Boundary conditions in fluent are ignored
 

Hi,


i am a beginner in fluent and have a problem with my polymer nozzle flow.


I set a velocity inlet condition: 0.01364 m/s, supersonic pressure = 0 Pa


and a pressure outlet condition (atmosphere pressure): gauge pressure = 0 Pa (my first try was: gauge pressure = 101325 Pa), Backflow Pressure Specification = total pressure.
The rest boundaries are walls with default settings.


- Solver: pressure-based
- steady
- laminar
- without energy Equation
- The material is pa12 as fluid
- solution method: coupled, Rhie-Chow: distance based, Least Squares Cell Based, Second Order, Second Order Upwind


During initialization the message: pressure information is not available at the boundaries.


And as result I do not have a flow, the velocity is zero. And the pressure is different from my outlet pressure.


Where is the error? Does anyone have an idea?


I hope you understand my words!


Thank you!

the boundary conditions are correct. I think something is screwed up in your geometry

Thank you LoGaL! I try it with a new mesh and will give a feedback :).

Which gauge pressure is the right one for the pressure outlet at atmospheric pressure?

Depends on what operating pressure you choose. (google the fluent button location)

The "real pressure" is operating pressure + gauge pressure. So if you set the operating pressure to 0, the gauge pressure is 1 atm, if the operating pressure is 1 atm, the gauge pressure is zero.

Keep in mind that, if you solve incompressible flow, the absolute value of pressure actually doesn't matter.

OK, I think I understand. Thank you!

But what does ist mean: "the absolute value of pressure doen't matter"?
I am student, simulate for our chair and am not yet so proficient in cfx application.

What could be the error of geometry. I am only simulating the fluid in the nozzle. There is a rod axially in the nozzle. Could that be the problem?

it is not abouyt cfx, it is about physics: in incompressible flow, what matters is the pressure gradient ( or difference) because it moves things. ( check navier stokes equations! there's only pressure gradient)

In compressible flow instead (e.g. air), the absolute pressure value matters because it goes into PV = nRT and determines the density


The error in your geometry: I have no idea, especially because you didn't post any pictures :)

Oh that's what it was referring to in your message. Sure, that's right.

I imported the geometry as a step (from inventor). The inlet is on the left/upper side; the outlet is at the bottom. The rest parts are walls (including the rod surface). (See pdf)