
[Sponsors] 
March 30, 2000, 06:22 
y+ and boundary layers.

#1 
Guest
Posts: n/a

Hi!
I am just wondering what y+ means with regards to boundary layers...I am doing a project in college using CFD and my lecturer has spoken about y+ and a few other parameters...could some one explain these or simple tell me where I can find a good simple explaination... thank you. Jack. 

March 30, 2000, 11:32 
Re: y+ and boundary layers.

#2 
Guest
Posts: n/a

"Turbulence Models and Their Applications To Prediction of Internal Flows: A Review", M.Nallasamy, 1987, "Computers and Fluids", v15,No.2 pp. 151194. Also Search papers by Prof. Rodi, AiAA, 1982.
All the best 

March 30, 2000, 11:58 
Re: y+ and boundary layers.

#3 
Guest
Posts: n/a

y+ is the nondimentional wall distance. What our fluid mechanics forefathers found was that they could express the velocity and distance from the wall using nondimentional numbers. Using these numbers they found that turbulent flow over a flat plate (for example) was always the same. You could have a small flat plate, or a flat plate the size of Manhattan. You could have a low free stream velocity, or a larger free stream velocity. When you plot the nondimentional velocity (u+) as a function of the nondimentional distance from the wall (y+) the curve you get in the boundary layer is the same.
As for books with good information, "Convective Heat and Mass Transfer" by Kays and Crawford has a good chapter on this stuff. Most other upper undergrad level fluid mechanics books should explain it also. 

April 1, 2000, 16:16 
Re: y+ and boundary layers.

#4 
Guest
Posts: n/a

Let me add a little bit to this. First of all, the definition of y+ is
y+ = y*u_tau/nu where y is the normal distance from a wall, u_tau is called the "friction velocity" and is defined by sqrt(tau_w/rho) where tau_w is the shearing stress at the wall and rho is the density of the fluid (assuming incompressible constant density flow). nu is the kinematic viscosity of the fluid. That's what y+ IS. But why is it useful? Its useful because when plotted against u+ which is u/u_tau, for a *turbulent* boundary layer, there is a region for sufficiently small y+ where the velocity profile is UNIVERSAL. You always get the same shape (for a flat plate with dp/dx=0, anyway). Any time you can find universal behavior such as this, it becomes a very powerful tool for analysis, because you can solve the universal relationship for variables which would otherwise be unknown. Hope this helps. Bob 

April 3, 2000, 12:58 
Re: y+ and boundary layers.

#5 
Guest
Posts: n/a

another suggestion  whenever your professor uses terms in class that he hasn't defined just ask him what they are. if you get lost in one part of the lecture the entire remainder of it is wasted time for you.


Thread Tools  
Display Modes  


Similar Threads  
Thread  Thread Starter  Forum  Replies  Last Post 
Water subcooled boiling  Attesz  CFX  7  January 5, 2013 04:32 
Import problem  ARC  Open Source Meshers: Gmsh, Netgen, CGNS, ...  0  February 27, 2010 11:56 
RPM in Wind Turbine  Pankaj  CFX  9  November 23, 2009 05:05 
Convective Heat Transfer  Heat Exchanger  Mark  CFX  6  November 15, 2004 16:55 
test cases for 2D boundary layers  roxana  Main CFD Forum  0  April 13, 2002 09:25 