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

Wall boundary condition in cylindrical co-ordinate

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

Reply
 
LinkBack Thread Tools Display Modes
Old   December 2, 2009, 01:11
Unhappy Wall boundary condition in cylindrical co-ordinate
  #1
New Member
 
Chandra
Join Date: Jul 2009
Posts: 5
Rep Power: 8
shekharc is on a distinguished road
Hi,

I am a bit confused about application of the wall BC at solid walls in cylindrical co-ordinate on staggered grid.

For the imaginary cells, the lateral velocity components v,w are set as negative of the adjacent inner cell to keep these zero on the wall.

The normal component u of this imaginary cell is determined from "Div = 0" at the wall.

i.e. DEL.V = 0 on the wall. However, v,w = 0 on the wall. So, this becomes:

[d(ru)/dr] = 0 ---- (1)

If r_i defines the cylindrical boundary and we discretize this, it gives:
u_(i+1) = [r_(i-1) / r(i+1)].u_(i-1); which is quite right.

However, if we take the equation (1) further, it gives:
du/dr + u/r = 0.

Since u = 0 at the wall, it becomes, du/dr = 0
i.e. u_(i+1) = u_(i-1), which is wrong in cylindrical co-ordinate.

Where is the mistake?

Thanks!
shekharc is offline   Reply With Quote

Old   December 2, 2009, 01:21
Default
  #2
Senior Member
 
ztdep's Avatar
 
p ding
Join Date: Mar 2009
Posts: 287
Rep Power: 9
ztdep is on a distinguished road
Send a message via Yahoo to ztdep Send a message via Skype™ to ztdep
Since u = 0
du/dr .NE. 0

u has gradient near the wall.
ztdep is offline   Reply With Quote

Old   December 2, 2009, 03:28
Default
  #3
New Member
 
Chandra
Join Date: Jul 2009
Posts: 5
Rep Power: 8
shekharc is on a distinguished road
Thanks! What you are saying is surely the case. However, why does the divergence-free condition lead to that incorrect condition? What's wrong?
shekharc is offline   Reply With Quote

Old   December 2, 2009, 09:20
Default
  #4
Senior Member
 
ztdep's Avatar
 
p ding
Join Date: Mar 2009
Posts: 287
Rep Power: 9
ztdep is on a distinguished road
Send a message via Yahoo to ztdep Send a message via Skype™ to ztdep
I think the problem is that the continuity equation is used for fluid. not for the wall
ztdep 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
How to set rotating wall as boundary condition babu FLUENT 7 December 14, 2013 05:48
Change wall boundary condition in multiphase? Jacques FLUENT 0 November 27, 2008 07:26
Wall boundary condition – UDF Ssn FLUENT 0 January 25, 2008 04:43
Variable temperature wall boundary condition Virginie FLUENT 2 October 2, 2007 09:23
the boundary condition of permeable wall lisa FLUENT 12 May 5, 2005 20:58


All times are GMT -4. The time now is 04:30.