
[Sponsors] 
What y+ value shall we choose for laminar flow? 

LinkBack  Thread Tools  Search this Thread  Display Modes 
October 4, 2012, 07:48 
What y+ value shall we choose for laminar flow?

#1 
Senior Member
lnk
Join Date: Feb 2011
Location: Switzerland
Posts: 118
Rep Power: 12 
Hi,
May I ask what y+ value shall we choose for laminar flow? Is that the same as for turbulent flow? Could I still use the calculator given by this forum? (http://www.cfdonline.com/Tools/yplus.php) Thank you very much. lnk Last edited by lnk; October 4, 2012 at 13:25. 

January 26, 2013, 05:58 

#2  
Senior Member
Meimei Wang
Join Date: Jul 2012
Posts: 494
Rep Power: 12 
Quote:
__________________
Best regards, Meimei 

January 27, 2013, 12:13 
viscoussublayer

#3 
New Member
Join Date: May 2011
Posts: 6
Rep Power: 12 
The y+ value is important for the turbulentmodel. It defines the height of the viscoussublayer where a laminar flow near the wall in turbulent flows occurs. I'm not a specialist but i would say it doesn't matter how big the height of your first cell is because there isn't a turbulence model.
regards 

January 27, 2013, 15:51 

#4 
New Member
mohsen
Join Date: Sep 2012
Posts: 6
Rep Power: 10 
hi the y+ is only important for turbulent flow because it used in turbulent flow.
boundary layer is three region in turbulent flow they are sub layer , buffer layer & log layer. velocity is linear in sub layer and it's logarithmic for log layer. if y+ was below 5 then the first cell is in sub layer. velocity for first cell obtain linearly if y+ was larger than 30 the the first cell is in log layer . velocity for first cell obtain logarithmic. however boundary layer of laminar flow is one region and it's velocity is defined we do'nt need y+ for laminar flow 

January 27, 2013, 17:24 

#5  
Senior Member
Lucky Tran
Join Date: Apr 2011
Location: Orlando, FL USA
Posts: 4,144
Rep Power: 50 
Quote:
Quote:
Quote:


January 28, 2013, 08:47 

#6 
Senior Member
Filippo Maria Denaro
Join Date: Jul 2010
Posts: 5,736
Rep Power: 60 
Consider the example of the plane channel flow. Then,
y+ = u_tau *y/ni = (u_tau *H/ni) * (y/H) = Re_tau* y' being y' the nondimensional position along the vertical direction. Assuming that y'=1 in the halfheigh of the channel, as in laminar flow one assumes Re_tau=O(1), you see that y+ will be very very small close to wall and will be O(1) in the halfheight. 

February 2, 2013, 08:24 

#7  
Senior Member
Meimei Wang
Join Date: Jul 2012
Posts: 494
Rep Power: 12 
Quote:
Thanks for your answer. But in order to resolve laminar boundary layer well, we still need a lowest wall cell width criteria, right? For example, what wall cell width would you choose for laminar pipe flow?
__________________
Best regards, Meimei 

February 3, 2013, 05:29 

#8 
Member
venki
Join Date: May 2011
Posts: 84
Rep Power: 12 
calculate first cell height based on boundary condition


February 6, 2013, 12:11 

#9 
Senior Member
Meimei Wang
Join Date: Jul 2012
Posts: 494
Rep Power: 12 
Could you briefly give a short example of how? Thank you very much.
__________________
Best regards, Meimei 

February 15, 2013, 10:17 

#10 
Senior Member
Lefteris
Join Date: Oct 2011
Location: UK
Posts: 241
Rep Power: 12 
You can calculate the maximum displacement of the boundary layer from the Blasius solution and then you can figure out how many points you need in order to cover this (with a geometric distribution let's say) after you decide your initial ds. Usually, the initial ds should be small enough, let's say of order 10^4.


February 15, 2013, 16:27 

#11  
Senior Member
Lucky Tran
Join Date: Apr 2011
Location: Orlando, FL USA
Posts: 4,144
Rep Power: 50 
Quote:
I usually take 3 orders of magnitude less the boundary layer thickness and then take half that height if I want to be conservative. El. K. suggested 10^4 which should also work. You just need to estimate the boundary layer thickness somehow (can always resort to Blasius solution). 

March 3, 2013, 18:54 

#12  
Senior Member
Meimei Wang
Join Date: Jul 2012
Posts: 494
Rep Power: 12 
Quote:
__________________
Best regards, Meimei 

March 4, 2013, 04:56 

#13  
Senior Member
andy
Join Date: May 2009
Posts: 177
Rep Power: 14 
Quote:
Laminar flows have no turbulence models and hence have none of these constraints on y+ for the first cell. What is required for the first cell and every other cell in the grid is that the gradients in the flow are adequately resolved. This is a function of the discretisation scheme and how well the gradients need to be resolved at the cell location. For example, a fully developed flow in a pipe may require only 1 cell to be fully resolved with a higher order numerical scheme or tens of cells for a low order scheme. A resolution criteria based on y+ is going to be rather limited in applicability compared to a normal one based on gradients of the solution variables. 

August 27, 2015, 13:09 
y+ value

#14  
New Member
AZ
Join Date: Jan 2014
Posts: 17
Rep Power: 9 
Quote:


August 4, 2016, 05:23 
y+ value or first cell height for laminar flow

#15 
New Member
Vijaya Kumar. G
Join Date: Jun 2016
Location: Chennai, India & Aachen, Germany
Posts: 20
Rep Power: 7 
This may help


February 15, 2017, 14:34 

#16 
Senior Member
Hamed Abdul Majeed
Join Date: May 2012
Location: New Orleans, LA, US
Posts: 147
Rep Power: 11 

Thread Tools  Search this Thread 
Display Modes  


Similar Threads  
Thread  Thread Starter  Forum  Replies  Last Post 
Different flow patterns in CFX and Fluent  avi@lpsc  CFX  6  April 17, 2012 01:22 
Flow meter Design  CD adapco Group Marketing  Siemens  3  June 21, 2011 08:33 
How to choose a good model for multiphasis flow ?  Yasmail AKARIOUH  FLUENT  2  April 21, 2008 02:12 
transform navierstokes eq. to eulereq.  pxyz  Main CFD Forum  37  July 7, 2006 08:42 
Plug Flow  Franck  Main CFD Forum  3  September 4, 2003 05:57 