CFD Online Logo CFD Online URL
www.cfd-online.com
[Sponsors]
Home > Forums > FloEFD, FloWorks & FloTHERM

Boundaries in external flow

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

Reply
 
LinkBack Thread Tools Display Modes
Old   August 10, 2009, 09:22
Default Boundaries in external flow
  #1
New Member
 
Carlos
Join Date: Aug 2009
Posts: 8
Rep Power: 8
crzbur is on a distinguished road
Hi,

I'm new in this community and first of all I would like to thank the administrator and team for this wonderful site dedicated to CFD.

I have to model an external flow on a plate at a certain speed. The plate simulates a flat-plate solar panel, and i want to set one of the walls as a no-slip boundary, so that the flow velocity be 0 there. The thing is that i just find 'default', 'symmetry' and 'periodicity' as the three possible options. How can i fix that problem?

Thanks.
crzbur is offline   Reply With Quote

Old   August 12, 2009, 05:32
Default
  #2
New Member
 
Juan Gerber
Join Date: May 2009
Posts: 11
Rep Power: 8
j.gerber is on a distinguished road
Hi,

The options "default", "symmetry" and "periodicity" are settings at the minimum and maximum of the three coordinate axes of the Computational Domain.

The default velocity boundary condition at solid walls is the no-slip condition.
For a slip condition you would have to specify an ideal wall boundary condition.

I hope this helps you.
j.gerber is offline   Reply With Quote

Old   August 13, 2009, 05:10
Default
  #3
New Member
 
Carlos
Join Date: Aug 2009
Posts: 8
Rep Power: 8
crzbur is on a distinguished road
Hi,

Well, the thing is that when i do a cut plot of the velocities at the tunnel floor, it appears that the velocity is similar to the inlet velocity specified at the inlet lid.

i kind of get confused with that. do you know if i'm doing something wrong or if i miss something?
crzbur is offline   Reply With Quote

Old   August 13, 2009, 06:28
Default
  #4
New Member
 
Juan Gerber
Join Date: May 2009
Posts: 11
Rep Power: 8
j.gerber is on a distinguished road
Which analysis type have you specified under General Settings, External or Internal?
j.gerber is offline   Reply With Quote

Old   August 13, 2009, 06:54
Default
  #5
New Member
 
Carlos
Join Date: Aug 2009
Posts: 8
Rep Power: 8
crzbur is on a distinguished road
internal... that's why it kind of gets me confused. it should behave like the flow though a pipe... but don't know why it does not. jesus!
crzbur is offline   Reply With Quote

Old   August 13, 2009, 09:42
Default
  #6
New Member
 
Juan Gerber
Join Date: May 2009
Posts: 11
Rep Power: 8
j.gerber is on a distinguished road
Have you got heat transfer in solids enabled?
And did you apply simmetry at any of the computational walls?
j.gerber is offline   Reply With Quote

Old   August 13, 2009, 10:18
Default
  #7
New Member
 
Carlos
Join Date: Aug 2009
Posts: 8
Rep Power: 8
crzbur is on a distinguished road
I didn't activate the option of heat transfer. If so, which material should i pick up from the list? The material of the wind tunnel could be interpreted as an insulator?

And yeah, i applied symmetry to the side walls of the tunnel (computational domain). The rest are under the default condition.

Thanks. It seems you know what's gotta be done!

Im trying with heat transfer activated right now to see if anything changes...
crzbur is offline   Reply With Quote

Old   August 13, 2009, 10:51
Default
  #8
New Member
 
Juan Gerber
Join Date: May 2009
Posts: 11
Rep Power: 8
j.gerber is on a distinguished road
You probably need to adjust your mesh at the tunnel walls in order detect the variation in velocity.

Compare the size of the cells at the tunnel walls with the boundary layer thickness. This could give you an indication of how much refinement is needed.
j.gerber is offline   Reply With Quote

Old   August 13, 2009, 11:14
Default
  #9
New Member
 
Carlos
Join Date: Aug 2009
Posts: 8
Rep Power: 8
crzbur is on a distinguished road
I did a cut plot of velocities at the bottom boundary of the CD and checked the "display boundary layer" button and got it. I could see the velocity there was 0, as I had set the default condition there. _Thank you very much for your help, j.gerber.

But my concern now is this one: In an external flow analysis, how can I set a velocity profile (given by a power law expression)?? So if i set different cut plots of the velocity at different heights I could see increasing speed with an increasing height measured from the bottom.

Last edited by crzbur; August 18, 2009 at 06:09.
crzbur is offline   Reply With Quote

Old   August 18, 2009, 11:37
Default
  #10
New Member
 
Juan Gerber
Join Date: May 2009
Posts: 11
Rep Power: 8
j.gerber is on a distinguished road
My apologies for my previous statement. FloEFD uses modified wall functions to characterize flow at the walls, thus a finer mesh in these regions are not needed to detect the velocity variation.

Are you saying that you need to set up a velocity profile at the wall irrespective of what the calculated profile should be?

Say if you make a horizontal cut plot close to the floor, just use the "Display Boundary Layer" option again and apply the plot. It seems to not show the boundary layer variation in velocity in preview mode.
j.gerber is offline   Reply With Quote

Old   August 19, 2009, 15:06
Default
  #11
New Member
 
Carlos
Join Date: Aug 2009
Posts: 8
Rep Power: 8
crzbur is on a distinguished road
yeah. I think FloWorks has a default uniform velocity profile, as opposed to a parabolic-kinda profile like the one in pipes flow. I am running now an external analysis, and was wondering if i could simulate the atmospheric boundary layer, where the velocity is 0 at the earth's surface but grows exponentially according to the power law (and consequently, the "ceiling" on the computational domain must be set up so that the velocity is not 0 there).

And about the display boundary layer, i could see how the velocity was 0 or almost 0 in the computational domain bottom boundary but if i applied another cut plot just 0.1 meters above, the velocity was almost fully developed, at the inlet's velocity.

j.gerber, you spanish?
crzbur is offline   Reply With Quote

Reply

Tags
boundary, no-slip

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
external flow meshing for a Sedan in pro*am rave CD-adapco 7 July 17, 2007 09:48
Why Re is different for external and internal flow john FLUENT 3 June 14, 2007 03:53
Reversed flow on external aero problem Ben FLUENT 2 August 22, 2006 10:05
external flow with mach 3 Roee FLUENT 2 January 9, 2006 09:12
Inviscid Drag at subsonic, subcritical Mach # Axel Rohde Main CFD Forum 1 November 19, 2001 13:19


All times are GMT -4. The time now is 08:57.