The Wind Tunnel (Page 8)

Aside from the visual results and showing you how to interpret the air patterns, what does this all mean? What would you use it for?

We've discussed drag (or air resistance) and the Re. We mentioned another use of Re as a parameter to determine when the boundary layer transitions from laminar to turbulent. This is called the critical Reynolds number and is given the shorthand symbol Re_crit. Specific geometries or objects have a different Re_crit. A turbulent boundary layer can actually stay attached to the object longer than a laminar boundary layer. That means flow separation is delayed and there is less drag. Less drag can mean the flight of the ball is longer and/or faster. This is exactly why there are dimples on golf balls. Originally, golf balls didn't have dimples, but people observed that used balls that were roughed up flew longer than new balls. Eventually they designed golf balls with dimples in them.

In our section on drag we mention briefly that drag is a force and a function of density, velocity, the area of the object and a drag coefficient C_d. Wind tunnel experiments are conducted to determine the C_d. The higher the C_d the higher the drag. The C_d for a brick for example is higher than that of a sphere. The sharp corners of the brick make it a less aerodynamic shape than that of a sphere. The C_d for an object is not a constant though. Below is a graph of the C_d versus the Re for a smooth sphere. So you can see that at low Re, the C_d is high, then it drops. What's that extra reduction in C_d between Re = 10⁵ and 10⁶? That's when the boundary layer transitions - this is Re_crit.

As surface roughness increases, these curves and Re_crit shift to the left (see the dotted lines). Transition and the onset of turbulence occurs at a lower Re number (or in general at a lower velocity).

So in a nutshell at a certain point turbulence can reduce drag.

What does this have to do with playing tennis for you and me?