# wall functions

 User Name Remember Me Password
 Register Blogs Members List Search Today's Posts Mark Forums Read

 April 12, 2012, 14:04 wall functions #1 New Member   Join Date: Nov 2011 Posts: 22 Rep Power: 5 Hi all, I'm currently writting up my disertation an am getting slightly confused about a few aspects of the wall functions. I generally understand the different velocity profiles within a turbulent boundary layer and the physical principles (viscous sublayer, buffer layer, law of the wall). However, the problem arises when I start to think about how the wall functions employ the use of these profiles. From my understanding, the law of the wall can be used to approximate the profile for values of Y+ > 30. What happens about the rest of the fluid below this point i.e. the viscous sub layer which doesn't exhibit this profile, as this is where the molecular viscous forces are dominant and surely contribute greatly to the skin friction. Does the wall function then assume a linear relationship between y and the fluid velocity to approximate the velocity profile below Y+ = 30 Any help would be great, Kind Regards, Kit

April 13, 2012, 02:03
#2
Super Moderator

Sijal Ahmed Memon (turboenginner@gmail.com)
Join Date: Mar 2009
Posts: 3,914
Blog Entries: 6
Rep Power: 38
Quote:
 Does the wall function then assume a linear relationship between y and the fluid velocity to approximate the velocity profile below Y+ = 30
No. Wall function (log law) does not assume the linear relationship by definition. However, two points are important to consider.

1) Standard wall functions ceases to be valid in linear profile.

2) Commercial codes assume the log profile above Y+ = 11.06 where both profile intersect each other.

So if you want to use the standard wall function then try to keep the Y+ alteast 11.06 otherwise your results wont be having any physical significance.

The solution to above problem is the use of scalable wall function for K-epsilon based models (Available in CFX and Fluent) which keeps the Y+ above 11.06.

OR

Use the automatic wall treatment (aka hybrid wall function or all Y+ wall function) which blends the both profiles and uses the linear profile for Y+ below 5, wall function for Y+ greater than 30 and mix of both in between Y+ = 5-30.

 April 13, 2012, 04:37 #3 New Member   Join Date: Nov 2011 Posts: 22 Rep Power: 5 Hi there, thanks for your reply. So is the law of the wall applied to the fluid in the cell next to the wall? even though it is not valid? or is the wall function turned off for the fluid in this cell? Regards, Kit

 Thread Tools Display Modes Linear Mode

 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 OffTrackbacks are On Pingbacks are On Refbacks are On Forum Rules

 Similar Threads Thread Thread Starter Forum Replies Last Post victorwfreire CFX 11 January 14, 2015 16:48 AlmostSurelyRob OpenFOAM 33 April 8, 2014 02:42 romekr ANSYS Meshing & Geometry 1 November 26, 2011 13:11 pierre OpenFOAM Running, Solving & CFD 0 October 1, 2005 13:13 barrat Main CFD Forum 10 September 1, 2004 08:29

All times are GMT -4. The time now is 12:38.

 Contact Us - CFD Online - Top