efore B the boundary layer becomes stalled and a turbulent wake extends downstream from the ball. In this turbulent wake there is considerable violent stirring of the viscous air and energy is dissipated. When a ball flies through air at rest, this energy dissipated in the turbulent wake comes from the energy of motion (kinetic energy) of the ball. There is thus a resistive force, called "drag," on the moving ball which is not related to the viscous force in the boundary layer but comes rather from the difference in the pressure on the front and on the back of the ball. This drag in this velocity range varies closely as the square of the relative velocity of the air and the ball. DIMPLES INCREASE TURBULENCE IN BOUNDARY LAYERNext consider the change in the flow line pattern for small masses of air when the surface of the ball is changed from smooth to the usual dimples found on a golf ball but still with the same large air velocity. The dimpled surface makes the boundary layer turbulent; it stirs the air up a bit. Instead of stalling near B, as in the previous example, the rapidly moving air carries the turbulent boundary layer along with it, helping it to extend further along the surface of the ball from the low pressure region at B toward the higher pressure region at C. This is indicated in Fig. 8.1(c). The turbulent wake starts farther back along the surface of the ball and is thus smaller in cross section than in the case of the smooth ball. The drag on the dimpled ball is considerably smaller than that on the smooth ball; less energy is dissipated in the smaller wake. This is shown to be true in actual experimental comparisons of the drag on smooth and rough spheres. THE SPINNING BALLSo far our discussion has been concerned with the drag on a ball which is not rotating. Professor Tait, and all those investigators who came after him, found that a properly hit golf ball acquires spin about a horizontal axis. The ball spins in such a ...
