Simple tech: Wind resistance

Shashank Singh  /
24 Dec 2015 18:26:21 IST

Aerodynamics is such a vast subject that it would take many, manySimple Techs to explain it fully. But in thesimplest of terms, aero refers to air anddynamics, refers to its flow. Aerodynamics,then, is the science of how air flows, inour case, around objects in its path, likea car or a motorcycle. In this context,automotive engineers worry specificallyabout two interactions between air andour car/motorcycle, drag and downforce.When a car starts moving, themovement causes friction between thesurface of the car and air. This resistsmotion and is called skin friction and isone component of drag. Drag dependson many things but topping the list of thebig contributors is the shape of the car,specifically its frontal area. The larger thearea, the greater the drag - this componentis called 'form drag'. That's why buses aren'tvery fast and aircraft are usually pointy atthe front. The density and velocity of theair also play a role and there's a constantcalled drag coefficient which we will comeback to. The point is, drag hinders motion.So when you are suffering drag, you haveto burn more fuel to overcome it. It getsworse. Air has a unique tendency of actinglike a liquid at very high velocities. Thisbehaviour increases drag exponentially asspeeds rise. It is the reason why very highspeed driving consumes an inordinateamount of fuel.Enter the study of aerodynamics. Anaerodynamics engineer's job is to takea car design and find tweaks that allowdrag to be reduced as far as possible. Thegeneral silhouette of all cars today, in fact,is a function of their need to be as dragfree as possible.

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The reduction of drag begins in awind tunnel, which is a closed-off spacewith a large fan that the engineers cancontrol to generate specific wind velocitiesaround a real-size car or a to-scale model.What the wind tunnel helps measureis the drag coefficient, a number thatengineers spend vast amounts of effortto reduce. In essence, the bigger the dragcoefficient (written as Cd - coefficient ofdrag), the more resistance that vehicle willencounter.

Typically, a current day production carusually has a Cd between 0.30 and 0.35.The slickest cars like the Jaguar XE or aMercedes B-Class can drop that to 0.26and still more aerodynamically efficientare cars like the Toyota Prius and a fewMercedes-Benz cars that manage 0.25 orlower. But while you would think thatmaking supercars slippery is why theyspend so much time in the wind tunnelbut they're in the hunt for downforce.Downforce is the exact opposite oflift, which is what aircraft use to fly andstay up in the air. In effect, the windtunnel time of a supercar is used to refineaerodynamic forms which cut drag andalso create a downward force that pushesthe car down harder on to its wheels asspeeds rise. This downward push generatesmore grip that fast cars can use, forexample, to go around a corner even faster.One way to understand drag anddownforce is that if you were to take afamily sedan and drive it in a vacuum, itwould report a higher top speed as wellas better economy. A supercar in thesame situation would do that as well butbe unable to corner at speeds it wouldmanage normally.Wind tunnels, however, are veryexpensive. The cheap solution increasinglyis to use computational fluid dynamics orCFD to predict (very accurately in mostcases) how wind and a car's shape wouldinteract. Next month, we'll dig deeper intodownforce and what it has come to meanin the world of motor racing.

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