lines drawn in the figure show paths of small masses of air in their motion past the ball. Several figures showing patterns of air flow past a ball are presented in this discussion. These should be looked upon as diagrams suggesting what the flow may be like. Flow patterns past some obstacles have been studied experimentally, but as far as I know, no one has thought of how a flow pattern past a high velocity spinning ball might be observed. These diagrams show patterns of flow that appear to me to be reasonable as a result of my studies and are to be considered only as a basis for discussion. STREAMLINE AND TURBULENT FLOWThe motion of the air shown in Fig. 8.1(a) is approximately streamline flow. Streamline flow is to be distinguished from turbulent flow. When a candle has been snuffed out, the smoke rises smoothly in still air for a few inches and then breaks into a disturbed motion. The lower part shows streamline flow while the upper part shows turbulent flow. As the air flows past the ball, the air close to the ball has its speed increased from A to B and then decreased from B to C. This means, according to Bernouilli, that the pressure of the air near the surface of the ball decreases from its value at A to a lower value at B and then rises to have at C the same value it had at A. THE BOUNDARY LAYERFor the example now being considered, that for air moving with a very small velocity, the forces of the air pressure on the ball are symmetrical forward and backward and therefore will result in no net force on the ball. But experiments show that there is a force on the ball for small air velocities. This force comes about because air is a viscous fluid. There is a thin layer of air near the surface of the ball, called the boundary layer, in which the speed of the air varies from zero at the surface to the larger value away from the surface out in the streamline flow. The air in contact with the surface of the ball does not slide over t...
