# Law of the Wall curve and Y+..for those of you who are bored.

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

 September 9, 2011, 13:21 Law of the Wall curve and Y+..for those of you who are bored. #1 Member   Join Date: Jun 2011 Posts: 48 Rep Power: 8 I'm a aero engineering student trying to put a bunch of things together...I am using Fluent with a K-e model for drag analysis over a vehicle. I still have numerous questions about y plus, even after searching these forums and google...my confusion mainly comes from applying the technical equations and definitions to how Fluent uses them. Ok so I guess there's no quick way around explaining my questions without sounding really basic, so I'll just have to tell you everything I think I understand so far.I get that Y+ is nondimesional and gives you an idea of how far your closest gridpoint is to the surface. For k-e models (used for fully turbulent flow) with no enhanced wall treatment, we want Y+ to be between 30-500. http://en.wikipedia.org/wiki/Law_of_the_wall shows me a graph of U+ (average velocity) vs Y+ I'm trying to make sense out of the graph, how it relates to my solutions in Fluent and the basic idea of what calculations are being carried out in CFD. Looking at the graph, I'm going to refer to Y+ as a "distance" even though it's a nondimensional quantity...it give you an idea of a scaled distance i guess...right? Ok so the curve in red shows me what the average velocity should be for a given "distance" from the wall. We need to use an average velocity because the flow is turbulent with lots of velocity fluctuations. The "Log-Law" blue curve is formed from the equation u+=(1/k)ln(y+)+C+. I see that for y+ from 30-500, the red and blue lines are very close. Ok so here's where I wonder what's going on. First off, for y+ values of 30-500, where are we in relation to the boundary layer? We are still inside the boundary layer right? We have not traveled far enough out to reach the freestream? So CFD will now know this average velocity, from using the Log-Law relation at that point/distance from the wall. Given this value, how does it approximate the underlying viscous sublayer/buffer layer? Does it approximate the nice red curve in the graph by using U+=y+ for Y+<5, and somehow guess the values for the buffer layers? Or, does it take the known avg velocity associated with the y+=30-500 and just take the du/dy slope at that point and interpolate all the way down to the wall? For example, if my y+ value for the k-e with no EWT is 20, and Fluent uses the log law relation, then my computed average U will be higher than it should be, so drag computed will be overestimated. OOOOrrrrrr, does fluent know that y+ is 20, so it knows it must lie within the buffer layer, and it does some kind of calculation from there... As if that wasn't enough, I heard that when you use enhanced wall treatment (EWT) you want y+ from 1-5. Is this because EWT is really solving what the boundary layer looks like, so you just need close enough grid points to model the viscous sublayer? And hey, what about if I were to be doing something else, and I wanted to use k-w, maybe for flow over an airfoil with Re=3e6? Does k-w or transition sst need different target y+ values (not in the 30-500 range) to model the boundary layer? If so, what are those values and why those numbers? One more thing...what the hell is y-star and how does that play in to any of this? Sounds like y+ takes care of alot. Alright well if anyone had the patience to read my ranting, I'll really appreciate any clarification on this stuff! Jomid, vs1 and Ionut G like this.

 September 12, 2011, 15:25 #3 Super Moderator   Sijal Join Date: Mar 2009 Location: Islamabad Posts: 4,352 Blog Entries: 6 Rep Power: 45 http://www.cfd-online.com/Forums/cfx...machinery.html check reply no 5 with attached pdf file

 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 Anorky ANSYS Meshing & Geometry 4 November 12, 2014 01:27 Robert@cfd ANSYS Meshing & Geometry 20 November 11, 2011 05:59 ruzfactor FLUENT 0 December 8, 2010 19:33 Chie Min CFX 5 July 12, 2001 23:19 Abhijit Tilak Main CFD Forum 6 February 5, 1999 02:16

All times are GMT -4. The time now is 02:22.