CFD Online Logo CFD Online URL
www.cfd-online.com
[Sponsors]
Home > Forums > Software User Forums > OpenFOAM

Should an empty MRF zone really induce this much cross-flow?

Register Blogs Members List Search Today's Posts Mark Forums Read

Like Tree2Likes
  • 1 Post By eugene
  • 1 Post By eugene

Reply
 
LinkBack Thread Tools Search this Thread Display Modes
Old   November 12, 2010, 06:28
Default Should an empty MRF zone really induce this much cross-flow?
  #1
Member
 
Roland
Join Date: Mar 2009
Location: Netherlands
Posts: 91
Rep Power: 17
sylvester is on a distinguished road
Hi,

As a test setup we have placed an empty MRF zone in a flow with with uniform freestream conditions, calculated using MRFSimpleFoam.
The MRF zone is a cylinder with the axis pointing in Z-direction. The freestream velocity is in X-direction. The cylinder has a radius of 1 m, see attached 'geometry-s.jpg'.

This is our MRF definition:
Code:
    cylinder
    {
        nonRotatingPatches ();
        origin  origin  [0 1 0 0 0 0 0] ( 0. 0. 0. );
        axis    axis    [0 0 0 0 0 0 0] ( 0. 0. 1. );
        omega   omega   [0 0 -1 0 0 0 0] 120.;
    }
The velocity is specified as:
Code:
internalField   uniform (50 0 0);

boundaryField
{
    sphere
    {
        type            freestream;
        freestreamValue $internalField;
    }
}
And the pressure as:
Code:
internalField   uniform 0;

boundaryField
{
    sphere
    {
        type            freestreamPressure;
    }
}
Our expectation was that, as the MRF zone is empty, the flowfield would be unaffected by it. Clearly this is not the case, as the screenshots of the resulting flowfield show.

We contacted OpenCFD about this (http://www.openfoam.com/mantisbt/view.php?id=78), but their reply was that an MRF region will always affect a cross-flow even if it is empty. But surely an empty MRF zone cannot have such a big effect?

Is this really what should happen, or is something going wrong here?

Sylvester

ps. OF 1.7.x is used, updated today. The mesh consists of tetrahedra only, created using ANSA.
Attached Images
File Type: jpg geometry-s.jpg (57.0 KB, 220 views)
File Type: jpg U-s.jpg (23.0 KB, 216 views)
File Type: jpg Glyps-s.jpg (57.7 KB, 197 views)
File Type: jpg p-s.jpg (22.1 KB, 180 views)
sylvester is offline   Reply With Quote

Old   November 15, 2010, 14:22
Default
  #2
Member
 
Masashi Ohbuchi
Join Date: Oct 2009
Posts: 74
Rep Power: 16
Ohbuchi is on a distinguished road
Hi,

I'm really surprised with your result.
Then, I've tried another case with sphere boundary as wall, and initial U=0.
In this case, fluctuation caused by MRF was considerably small.
So I suppose this phenomena was caused by interaction free stream boundary with MRF.
Ohbuchi is offline   Reply With Quote

Old   November 16, 2010, 05:49
Default
  #3
Senior Member
 
Eugene de Villiers
Join Date: Mar 2009
Posts: 725
Rep Power: 21
eugene is on a distinguished road
Unfortunately, empty MRFs do have a huge influence. I bashed my head against the problem for more than a week and dissected the governing equations down to the bones. What you see is "correct". In general, I would say an MRF with spin axis perpendicular to the flow does not behave very nicely unless it is encased (i.e. has some kind of solid rim).

A free MRF with flow perpendicular spin axis and no rim will produce non-physical (but correct according to the model) results.
bennn likes this.
eugene is offline   Reply With Quote

Old   November 16, 2010, 18:00
Default
  #4
Senior Member
 
BastiL
Join Date: Mar 2009
Posts: 530
Rep Power: 20
bastil is on a distinguished road
Interesting quote Eugene. I was not aware of this. C0uld you please go little more into detail?
This means there should be not MRF-Zone exceeding solid rims, right?

Regards Bastian

Quote:
Originally Posted by eugene View Post
Unfortunately, empty MRFs do have a huge influence. I bashed my head against the problem for more than a week and dissected the governing equations down to the bones. What you see is "correct". In general, I would say an MRF with spin axis perpendicular to the flow does not behave very nicely unless it is encased (i.e. has some kind of solid rim).

A free MRF with flow perpendicular spin axis and no rim will produce non-physical (but correct according to the model) results.

Last edited by bastil; November 17, 2010 at 03:48.
bastil is offline   Reply With Quote

Old   November 16, 2010, 19:49
Default
  #5
Senior Member
 
Eugene de Villiers
Join Date: Mar 2009
Posts: 725
Rep Power: 21
eugene is on a distinguished road
Hi Bastil,

I can speculate on what the all this means, but I am not really sure, so don't take anything I say here as the gospel truth.

If you take the MRF equations and write them all in terms of the absolute velocity, you end up with at least one term that does not contain a gradient and will be non-zero in a uniform cross-flow: (omega x Ui). If the flow is purely swirling around the axis, this term exactly balances another term that does have a gradient of U in it.

If the directions of omega and Ui are the same, then the cross product is zero. If not, then it results in a net force. So a perfectly uniform flow field passing through an empty MRF can be deflected, just as if there was some kind of rotor occupying the volume. In general, I think an MRF should be constructed such that the cell zone matches the swept volume as closely as possible. Extending the MRF to any volume outside this, could potentially lead to additional non-physical forces.

In an ideal world, we would have automatic detection of the swept volume based on solid body rotation and some kind of flux jump boundaries to take care of the non-perpendicular MRF internal boundary faces. Maybe, we could just turn off the additional MRF momentum term in non-swept parts of the MRF volume. The main thing to note, is that there is a potential source of error inherent in using MRF and caution should be exercised when it is being applied.
Raphael_Santos likes this.
eugene is offline   Reply With Quote

Old   November 18, 2010, 04:48
Default
  #6
Member
 
Roland
Join Date: Mar 2009
Location: Netherlands
Posts: 91
Rep Power: 17
sylvester is on a distinguished road
Eugene, thanks for your excellent explanation. We tested it on a spinning wheel and indeed when we use a non-protruding MRF zone the cross-flow no longer appears.
sylvester is offline   Reply With Quote

Reply

Tags
mrf, mrfsimplefoam

Thread Tools Search this Thread
Search this Thread:

Advanced Search
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are On
Refbacks are On


Similar Threads
Thread Thread Starter Forum Replies Last Post
[blockMesh] StitchMesh on two patches anita OpenFOAM Meshing & Mesh Conversion 31 April 4, 2013 12:51
how to treat the rotating zone in multiple reference frame (MRF) Jiuan FLUENT 4 May 24, 2010 22:58
Aerodynamics of Cross flow fan Yerram Ravinder Main CFD Forum 1 September 6, 1999 02:07
Flow visualization vs. Calculated flow patterns Francisco Saldarriaga Main CFD Forum 1 August 3, 1999 00:18
Question on 3D potential flow Adrin Gharakhani Main CFD Forum 13 June 21, 1999 06:18


All times are GMT -4. The time now is 23:41.