Coefficients in Aerodynamic Engineering
If you read anything online about aerodynamics, you will come across something called a drag coefficient. Often, articles on blogs, magazine websites, or Youtube videos will explain this drag coefficient as a measure for comparing the aerodynamic efficiency of two cars or trucks. But what is a “drag coefficient” exactly?
Coefficients in Engineering To answer that, we need to step back and look at the
concept of coefficients more generally. Coefficients are nondimensional numbers—that is, they
have no units and don’t represent a measure of something physical like
speed or force. For example, engineers working with compressible gases use several coefficients to determine properties of these
gases, including reduced pressure (pR), reduced
temperature (TR), and reduced specific volume (v’R),
Coefficients of performance are used to calculate the efficiency of refrigeration or heat pump cycles, or the thermal efficiency of power cycles (like the homework problem above):
In aerodynamics, one common coefficient is Reynolds number, which is the ratio of inertial force to viscous force in a flow:
Something you may have noticed about all these coefficients is: they are all ratios (of pressure, temperature, specific volume, energy, force, and area, respectively, for the examples above). The reason for this is simple. If you compare two measures with the same units in a ratio (fraction), the units cancel and you will be left with a nondimensional number—a coefficient. Coefficients are useful for summarizing a comparison of one parameter with another of the same units in one number.
Aerodynamic Coefficients Coefficients in aerodynamics work exactly the same way;
they are ratios. Aerodynamic drag force can be
described as the product of pressure and area:
Pressure here is symbolized by q rather than p because it is the dynamic pressure of the airflow (dynamic pressure is given by static pressure subtracted from total pressure, and it is derived from the kinetic energy of the air in motion), and A here is something we call “drag area.” You can think of the drag force measured on a car in a wind tunnel as being represented by the dynamic pressure of the air acting as a pressure difference between two sides of an imaginary plate of area A, which produces the same force acting on the plate as acts on the car.
So how do we get a drag coefficient out
of that? We use a ratio of the drag area to some reference area (S)
that is based on the actual dimensions of the car; in car aerodynamics, this is
typically the projected frontal area, or the area covered by the car’s
shadow if a uniform light source directly behind it shines onto a wall in front
of it. Note that projected frontal area is simply the maximum cross section area and has nothing to do with the shape of the front surfaces of a car—something even experienced engineers get wrong.
|"Frontal area" does not refer to the shape or surface area of the bumper, hood, and windshield. The new Prius has ~10% higher aerodynamic drag than the previous generation, based on Toyota's published dimensions and drag coefficients.
Our reference area doesn’t have to be frontal area, either; in aeronautics, reference area is usually wing plan area (as in the lift-induced drag equation above; S in the denominator is wing plan area, and the coefficient is proportional to the ratio of plan area and span, b, squared). But conventionally, frontal area is used to calculate a drag coefficient for an automobile:
Drag coefficient is properly equated to a ratio of forces (total drag divided by [dynamic pressure times reference area]; this is what a proper dimensional analysis will produce), but it simplifies to a ratio of areas. You can also think of the drag coefficient as a percentage; that is, how big a car's drag area is as a percentage of its reference area. A car like my Prius (CD = 0.25) has a drag area 25% as large as its reference area; a Rivian R1T (CD = 0.31) has a drag area 31% as large as its reference area; a Formula SAE car (CD = as high as 1.30) can have a drag area 130% as large as its reference area.
|To get an accurate frontal area for your car, you’ll need a better picture than this. Take a photo from as far away as possible to minimize distortion.
But if that car is a big SUV with S = 3.38 m2 (Chevrolet Suburban-sized), its drag force at the same speed will be:
…or 64% greater despite having the same drag coefficient.
Where drag coefficients are useful is in
comparing vehicles of similar size/class e.g. full-size trucks or mid-size
SUVs, etc. where vehicles in the same size class tend to have similar dimensions. Where they are not useful is in comparing vehicles of differing
size because this leads to incorrect conclusions. And the size difference doesn’t
need to be extreme to throw things off. For
example, one might conclude based on drag coefficients that the current Toyota Prius (CD = 0.27) is “more aerodynamic” than the
GT86 (CD = 0.28). In reality the GT86 has lower aerodynamic drag due to its smaller size and thus smaller reference
area used to calculate its drag coefficient, which we infer from the relationship between drag area and reference area will make CD appear larger for the same drag force.
|Modifications like this large air dam that reduce drag force but increase S mean the percent change in drag coefficient does not scale with percent change in drag area. In this case, since S has increased, CD will decrease more than if S remained constant for the same reduction in drag.
(*This test wasn’t without its problems: the tunnel used has no moving ground plane and is on the small side, giving a high blockage ratio for full-size cars tested in it. But it is useful here in comparing the drag force vs. coefficient of several cars measured in the same tunnel and same conditions; just take all these numbers with a grain of salt if you use them to compare to other tests).
|This tailgate spoiler, unlike the air dam above, does not change the reference area of the truck since it sits entirely within its projected frontal area already—so here, percent change in drag coefficient will equal percent change in drag area (+8% in this configuration).