Common Misconceptions in Aerodynamics: Part 4
Now that we've looked at the fallacy of using templates to guide aerodynamic modifications, the necessity of testing, and the complicated nature of aerodynamics, today these all come together in the false idea that airflow can be predicted or intuited.
Intuitive Design Is Not an Effective Approach to Reducing Drag
The claim: Since airflow can be predicted with complete certainty and behaves according to simple principles, it is similarly possible to predict the aerodynamic performance of any design by using intuition.
|It's so easy--just absorb these (oversimplified) schematic representations of the theoretical flow over prismatic bodies. Just like a real car!|
The reality: Along with a severe phobia of
testing aerodynamic changes, it seems that most—perhaps all—of us harbor the
innate idea that we can ascertain whether a car has low drag or not simply by
looking at it. In some cases this works: look at a 1932 Ford Model A coupe, for
example, and you don’t need to see any numbers to know that it won’t be as
low-drag as a modern car like the Toyota Prius.
But how about that Prius compared to, say, a Tesla Model 3? A
10th-generation Civic hatchback? Which one has lower drag? Lower
lift? Supporters of “template” theory might compare the profiles of these with
a Klemperer-esque shape and conclude that one or the other is more
aerodynamically efficient. In fact, one thread on a well-known web forum years
ago predicted flow separation over the rear window of the Prius because it did
not conform to the “template,” a conclusion that led commenters to chide Toyota
for making such a simple mistake! Never mind that the company’s own press
material for the 2010 Prius stated that it had spent more hours in wind tunnel
development than any other Toyota model to date, and that a simple tuft test
shows immediately that, yes, the car has attached flow over the backlight.