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

Boundary conditions for rotating reference frame

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

Reply
 
LinkBack Thread Tools Display Modes
Old   August 24, 2011, 07:54
Default Boundary conditions for rotating reference frame
  #1
New Member
 
Borna Obradovic
Join Date: Aug 2011
Posts: 1
Rep Power: 0
Borna is on a distinguished road
Hi All,

I'm a new OpenFOAM user and new to the community, so my apologies in advance if this is overly simple a question.

I'm trying to simulate a rotating fan blade, using the single rotating frame approach. The results look wrong, and I suspect I'm missing something in the boundary conditions. My approach is the following

1. Modify icoFoam to include Coriolis and centrifugal terms (as discussed elsewhere in the Forums) as follows:

dimensionedScalar angularVel(transportProperties.lookup("angularVel" ));
dimensionedVector Omega("Omega",
dimensionSet(0, 0, -1, 0, 0),
vector(0, angularVel.value(), 0)
);

volVectorField Fcent = (Omega ^ (Omega ^ mesh.C()));

fvVectorMatrix UEqn
(
fvm::ddt(U)
+ fvm::div(phi, U)
- fvm::laplacian(nu, U)
+ (2*Omega ^ U) // Added Coriolis force
+ Fcent // Added centrifugal force
);


2. Geometry:
My first testcase is just a cylindrical geometry which represents the bounding box of the simulation. It will eventually contain the fan geometry, but I wanted to verify the behavior on a very simple testcase. The cylinder axis is along "y", and is the axis of rotation. This is generated using blockMesh, and looks good.

3. Boundary conditions
The "side" of the cylinder has a fixed pressure condition (arbitrary fixed value, incompressible flow), since I'm assuming that the cylinder boundary is far enough away from the future geometry in the center that it is essentially free stream. Given that it is observed from a rotating frame, it should be uniform along the cylinder boundary (right?)

The top and bottom of the cylinder have zeroGradient pressure and velocity conditions. The velocity boundary condition on the side of the cylinder is set to slip - I'm trying to let the UEqn naturally set the velocity.


The results are not at all what I expected. Setting a finite rotational velocity, I expected to find a rotational U field, azimuthally symmetric, linearly varying radially. Instead, I have zero velocity everywhere. Interestingly though, the pressure field looks qualitatively OK, azimuthally symmetric with radial variation roughly as expected (correct value at the boundary).


Are my expectations incorrect? It seems to me that after adding the Coriolis and cent. terms, the "U" we are solving for is the velocity in the rotating frame, which should be non-zero.
What am I missing with the BCs? BTW, the solution looks fully converged, many more steps taken than were actually required for convergence.

Many thanks,

Borna
Borna is offline   Reply With Quote

Old   August 24, 2011, 10:25
Default
  #2
New Member
 
Jianxi Yao
Join Date: Apr 2011
Posts: 15
Rep Power: 6
jianxiyao is on a distinguished road
Quote:
Originally Posted by Borna View Post
Hi All,

I'm a new OpenFOAM user and new to the community, so my apologies in advance if this is overly simple a question.

I'm trying to simulate a rotating fan blade, using the single rotating frame approach. The results look wrong, and I suspect I'm missing something in the boundary conditions. My approach is the following

1. Modify icoFoam to include Coriolis and centrifugal terms (as discussed elsewhere in the Forums) as follows:

dimensionedScalar angularVel(transportProperties.lookup("angularVel" ));
dimensionedVector Omega("Omega",
dimensionSet(0, 0, -1, 0, 0),
vector(0, angularVel.value(), 0)
);

volVectorField Fcent = (Omega ^ (Omega ^ mesh.C()));

fvVectorMatrix UEqn
(
fvm::ddt(U)
+ fvm::div(phi, U)
- fvm::laplacian(nu, U)
+ (2*Omega ^ U) // Added Coriolis force
+ Fcent // Added centrifugal force
);


2. Geometry:
My first testcase is just a cylindrical geometry which represents the bounding box of the simulation. It will eventually contain the fan geometry, but I wanted to verify the behavior on a very simple testcase. The cylinder axis is along "y", and is the axis of rotation. This is generated using blockMesh, and looks good.

3. Boundary conditions
The "side" of the cylinder has a fixed pressure condition (arbitrary fixed value, incompressible flow), since I'm assuming that the cylinder boundary is far enough away from the future geometry in the center that it is essentially free stream. Given that it is observed from a rotating frame, it should be uniform along the cylinder boundary (right?)

The top and bottom of the cylinder have zeroGradient pressure and velocity conditions. The velocity boundary condition on the side of the cylinder is set to slip - I'm trying to let the UEqn naturally set the velocity.


The results are not at all what I expected. Setting a finite rotational velocity, I expected to find a rotational U field, azimuthally symmetric, linearly varying radially. Instead, I have zero velocity everywhere. Interestingly though, the pressure field looks qualitatively OK, azimuthally symmetric with radial variation roughly as expected (correct value at the boundary).


Are my expectations incorrect? It seems to me that after adding the Coriolis and cent. terms, the "U" we are solving for is the velocity in the rotating frame, which should be non-zero.
What am I missing with the BCs? BTW, the solution looks fully converged, many more steps taken than were actually required for convergence.

Many thanks,

Borna

you can try the SimpleSRFFoam for you case.
jianxiyao is offline   Reply With Quote

Reply

Tags
bcs, rotating frame

Thread Tools
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 On
Pingbacks are On
Refbacks are On


Similar Threads
Thread Thread Starter Forum Replies Last Post
Installing OpenFOAM-1.5-dev on a cluster ZKM OpenFOAM Installation 4 December 25, 2010 16:59
Constant velocity of the material Sas CFX 15 July 13, 2010 08:56
RPM in Wind Turbine Pankaj CFX 9 November 23, 2009 05:05
CFX doesn't continue calculation... mactech001 CFX 6 November 15, 2009 22:25
How to apply negtive pressure to outlet bioman66 CFX 5 June 3, 2006 01:40


All times are GMT -4. The time now is 00:32.