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

Eddy viscosity ratio to get the boundary layer thickness

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

Reply
 
LinkBack Thread Tools Display Modes
Old   May 28, 2013, 09:23
Question Eddy viscosity ratio to get the boundary layer thickness
  #1
Member
 
Francesco
Join Date: Jan 2012
Posts: 31
Rep Power: 4
Cecco is on a distinguished road
Hi,
I have simulated the flow into a centrifugal fan.
Now I have plotted the Eddy viscosity ratio (Eddy viscocity/dynamic viscosity) in order to get the thickness of the boundary layer (see the figure).
I surprised to watch that the EVR increase also outside the boundary layer.
I was expecting to see EVR=0 adjacent to the blade, then an increasing inside the boundary layer and then again EVR=0 ouside the boundary layer.

Is it strange? What do you think about? This would be that between two blades the flow is a continuing mixing?

Thanks in advance
Attached Images
File Type: jpg EVR.jpg (76.2 KB, 53 views)
Cecco is offline   Reply With Quote

Old   May 28, 2013, 10:29
Default
  #2
Senior Member
 
RicochetJ's Avatar
 
Mr CFD
Join Date: Jun 2012
Location: Britain
Posts: 168
Rep Power: 4
RicochetJ is on a distinguished road
Curious: what value of eddy viscosity ratio will give you the thickness of the boundary layer?

The eddy viscosity ratio is simply the ratio of turbulent to laminar (molecular) viscosity. If you have values of eddy viscosity ratio outside of your boundary layer then it just indicates some sort of laminar or turbulent regime.

To be honest I don't see why this is strange.
RicochetJ is offline   Reply With Quote

Old   May 28, 2013, 10:41
Default
  #3
Senior Member
 
Lance
Join Date: Mar 2009
Posts: 472
Rep Power: 10
Lance is on a distinguished road
As far as I know the eddy viscosity ratio should have its maxium value close to the middle of the boundary layer, at least for external flows. In the free stream the eddy viscosity is governed by the turbulence at the inlet, but it shouldnt necessarly be zero.

Also, your color range is [0,1] which is normally too low for RANS models. Try something on the order of [0,100] or more.
Lance is offline   Reply With Quote

Old   May 28, 2013, 11:17
Default
  #4
Member
 
Francesco
Join Date: Jan 2012
Posts: 31
Rep Power: 4
Cecco is on a distinguished road
@ RicochetJ:
I expected to see the same of this one: http://www.computationalfluiddynamic.../2013/04/2.png

probably it means this is laminar outside boundary layer, while mine is turbulent so explain the ratio doesn't come back to zero outside.

@Lance:
You can see image below with EVR btw [0;70]. I plotted [0;1] just to underline the bondary layer.

Thanks guys.

????????????????????
I have no much experience in internal flows. Do you think it is a normal way to work (i mean in a turbulent regime) for a tangential blower (max volume flow rate 70 m3/h)?
????????????????????
Attached Images
File Type: jpg EVR2.jpg (87.2 KB, 26 views)
Cecco 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
Low Mixing time Problem Mavier CFX 5 April 29, 2013 00:00
Wall coordinates and boundary layer thickness bentmol Main CFD Forum 1 July 3, 2011 20:44
Question about eddy viscosity ratio limit Piti CFX 2 January 17, 2008 16:49
errors Fahad Main CFD Forum 0 March 23, 2004 13:20
length scales at inlet for internal flows Anne-Marie Giroux Main CFD Forum 3 July 5, 1999 21:28


All times are GMT -4. The time now is 01:23.