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

Mixed/Robin boundary condition

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

Like Tree1Likes
  • 1 Post By aaev

Reply
 
LinkBack Thread Tools Display Modes
Old   February 10, 2010, 16:09
Default Mixed/Robin boundary condition
  #1
New Member
 
Anton Evgrafov
Join Date: Jun 2009
Posts: 3
Rep Power: 8
aaev is on a distinguished road
I am quite new to OpenFOAM and therefore am not entirely sure whether this is a bug or a feature, but results one obtains when selecting valueFraction in ]0,1[ when using 'mixed' boundary condition (i.e., a truly mixed/Robin-type boundary condition, not a pure Dirichlet or a Neumann one) depend on the grid size, which seems quite startling to me. For example, I attach a small effectively one-dimensional test-case (tar-ed gzip-ed test_bc.tar.gz) of a steady-state heat conduction problem on a unit square, where at the bottom boundary a mixed BC is prescribed with refValue=-1.0, and refGradient=2.0. The solution to this problem varies linearly from top (where T=0 is prescribed) to bottom, so for a purely Dirichlet problem (valueFraction=1) T_Dir=-1.0 at the "mixed" boundary, and for purely Neumann case (valueFraction=0) T_Neum=n*grad(T)=2.0 there. Therefore, say for valueFraction = 0.5 I would expect T=0.5*(T_Dir + T_Neum) = 0.5; however, one may easily check that this value depends on the number of elements in the mesh and in fact converges to T_Dir as the mesh is refined.

Such behaviour can be fixed if one uses

newValueFraction_=valueFraction_/(valueFraction_ + (1.0-valueFraction_)*this->patch().deltaCoeffs());

in place of valueFraction_ in the implementation of the mixed boundary condition (I can sketch a small note why, and I tried to write a comment about this in my amateurish C++ implementation of mixed1 boundary condition-also in the attached archive), which now produces expected mesh-independent results. You can try it out by building the corrected mixed1 boundary condition (wmake libso in mixed1BC.d); and then changing mixed->mixed1 in test_bc/0/T file and uncommenting the 'libs' line in test_bc/system/controlDict to load the mixed1 BC.

Please let me know if I mis-understand the purpose or the expected behaviour of the original mixed boundary condition in OpenFOAM.

Sincerely,
--Anton Evgrafov
Attached Files
File Type: gz test_bc.tar.gz (4.6 KB, 80 views)
mm.abdollahzadeh likes this.
aaev is offline   Reply With Quote

Old   December 15, 2011, 14:18
Default Thanks
  #2
Member
 
Ilya
Join Date: Dec 2011
Location: Russia
Posts: 82
Blog Entries: 38
Rep Power: 5
skeptik is on a distinguished road
Today I found that Robin BC give incorrect temperatures on the wall Twall>Tref.

Your post could be useful. Tomorrow I'll try It.

А ещё, Вы говорите по-русски?)
skeptik is offline   Reply With Quote

Old   December 15, 2011, 15:03
Default
  #3
New Member
 
Anton Evgrafov
Join Date: Jun 2009
Posts: 3
Rep Power: 8
aaev is on a distinguished road
Quote:
Originally Posted by skeptik View Post
А ещё, Вы говорите по-русски?)
da, govoru
aaev is offline   Reply With Quote

Reply

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
Boundary Conditions Thomas P. Abraham Main CFD Forum 20 July 7, 2013 05:05
Transient outlet boundary condition problem jwillie2000 CFX 1 December 7, 2009 18:07
Axis Boundary Condition..what is it? CFDtoy FLUENT 6 February 13, 2007 06:51
How to set boundary condition in Fluent for the fo Peiyong FLUENT 1 November 10, 2006 12:44
How to resolve boundary condition problem? sam FLUENT 2 July 20, 2003 02:19


All times are GMT -4. The time now is 06:02.