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sports science J P SINGH ASSISTANT PROFESSOR PHYSICS POSTGRADUATE GOVT. COLLEGE SECTOR-11, CHANDIGARH Aerodynamics in racing The importance of aerodynamics to automobile racing has been known throughout most of the sports history. In particular, the significance of aerodynamic drag has been known since the early days when streamlined shapes were commonplace for race cars. However, aerodynamic effects were a secondary concern to engine, suspension, and tire technologies. The effect of aerodynamic lift on a race car was not examined in detail . Today, the production of an aerodynamic downforce is considered to be more important than drag reduction.By examining the aerodynamics of the javelin, we will begin to understand how a javelin might be designed to limit the time of flight. Javelin Throw First of all, a javelin does not produce lift in the same manner as a discus. As the air travels around the shaft the flow tends to separate on the upper surface. One normally thinks of separation as increasing the drag force. That is what happens here, only the direction of the force is opposite that of the gravitational force. Therefore, the separation of the flow from the upper surface of the javelin actually increases the flight time. The modern javelin is designed such that the center of pressure is behind the center of gravity. This induces a nose down pitching moment, thereby reducing the flight time of the javelin. The location of the center of pressure will vary in flight. However, the center of pressure remains behind the center of gravity and the nose down pitching moment remains throughout the flight. The nose down pitching moment of the javelin also ensures that the javelin lands point first. A point first landing makes the javelin a safer event as well as making it easier to measure the distance of a given throw. Since the javelin can no longer slide across the ground, there is no question about how far the javelin was thrown. The javelin also experiences a spin about its longitudinal axis during flight. This spin can be as high as 25 revolutions per second. This spin tends to stabilize the javelin in flight. Another phenomena of the javelin in flight is an oscillation about the length of the javelin. This oscillation has a frequency of about 25 hz. The oscillation is detrimental to the flight of the javelin and therefore needs to be minimized by the thrower. This is done by ensuring a delivery which is in the same vertical plane as the flight path of the javelin. History of the Golf Ball A golf ball can be driven great distances down the fairway. How is this possible? Is the drive only dependent on the strength of the golfer or are other factors at play? As we will see, the aerodynamic forces play a key role in the flight of the golf ball. We will start by looking at the history of the golf ball, show why a golf ball has dimples, then explain how lift is formed by the spin imposed on the golf ball. We will also look at how experimental tests can be performed using a spinning ball in a wind tunnel. The early golf ball, known as a featherie, was simply a leather pouch filled with goose feathers. In order to obtain a hard ball, the pouch was filled while wet with wet goose feathers. Since people believed a smooth sphere would result in less drag (and thus fly farther), the pouch was stitched inside out. Once the pouch was filled, it was stitched shut. Therefore there were a few stitches on the outside of the ball. The ball was then dried, oiled, and painted white. The typical drive with this type of ball was about 150 to 175 yards. Once this ball became wet, it was totally useless. The Dimples- Why, then, does a golf ball have dimples? The answer to this question can be found by looking at the aerodynamic drag on a sphere. There are two types of drag experienced by a sphere. The first is the obvious drag due to friction. This only accounts for a small part of the drag experienced by a ball. The majority of the drag comes from the separation of the flow behind the ball and is known as pressure drag due to separation. For laminar flow past a sphere, the flow separates very early as shown in Figure . However, for a turbulent flow, separation is delayed as can be seen in Figure . The difference in the size of the separation region behind the spheres. The separation region in the turbulent case is much smaller than in the laminar case. The larger separation region of the laminar case implies a larger pressure drag on the sphere. This is why the professor experienced a longer drive with the marked ball. The surface roughness caused the flow to transition from laminar to turbulent. The turbulent flow has more energy than the laminar flow and thus, the flow stays attached longer. So, why dimples? Why not use another method to achieve the same affect? The critical Reynolds number, R, holds the answer to this question. As you recall, R is the Reynolds number at which the flow transitions from a laminar to a turbulent state. For a smooth sphere, R is much larger than the average Reynolds number experienced by a golf ball. For a sand roughened golf ball, the reduction in drag at R is greater than that of the dimpled golf ball. However, as the Reynolds number continues to increase, the drag increases. The dimpled ball, on the other hand, has a lower R, and the drag is fairly constant for Reynolds numbers greater than R. Therefore, the dimples cause R to decrease which implies that the flow becomes turbulent at a lower velocity than on a smooth sphere. This in turn causes the flow to remain attached longer on a dimpled golf ball which implies a reduction in drag. As the speed of the dimpled golf ball is increased, the drag doesn't change much. This is a good property in a sport like golf. . How a Golf Ball Produces Lift . Lift is another aerodynamic force which affects the flight of a golf ball. This idea might sound a little odd, but given the proper spin a golf ball can produce lift. Originally, golfers thought that all spin was detrimental. British scientist learned that a ball, driven with a spin about a horizontal axis with the top of the ball coming toward the golfer produces a lifting force. This type of spin is know as a backspin. The backspin increases the speed on the upper surface of the ball while decreasing the speed on the lower surface. From the Bernoulli principle, when the velocity increases the pressure decreases. Therefore, the pressure on the upper surface is less than the pressure on the lower surface of the ball. This pressure differential results in a finite lift being applied to the ball. The dimples also help in the generation of lift. By keeping the flow attached, the dimples help promote an asymmetry of the flow in the wake. This asymmetry can be seen in Figure 5. In this figure, the smoke shows the flow pattern about a spinning golf ball. The flow is moving from left to right and the ball is spinning in the counter-clockwise direction. The wake is being deflected downwards. This downward deflection of the wake implies that a lifting force is being applied to the golf ball. Hook and Slice A hook or a slice can be explained in the same way. If the golf ball is given a spin about it's vertical axis, the ball will be deflected to the right for a clockwise rotation and to the left for a counter-clockwise rotation. The generation of an aerodynamic force by a spin about the axis perpendicular to the flight path is known as the Magnus effect. The Magnus effect is important in most ball games.
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