlet. The atmosphere pressure is the same as the one found in operation condition in FLUENT (101325 Pa) in standard condition. This pressure changes depending on the altitude or the temperature.
Compute the viscosity of your fluid from the Reynolds number equation knowing the Re, the velocity and the diameter of your tube.
Go to material and change the viscosity of the fluid.
I hope that help.

John222 August 10, 2012 18:04

The reference pressure is 1.2 atmospheric for whole bend at a temperature of 21C and Re= 10^5.

My Tube diameter is 3.81cm.

So, you mean that I have to set the 30.1m/s at velocity inlet and pressure of 1.2atm for velocity outlet? Then set the viscosity in references.

If I changed the viscosity in references after simulation is done, does it take effect? Thanks alot!

Touré August 10, 2012 20:16

In general for this case, there are three options to define the flow:

1. Give the velocity inlet and the pressure outlet
2. Give the pressure inlet and the velocity outlet
3. Give the pressure at inlet and outlet

The first option is used most of the time.
You can not change viscosity in references. This is for dimensionless results.
You change the viscosity in Materials.

John222 August 10, 2012 20:30

I will go with first option. It's just confusing the value of pressure at outlet boundary condition. Is it the dynamics pressure or the reference pressure. I know that the flow has to be 1.2atm along the whole pipe. Do I have to subtract the dynamic pressure from the reference pressure of 1.2atm?

Touré August 10, 2012 23:44

The pressure stay the same value along the pipe only if you don't have loss by friction, the height remain the same and the velocity also remain the same, according to the Bernoulli equation. Most of the time the difference of pressure create the suction that drive the fluid.

If you chose Pressure outlet, let Gauge pressure to 0. That means the relative pressure between both side of the outlet is 0. According to Bernoulli's equation the summation of static, dynamic and potential pressures is constant. What does reference pressure means to you? If the reference pressure is the value of the ambient air pressure, you can put 1.2 atm in Operating conditions -> Operating pressure (Don't forget to convert in Pa or change the unit of pressure in atm with Set units)

mohammadkm August 12, 2012 17:52

Dear John22
Due to the Secondary flow which induces by centrifugal force, your problem should be modeled 3D.

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