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.
 
This idea—that one can predict the aerodynamic performance of a car by observing its shape and/or details, and by extension, and perhaps more importantly, that the behavior of airflow over a car can be predicted or intuited—is perhaps the most insidious in amateur aerodynamics study. It leads, after all, to other misconceptions I have discussed here: that tacking an “ideal” shape onto any car geometry will reduce its drag, no questions asked; that testing of aerodynamic modifications is not necessary; that simple thought experiments are enough to determine principles of aerodynamic design; that the “goodness” or “badness” of a particular design can be ascertained simply by looking at it.
 
Why can’t we? RH Barnard points out, “An important step in understanding air flows is to appreciate that air and other fluids do not behave in the same way as solid objects or even streams of solid particles; air molecules do not impact on the front of a vehicle, they flow over it.” Doug McLean, retired Boeing aerodynamics engineer, writes in his Understanding Aerodynamics: An Argument from the Real Physics (2013), “It is natural to expect that complexity in the flow requires complexity in the basic physics and that complex behavior in the flow must have its origin at a ‘low level,’ in the statistical behavior of the molecules that make up the gas or in the behavior of the particles that make up the molecules. But this natural expectation is wrong. Instead, the complexity we see arises from the aggregate behavior of the fluid represented by the continuum equations.”
 
Fluids, simply put, do not behave like solids despite us thinking they should. Where solids—even flexible solids—can be analyzed as discreet bodies or collections of discreet bodies, fluids behave continuously. When a fluid flow encounters a solid body like a car, it doesn’t “hit” the front of it like a bunch of particles, it flows around it as a continuous substance. The complexity of fluid flows arises from this fundamental difference between solids and fluids. The properties of the flow at the rear of a car can have as much of an impact on the front as vice versa. This difference is not intuitive; we instinctively believe that if something moves past a car from front to back, its effects must also move in the same direction. This speaks to the unintuitive nature of air flows; since our lived experience deals almost entirely with solids, they do not behave as we expect and thus are not predictable by simple guessing, least of all by unpracticed amateurs.
 
This is a common theme in textbooks and exists at the heart of the science of aerodynamic design. Barnard again: “An important feature of the subject of road vehicle aerodynamics is that it does not lend itself readily to mathematical analysis; there are no straightforward methods for predicting how air will flow around a given vehicle shape” (emphasis added). Hucho, in a paper given at a 1976 conference on aerodynamic drag at the GM Technical Center: “Despite the similarities of shape, the variance of drag among cars is remarkable.” GM aerodynamicist Gino Sovran gave the concluding address at that conference, and I think it’s worth quoting here at length. He said:
 
“We have explored the heretofore distinct possibility that answers to some of the key questions about the three-dimensional flow fields of road vehicles lie, for the taking, in the vast bluff body literature on simple, aeronautical and architectural configurations. Is it possible that the knowhow already exists and all that is necessary is to pose the proper questions to the right people? I think it is now clear that this is not the case. We might have hoped for better, but at least we can check off that possibility and get on with the specifically directed research that is required.”
 
That quote, from a respected aerodynamicist at the top of his field, should be enough to put to bed any assertions from know-it-alls on the internet that we learned everything we need to about aerodynamics in the 1920s, or that we can simply look at a car and guess what the air does as it moves around that particular shape, or that we can follow some simple guidelines for shaping or a particular template when designing aerodynamic modifications to our cars. Those assertions are all bullshit, plain and simple, “a tale told by idiots.”


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