topic Betreff: Very simple propeller disk in CFD Forum

Very simple propeller disk

Willa1980 — Mon, 11 Dec 2017 22:11:14 GMT

Hello everybody,

sorry for asking a dumb question. I feel a bit ashame, but I am totally new to CFD (but pretty experienced in eperimental flow analysis).

I want to simulate a very simple propeller disk.

I loaded a custom stp model that contains

a motor (just to test if I manage to load geometry),
a propeller disk (with a certain thickness, because I think I need a face on the inflow side with a negative pressure, and a face on the outflow side with positive pressure)
a fluid volume (the volume in which the calculation takes place).

The results that I get actually look surprisingly ok (I chose a low mesh resolution however). I know that the blue horizontal line (which is my propeller disk in side view) should not be there.

--> Unlike my simulation attempt, the propeller disk should actually have zero thickness, correct? And on this zero-thickness disk, I would ideally select the inflow face and apply negative pressure, and then the outflow face and apply positive pressure. But even when I sometimes manage to import a 3D model of a zero thickness circular face, then this face only has one side, and I can only assign one boundary condition to it. But I think I need two...?

I am sure that I am doing sth terribly wrong - who can help me with this issue?

Thank you!!!

Betreff: Very simple propeller disk

Willa1980 — Mon, 11 Dec 2017 22:11:57 GMT

last image...

Betreff: Very simple propeller disk

matt.bemis — Tue, 12 Dec 2017 01:50:37 GMT

Hi @Willa1980,

Thanks for reaching out to us on the forums! No need to be ashamed about being unfamiliar with CFD- this is the perfect place to learn and ask questions.

I believe you want a rotating region around the propeller. This is some kind of pump blade/impeller, right? In CFD we put a fluid volume around the impeller geometry and then the volume is rotated while the impeller stays stationary.

The blue line is a low velocity because of the ABS disc you have above the impeller device, yes?

There's a great webinar on modeling pumps/fans with rotating regions. Check it out here. I also suggest you go through the centrifugal pump tutorial model. This uses a rotating region and I think you will find it very helpful.

Thanks,

Betreff: Very simple propeller disk

Willa1980 — Tue, 12 Dec 2017 07:16:20 GMT

Dear Matt,

thanks for your reply. I want to neglect the rotation of the propeller and only model it a s an actuator disk:

Neglect rotation imparted to the flow.
Assume the Mach number is low so that the flow behaves as an incompressible fluid.
Assume the flow outside the propeller streamtube has constant stagnation pressure (no work is imparted to it).
Assume that the flow is steady. Smear out the moving blades so they are one thin steady disk that has approximately the same effect on the flow as the moving blades (the ``actuator disk'').
Across the actuator disk, assume that the pressure changes discontinuously, but the velocity varies in a continuous manner

Any ideas how to do this? Sounds actually so simple, but I don't find a way to do it...

Betreff: Very simple propeller disk

David.Short. — Tue, 12 Dec 2017 09:20:02 GMT

Hi @Willa1980,

Please correct me if I'm wrong but i thought that an actuator disk is more of a theoretical concept rather than something that can actually be modeled in reality and as CFD is attempting to model reality I don't think this will be possible. Firstly we can't use an infinitely thin part and also we cannot apply internal pressure boundary conditions so you might be out of luck here (but I could be wrong).

What are you trying to approximate / represent using an actuator disk?

All the best,

David

Betreff: Very simple propeller disk

Willa1980 — Tue, 12 Dec 2017 11:07:12 GMT

Hi,

I think, I was searching at the wrong location... Currently it seems that Autodesk CFD is offering exactly what I want (an actuator disk) in the material database: "Internal fan". The german translation of the whole UI is pretty bad imho, so I really have difficulties to find what I am looking for.

But here, I can assign a velocity profile across the disk to model a (simplified) propeller. That should work.

BTW, is there a way to switch the UI language to english? I downloaded the english version, but everything is in german now.

Betreff: Very simple propeller disk

David.Short. — Tue, 12 Dec 2017 11:56:45 GMT

Excellent news. Yes an internal fan device will act just like an actuator disk described in theory. I thought you actually wanted to model an actuator disk to prove the theory...sorry for the confusion!

To change the language you can run the CFDConfig.exe file as administrator. This will give the option again for language selection.

Best,

David

Betreff: Very simple propeller disk

matt.bemis — Tue, 12 Dec 2017 13:35:59 GMT

Hi @Willa1980,

Okay, it makes sense to use an internal fan. I am glad you found that as it may have not been something I would think of.

Good luck with the modeling!

Thanks,