[Janfi]

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!

[LoGaL]

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

[Janfi]

Originally Posted by LoGaL

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?

[LoGaL]

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.

[Janfi]

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?

[LoGaL]

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

[Janfi]

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)