Friction Velocity from Law of the Wall

[pobatso]

Hi all,

Am currently studying a set of data from a Hot Wire Anemeotry lab that was done a few days ago. The experiment was investigating a turbulent flow of air in a wind tunnel near a flat wall.

As part of the write up, we're ment to find the friction velocity from the laminar near wall region of the data, specifically Y+<10, on a method based on the one used in a paper by Kline et al.

Now it appears that they are finding the frictional velocity from a direct measurement of wall shear stress (t), as u*=(t.rho)^0.5. This is done by plotting a graph of U (velocity) Vs. Y for the laminar near wall region and somehow getting a value for t from the slope of the near wall region.

I'm a bit confused as to the exact steps of how they went about this and was wondering if anyone could help shed some light. Hope I've explained it adequately.

Cheers,
pobatso

 

[RandomGuy88]

Very near to the wall (typically y+ < 5-10) in a turbulent boundary u+ = y+

If you understand what u+ and y+ are then you can use the above equation and your measurments to determine the wall shear stress.

Since this is probably for school I won't tell you exactly what to do. But it's relatively straight forward.