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.
|(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).
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