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-   -   Rotation Reference Frame Boundary Conditions (http://www.cfd-online.com/Forums/fluent/116022-rotation-reference-frame-boundary-conditions.html)

 Brathmann April 11, 2013 05:46

Rotation Reference Frame Boundary Conditions

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Hello everybody.

For a propeller-like geometry in a closed volume I want to model the air flow, namely dynamic pressure, resulting from the rotor movement. Most hints how to do that which I found pointed out to use a rotating reference frame. I did that but end up with results that don't match each other at the contact faces between the moving and the rotating fluid zone. Do you have any idea what the problem might bee?

Here is what I did so far: (If you lack information, I's be happy to provide more. I'm still not so sure, which information is relevant for the problem)
I modeled a cyclic periodic region of the problem, meshed it with Face Sizings and obtained a mesh of about 0.2 othogonal quality and 10 aspect ration. Which isn't super, as far as I got it, but did lead to convergence. After defining the small, disk like fluid zone (s. pic: PeriodicRegion) as rotationally moving frame with rot. velocity of 90 rad and the adjacent rotor-wall as rotationally moving wall with relative velocity of 0m/s i sucessfully made periodic boundaries for the symmetry boundaries in the model and set the contact region between the moving fluidzone and the rest of the flow channel as standard interfaces.
With the standard visous k-e-model, Coupled Scheme as Solution Methods using Second Order Upwind discretizations for Momentum, Turbulent Kinetic Energy and Turbulent Dissipation Rate convergence is quick (worst convergence is Continuity with 0.001) and results seem reasonable.

The problem is, that the calculated values clearly don't match in the contact area of the two fluid zones as you can see in pic. PressureDyn. The spare area in the plane is the airfoil, rotating away from the spectator around the y-axis situated at the origin to the left of the countour plane.

Any hints? I would appreciate a lot.
Have a nice day
Brathmann

ANSYS Version: 14.0 workbench

 A CFD free user April 11, 2013 11:43

Hi pal
I didn't get the exactly geometry of your model. But, if you have something like a stirred tank( an impeller inside a tank which rotates) I already have done it. You have to get a distinguished circulating zone. I can't recall the details exactly. I might be able to send you something useful if your model is similar to what I described. Contact me via:
aboozar.azarafza@gmail.com
Good luck

 ghost82 April 12, 2013 02:55

Hi, Azarafza is right, this is similar to a stirred tank simulation: steps are:
- defining the rotating fluid zone with rotational velocity and rotational axis;
- assign to the rotational wall of the propeller a relative velocity adjacent to the fluid zone of 0 rad/s
- define the other fluid zone (external to the rotating zone) as stationary
- define periodic condition
- define outer fixed wall as stationary or moving zone with absolute velocity of 0 rad/s

If you can try to avoid interfaces by making a structured mesh, since in some cases you can observe some discontinuities at interfaces.

Daniele

 Brathmann April 12, 2013 05:05

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Hey,

@Azarafza, you got the geometry right. Basically it's an impeller in a Fluid only not in a normal tank but in a hollow cylinder. To reduce calculation time I only modeled a part of the geometry (1/8). Thanks for you Mail adresse, I'll contact you for you model.

@ghost82:
All of the points you mention I did.
"- define outer fixed wall as stationary or moving zone with absolute velocity of 0 rad/s" for me means, leaving the conditions at default,

About the structured mesh, i'm not very sure:
The mesh on the interfaces between the two fluid zones are both defined by a face sizing of type element size which results in triangular mesh on the rotating surface and rectangular mesh on the stationary surface. I think, that's not stuctured, so as you recommended. Is that right?
Or do you mean, I can avoid the existence of the interface by using a structured mesh? But in this case, how do I define a rotating and a stationary fluid zone, if I don't have two bodies with contact surfaces?

For the Mesh Interfaces in FLUENT I tried with and without the Periodic Repeats Option. But the discontinuity between the zones is there in both cases.

Attached you find another picture of the geometry. I modeled 1/8 of the whole tank with the rotor blade as moving wall and the moving fluid zone as the disc around it .