Common Misconceptions in Aerodynamics: Part 5
Continuing this series of posts, now we'll look at a more practical--and more popular--fallacy.
Waxing Your Car Does Not Reduce Its Drag
The claim: Clean and wax your car’s paint and the air will slip smoothly past it, reducing drag.
|This screenshot comes from--surprise!--a website for a carwash chain.
|There you have it; Internet wisdom at its finest ("we agree, so this must be true").
The reality: This one pops up on forums and online articles all the time. Unfortunately, it is a misconception that seems like it should be true: if fluid/air flows past the car body, making that body more “slippery” by waxing it should help the air move past more easily, right? Wrong.
What we’re dealing with here is a fluid boundary or
“interface.” Interfaces can occur between fluids and solids or between different
fluids (think of an oil slick floating on water or better yet, wind rushing
over the open ocean; these are both examples of fluid interfaces). At these
interfaces, strange, non-intuitive things happen. In the case of a car, the air
moving past it achieves equilibrium of temperature and velocity at the
interface. Friction between the streams of air creates what is called a
“boundary layer” that surrounds the car body; outside of this boundary layer
the air moves (relative to the car) at the car’s speed, or “freestream
|(Image source: NASA)
|The Mercedes-Benz aeroacoustic wind tunnel. (Image source: Daimler AG)