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.