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 velocity.”

(Image source: NASA)

The study of boundary layers is a fascinating topic in itself. Boundary layers have to be carefully controlled in wind tunnel testing; as air is blown past the floor a boundary layer develops there that is not present in the real world where the relative velocity of the road and air is zero. This boundary layer must be corrected or removed to accurately simulate real-world conditions; often, a slot just upstream of the car is used to suck out the developing boundary layer. Fixed-floor tunnels, however, still suffer from the inaccuracy of a boundary layer building up under the car, skewing results. Additionally, boundary layers on the walls of the tunnel can influence test results. Because of this, modern tunnels are quite large to keep the “blockage ratio,” the ratio of the car’s size to the tunnel opening, small—usually less than 5%--and use a “rolling road” or belt under the car that moves at the same speed as the airflow.

The Mercedes-Benz aeroacoustic wind tunnel. (Image source: Daimler AG)

The boundary layer on a car also increases in thickness moving from front to back, as energy is lost to friction between the layers of air. But no matter its thickness, the “side” of the boundary layer touching the car is subject to the “no-slip condition” i.e. its velocity relative to the car body is so close to zero that it can be ignored. Doug McLean, in Understanding Aerodynamics: Arguing from the Real Physics, explains, “The upshot is that gas molecules impinging on real surfaces bounce off in effectively random directions, which forces the average tangential velocity of molecules near the surface to be very small. Kinetic theory can be used to estimate the effective slip velocity, showing that in practical situations it is practically zero.” Dietrich Hummel, in Aerodynamics of Road Vehicles (4th ed.), says, “Within the boundary layer the velocity decreases from the value of the inviscid external flow at the outer edge of the boundary layer to zero at the wall, where the fluid fulfills a no-slip condition.” T. Yomi Obidi, in Theory and Applications of Aerodynamics for Ground Vehicles, writes, “Consider the flow past a flat plate shown in Fig. 1.11. At the fluid-plate interface, there is no relative motion between the fluid (air in this case) and the plate.” RH Barnard, in Road Vehicle Aerodynamic Design, says, “An important feature of the flow past a vehicle is that the air appears to stick to the surface. Right next to the surface there is no measurable relative motion. You may have noticed that loose dust particles are not blown off a car’s surface even at high speeds. Individual molecules do not actually physically stick, they move around randomly, but their average velocity component parallel to the surface is zero.” (Emphasis added).
Now, aerodynamics references frequently discuss “surface drag,” but in this case the terminology is confusing to laypeople because it does not indicate a friction force generated between the fluid and a solid body. “Surface drag” in aerodynamics refers to energy losses in the boundary layer. Because there is effectively no relative movement between the car and air at the body/fluid interface, there is no mechanism to generate friction drag from the moving airstream there. Thus, waxing the car doesn’t do anything to reduce its drag (except make it look faster, of course).