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

Boundary Layer in Duct Flow

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

Reply
 
LinkBack Thread Tools Display Modes
Old   January 26, 2011, 16:50
Default Boundary Layer in Duct Flow
  #1
New Member
 
Brady
Join Date: Jan 2011
Location: Calgary, Alberta, Canada
Posts: 12
Rep Power: 6
recon9 is on a distinguished road
I am trying to verify the accuracy of CFX by modelling some simple flows. One such is flow through a square duct. I am noticing an odd velocity profile within the boundary layer. It starts at zero at the no-slip wall, climbs and peaks at a velocity greater than the free stream velocity, then lowers to a steady value approximately equal to the free stream velocity within the core of the duct.

As far as I understand flow theory, the velocity within the boundary layer should not exceed the inlet velocity. Does anyone know why this happens and if it can be fixed?

Thank you,

Brady
recon9 is offline   Reply With Quote

Old   January 26, 2011, 19:27
Default
  #2
Super Moderator
 
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 10,951
Rep Power: 85
ghorrocks has a spectacular aura aboutghorrocks has a spectacular aura aboutghorrocks has a spectacular aura about
Yes, the result is correct.

If your inlet is a plug flow (ie constant velocity over the whole boundary), as the boundary layer grows it is a region of slower velocity. To maintain continuity then the flow in the middle must be faster than the inlet.
ghorrocks is offline   Reply With Quote

Old   January 26, 2011, 19:44
Default
  #3
New Member
 
Brady
Join Date: Jan 2011
Location: Calgary, Alberta, Canada
Posts: 12
Rep Power: 6
recon9 is on a distinguished road
BL.JPG

That makes sense, but should the boundary layer show a profile such as this? Sorry for the crude drawing, it should be symmetric.
recon9 is offline   Reply With Quote

Old   January 26, 2011, 19:47
Default
  #4
Super Moderator
 
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 10,951
Rep Power: 85
ghorrocks has a spectacular aura aboutghorrocks has a spectacular aura aboutghorrocks has a spectacular aura about
Yes, that is normal. As the boundary layer develops the "bump" smooths itself out and ends up being a small increase in the duct centre velocity. But in the early stages of BL grow it starts as a bump in the velocity profile.

Don't take my word for it, do a literature search and find out for your self. Also, do a sensitivity study on your simulation (mesh, convergence, time step) and if it still gives the bump you can be confident it is real.
ghorrocks 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
Turbulent Boundary Layer on a Flat Plate Hoshang Garda FLUENT 1 November 27, 2013 11:24
To thin boundary layer? Luk CFX 3 February 27, 2009 04:22
Trimmed cell and embedded refinement mesh conversion issues michele OpenFOAM Other Meshers: ICEM, Star, Ansys, Pointwise, GridPro, Ansa, ... 2 July 15, 2005 04:15
Boundary Layer Meshing of a nosecone Chris FLUENT 1 March 15, 2005 13:42
Unsteady Boundary Layer Flow Wen Long Main CFD Forum 0 July 29, 2002 23:08


All times are GMT -4. The time now is 10:26.