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

boundary condition: alpha*dp/dt = u&n

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

Reply
 
LinkBack Thread Tools Display Modes
Old   July 22, 2013, 05:39
Question boundary condition: alpha*dp/dt = u&n
  #1
New Member
 
Andreas Ruopp
Join Date: Aug 2009
Location: Stuttgart / Germany
Posts: 29
Rep Power: 8
andyru is on a distinguished road
Dear Foamers,

normally, on wall boundaries for example, we must guarantee:
Code:
0 = U&n
where U is the velocity vector and n is the patch normal surface vector,
and the inner product has to be zero.
On a specific patch, I impose a diriclet boundary condition for the pressure.
For the velocity, I want to have:
Code:
alpha*dp/dt = U*n
I'm thinking about treating this patch as a modified free slip wall, but I'm wondering, how I can impose that mentioned boundary condition for U.
A slip wall uses in the end the basicSymmetryFvPatchField method, resulting in 0 = u&n
Code:
tmp<vectorField> nHat = this->patch().nf(); 
00116  
00117     const Field<Type> iF(this->patchInternalField()); 
00118  
00119     Field<Type>::operator= 
00120     ( 
00121         (iF + transform(I - 2.0*sqr(nHat), iF))/2.0 
00122     ); 
00123  
00124     transformFvPatchField<Type>::evaluate();
For example if the cell value next to patch face is in 2D:
if=(1 1)
and nf is normal patch face vector pointing outward with
nf=(0 1)
Then evaluate returns a vector
(1 0)
resulting from,
((1 1) + transform([1 0;0 1] - [0 0 ;0 2],(1 1)])/2
ok.
And then
n&U is zero, hence 0=1*0+0*0
and
0 = U&n is fullfilled.

But how can
Code:
alpha*dp/dt = U*n
be achieved?

Any help will be appreciated!

Best regards,

Andy
andyru is offline   Reply With Quote

Old   July 23, 2013, 05:59
Default After a long night... direction mixed boundary condition approach, maybe?!
  #2
New Member
 
Andreas Ruopp
Join Date: Aug 2009
Location: Stuttgart / Germany
Posts: 29
Rep Power: 8
andyru is on a distinguished road
Hello,

here my update, maybe things become clearer:

since I want to impose
Code:
alpha*dp/dt = U*n
where alpha*dp/dt represents a velocity term for that boundary in normal direction to that face, I was digging in under
.../src/finiteVolume/fields/fvPatchFields/basic/
where I found directionMixedFvPatchField.C

If I understand it right:
For a vector (like in my case it is velocity U), this boundary condition is decomposed into the normal and tangential component.

I have to set three reference values for that boundary condition (good description under DirectionMixed mixed bc:
1) refValue = which will be the value on face in normal direction
2) refGradient = will be enforced in the patch tangential direction
3) valueFraction = which is the symmetrical Tensor, which I have to define for transformation purposes

I think this b.c. is a good starting point for manipulating my boundary condition for U...

Best regards,

Andy
andyru is offline   Reply With Quote

Reply

Tags
boundary, condition, slip, treament, wall

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
Question about heat transfer coefficient setting for CFX Anna Tian CFX 1 June 16, 2013 06:28
Error finding variable "THERMX" sunilpatil CFX 8 April 26, 2013 07:00
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
Boundary conditions? Tom Main CFD Forum 0 November 5, 2002 02:54


All times are GMT -4. The time now is 18:15.