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CFdesign在赛车NASCAR外流场计算中的应用

By Mike Snow

       Legend has it Dale Earnhardt was a master at plate racing because he could “see” the air as it swirled over cars, giving him an advantage in the draft. Unlikely, but Earnhardt had a superb feel for how his car was buffeted in an opponent’s wake. Racecar builders, however,had to rely on simple wind tunnels to learn with any precision how airflow affected a car—until Computational Fluid Dynamics (CFD) came along. Fluid dynamics is the study of how liquids and gases interact with a surrounding solid—think of air flowing over the complex shape of a Cup car. Recent CFD developments bring computer power to bear, modeling virtual cars in virtual air. Builders can study reams of data before shaping metal, and that saves time and money.
       Dodge used CFD extensively when designing its new Charger;some results of that computer work are seen in the image above,which has been modified slightly to protect proprietary information.
       According to Jim Spann of Blue Ridge Numerics, a firm that provides CFD software called CFdesign to several NASCAR teams, it’s all about gaining an information edge. “Physical tests [in a wind tunnel] tend to yield only limited data. With CFD, you can see a full product performance picture; literally every inch of the car has data attached to it. A conventional test might yield five data points.”
       The data point that matters most, of course, is lap time: With qualifying times at Cup races shaved to as little as 1/1000th of a second, knowing precisely how a car responds to air is hugely important. CFD allows a builder to see, for example, that air is not flowing over the A-pillar on the driver’s side as well as it should. A slight change is made to the virtual car and another simulation is run. Then the real car is built and tested.
       Racecar design, however, is much more than the single-minded pursuit of aerodynamic perfection; builders must first plug in the NASCAR-mandated specifications—designed to equalize performance between cars. And then there are cosmetic issues. According to John Fernandez, director of Dodge Motorsports, “Once NASCAR gives us the specs that define the actual three-dimensional area of the car, we then factor in our identity boundary,” or the minute differences that distinguish one brand from another—check out the Charger’s nose.
       CFD has accelerated the whole design process. According to Jean-Michel Esclafer de La Rode, Dodge’s chief CFD manager, “Drafting studies [that once] took a couple of weeks to simulate, we can do in a few days today.”


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