Читать книгу Flight Theory and Aerodynamics - Joseph R. Badick - Страница 98

A stationary (nonrotating) cylinder is located in a wind tunnel as shown in Figure 3.22a. The cylinder is equipped with static pressure taps. These measure the local static pressure with respect to the ambient static pressure in the test chamber. When the tunnel is started, the airflow approaches the cylinder from the left as shown by the relative wind vector. Arrows pointing toward the cylinder show pressures that are higher (+) than ambient static pressure; arrows pointing away from the cylinder show pressures that are less (−) than ambient static pressure. Figure 3.22a shows that the upward forces are resisted by the downward forces and no net vertical force (lift) is developed by the cylinder.

Now consider if the wind tunnel is stopped, and the cylinder begins to rotate in a motionless fluid. We begin to see the factors of viscosity and friction at work. The more viscous the fluid, the more it is resistant to flow, and since air has viscosity properties it will resist flow. Similar to a wing, the surface of the cylinder has some “roughness” to it, so as the cylinder turns some molecules adhere to the surface. The closer to the surface of the cylinder (airfoil), the greater the possibility the molecules are drawn in a clockwise direction by viscosity, so now substituting air we see the velocity increase in the direction of rotation above the cylinder. This circular movement of the air is called circulation.

Finally, let us consider a rotating cylinder in a moving fluid as the cylinder continues rotating in the clockwise direction when the wind tunnel is once again started (Figure 3.22b). The air passing over the top of the cylinder will be speeded up by circulation, while the air passing over the bottom of the cylinder will be retarded. According to Bernoulli’s equation, the static pressure on the top will be reduced and the static pressure on the bottom will be increased, similar to an airfoil with a positive angle of attack. The new pressure distribution will be as shown in Figure 3.22b, where a low‐pressure area produces an upward force. This is called the Magnus effect, named after Gustav Magnus, who discovered it in 1852. It explains why you slice (or hook) your golf ball or why a good pitcher can throw a curve. The principles of the Magnus effect are even utilized in space as a spinning satellite can maintain its orbit longer due to the Magnus force, minimizing orbital decay.