Monday, January 3, 2011

aerodynamics - everything you want to know


Also known as dive planes or dive plates, since they resemble the winged appendages on submarines, canards help generate downforce in two different ways. First, the canard redirects the oncoming air's momentum upwards, which causes a downward force on the canard. This is only moderate, since the velocity near the skin is significantly slower than in the free stream. In addition, canards generate strong vortices that travel down the sides of the car and act as a barrier. If the canards are positioned correctly, these strong vortices act to keep high-pressure air around the car from entering the low-pressure underbody region, thus maintaining more downforce. If air was allowed to enter the underside, the pressure would inevitably rise, reducing downforce. Therefore, these strong vortices act like a virtual curtain or dam, restricting higher-pressure air around the car's sides from entering the underbody region. As a result, the low pressure under the car is maintained and downforce is maximized. Unfortunately, canards are not that efficient , since the strong vortices create a significant amount of drag. They are more useful for fine-tuning aerodynamic balance.




Side ducts are primarily seen on race cars for two reasons, because brake and engine cooling is crucial, and because most serious race cars will use a front underbody diffuser that channels airflow toward the rear of the front wheel well. Conventional fender designs trap much of the turbulent air coming off the top and back of the tire generated by the counter rotation of the tires and wheels. Combined with hot air moving through the engine bay and brakes, this generates losses and drag. Side ducting not only provides a smooth outlet for these hot and turbulent gasses, but also turns the flow to exit smoothly along the side of the car instead of directly outward, which would interfere with the turbulent curtain generated by the canards. This reduction of air stagnation inside the bay also helps pull more fresh air through the cooling system.
The air dam's job is to restrict the amount of air going under the car. By using a vertical barrier made from either a composite material or aluminum sheet, the air dam effectively reduces the opening leading to the underside of the car. By restricting flow under the car, more air is forced around the sides and over the top of the bodywork at higher pressure. The limited air forced underneath has to pass through faster and thus at a lower pressure which causes a suction effect. Air dams are more common in production cars with higher ride heights and bumpers.

Splitters, the horizontal plate extending forward and underneath the air dam, use the same principle but operate differently. Since the front of the car is a blunt shape, the oncoming air is slowed substantially, resulting in a high-pressure zone known as a stagnation point. By placing a horizontally protruding splitter plate right in the thick of this high-pressure zone, a large amount of efficient downforce can be generated. The splitter, hence its name, splits the high-pressure zone from the low-pressure high-speed flow moving under the car. Pressure varies with the car's speed squared, so downforce increases quickly as the speed increases. Generally, the effects are felt at speeds over 75mph. Downforce can be increased or decreased, depending on the amount of exposed splitter area, and an adjustable splitter area can be used to fine-tune the aerodynamic balance. As is true with most race cars, the Nismo GT-R uses a splitter only, on account of its low ride height and large ducts that feed its engine bay.



Unlike the prominent ram-type intakes seen behind the driver's head in Formula One cars, NACA ducts are submerged into the bodywork. When they were developed for the National Advisory Committee for Aeronautics (NACA) in 1945, these ducts where called 'submerged-duct entrances'. NACA ducts are low-drag intake channels used for a variety of cooling requirements such as brakes, engine, and even the poor overheated driver. The NACA duct's distinctive geometry includes a widening mouth at the inlet, with the duct floor slightly opening up the flow area. Extensive wind tunnel testing of various designs has resulted in the best compromise of flow rate to drag. In the case of this GT car, the NACA duct on the hood feeds small airboxes that direct cool air into the front brakes. Sharp wall-edges effectively generate vortices that help keep the flow attached to the diffuser-like slope floor. These edges have to be sharp (unlike many aftermarket parts copies), otherwise the flow would separate, reducing the duct's efficiency.


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