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.
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| 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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