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to increase fuel efficiency. The rougher a vehicle’s surface, the more drag the vehicle experiences. By eliminating even microscopic rough spots in a car’s paint coating, engineers help conserve fuel.
New coatings that cover a vehicle’s paint have been investigated to reduce skin friction drag. For example, thin plastic coatings overlaid on paint can affect the roughness of a vehicle’s surface and so affect drag. The best coatings create a smooth surface for airflow and resist scratches and corrosion, causes of surface roughness. A smooth surface then allows air to pass over the vehicle in laminar sheets, meaning that streams of air line up in parallel sheets as they move. Turbulent air, by contrast, contains eddies and swirls that cause skin friction drag.
Engineers study the boundary layer of air just next to a vehicle’s surface to determine why most of the boundary layer contains laminar flow but some parts contain turbulence. The German physicist Ludwig Prandtl introduced the idea of boundary layers in 1904 and explained that the friction between a moving object and air (or water) developed only in the thin boundary layer that existed very close to the surface. Scientists since Archimedes (287–212 b.c.e.) have pondered the way flow moves over and around solid objects, but Prandtl’s boundary layer theory proposed that all flow does not behave the same in the presence of a surface. Prandtl explained that flow nearer a surface moves slower than
flow farther from the surface: “A very satisfactory explanation of the physical process in the boundary layer between a fluid [or airflow] and a solid body could be obtained by the hypothesis of an adhesion of the fluid to the walls . . . If the viscosity [thickness of a fluid] was very small and the fluid path along the wall not too long, the fluid velocity ought
to resume its normal value at a very short distance from the wall. In the thin transition layer however, the sharp changes of velocity, even with small coefficient of friction, produce marked results.” In other words, air sheets close to the solid surface move at slower speeds and contribute to skin friction drag more than the airflow outside that boundary layer,
which moves at normal speed.
Surfacing engineering now focuses on substances to increase airflow at the boundary layer. Engineering experiments have discovered that a little roughness at certain points along the boundary layer actually helps with airflow by weakening the boundary layer–surface association. For this reason, some cars contain rough outer body components just under
the doors and below the radiator grill. Specialists in surface science now study the structure of sharkskin, which reduces a shark’s drag and allows it to speed through the water. Sharkskin contains a series of tiny rigid ridges called riblets that feel rough to the touch but greatly reduce drag in
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| Surface coating technology is a specialty in vehicle design and engineering. Surfaces should reduce drag and assist airflow. All sources of turbulent air movement, which decrease aerodynamics, must be eliminated or corrected. The vortex generator is a simple row of fins that smooth the airflow off the back of a vehicle where turbulence is common. |
water by interacting with the skin-water interface. The German engineer Volkmar Stenzel has been creating lacquers for vehicles and watercraft based on this riblet effect, sometimes by building the riblet structure into the lacquer itself. Stenzel said, “Our trial lacquer is based on the chemistry used in aviation paints.” Surface engineering to improve the aerodynamics
of a moving vehicle therefore remains an intricate science that borrows from technologies used in aircraft and watercraft. New vehicles will require the best combination of smooth surfaces, flow-direction surfaces, and vehicle shape.
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