2.2 Interpretation of the graph of log CD vs log Re

The first section of the graph, for Re < 0.5, corresponds to flows like that in Figure 1a. The drag coefficient for these low Reynolds number flows was theoretically derived by G. G. Stokes in 1851. He predicted that CD = 24/Re. We will soon compare this with the experimental results. As the Reynolds number increases, a wake develops behind the sphere. This wake is wide for 10^3 < Re < 10^5, as illustrated in Figure 1b. The large dip in CD between Re = 10^5 and Re = 10^6 corresponds to the formation of a turbulent boundary layer in front of the sphere accompanied by a narrower wake behind the sphere, as in Figure 1c. This narrower wake causes the drag force to actually decrease. Golf balls are corrugated in order to induce a turbulent boundary layer and reduce the drag force. The seams on a baseball allow a knuckleball to oscillate.

Up to 2. The Graph of the Drag Coefficient verses Reynolds Number