Boundary Layer in Duct Flow

recon9 · January 26, 2011, 15:50

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

ghorrocks · January 26, 2011, 18:27

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.

recon9 · January 26, 2011, 18:44

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.

ghorrocks · January 26, 2011, 18:47

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.

January 26, 2011, 15:50	Boundary Layer in Duct Flow	#1
recon9 New Member Brady Join Date: Jan 2011 Location: Calgary, Alberta, Canada Posts: 12 Rep Power: 15	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

Thread Tools	Search this Thread
Show Printable Version Email this Page	Search this Thread: Advanced Search
Display Modes
Linear Mode Switch to Hybrid Mode Switch to Threaded Mode

Similar Threads
Thread	Thread Starter	Forum	Replies	Last Post
Turbulent Boundary Layer on a Flat Plate	Hoshang Garda	FLUENT	1	November 27, 2013 10:24
To thin boundary layer?	Luk	CFX	3	February 27, 2009 03:22
[Commercial meshers] Trimmed cell and embedded refinement mesh conversion issues	michele	OpenFOAM Meshing & Mesh Conversion	2	July 15, 2005 04:15
Boundary Layer Meshing of a nosecone	Chris	FLUENT	1	March 15, 2005 12:42
Unsteady Boundary Layer Flow	Wen Long	Main CFD Forum	0	July 29, 2002 23:08

January 26, 2011, 18:27		#2
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,692 Rep Power: 143	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.

January 26, 2011, 18:44		#3
recon9 New Member Brady Join Date: Jan 2011 Location: Calgary, Alberta, Canada Posts: 12 Rep Power: 15	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.

January 26, 2011, 18:47		#4
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,692 Rep Power: 143	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.