Caught in the Draft

摘要

We've all seen (in movies, newscasts, or perhaps in person) the violent effect of the downwash that occurs when a helicopter hovers over the ground. Leaves, grass, and debris are dramatically blown about. We've also sat in front of circulating room fans and felt a large draft, whereas there seems to be very little air movement behind the fan. The cause of this is a delightful manifestation of .Bernoulli's principle. The fan blades, or helicopter rotor blades, produce a pressure differential as air passes through them-let us say^>i before andj>2 after, as shown in Fig. 1, with p2 greater than p. Ifpo is the ambient pressure, Bernoulli's equation gives.