A Glossary of Aerodynamics: Part 1

Cicero, in his letter On Moral Duties, wrote, “The definition of terms must in fact form the basis of every scientific exposition if the scope of the argument is to be clearly understood.”

Terminology in aerodynamics is often not well understood even by people who think they have a good grasp on it, let alone those mostly or wholly unfamiliar with the field. You will see people throw around words with apparently little or no understanding of what they actually mean all the time or use vague terminology in an attempt to impress or obfuscate (I was guilty of this for years, aping what I saw modeled in online forums).

To help dispel some of this uncertainty, here’s a primer on words and phrases you’ll see used in forum discussions and online articles about aerodynamics, often incorrectly—with accurate definitions pulled straight out of car aerodynamics textbooks.

Aerodynamics: “Aerodynamics is simply the study of the forces involved in the movement of an object through the air. Various objects—airplanes, cars, trains, footballs, cricket balls, tennis balls, baseballs, feathers...—interact with and are therefore affected by the dynamics of the surrounding air” (Obidi xvii).

This is a big one. You will see people refer to cars as “more aerodynamic” or “less aerodynamic” all the time when what they really mean is higher- or lower-drag. “Aerodynamic” simply means the object moves through air (or that the air moves past the stationary object, e.g. around a building) and is subjected to forces created by that moving air.

Drag: “The resultant aerodynamic force is resolved for convenience of the analysis into the following three components: (1) The aerodynamic drag force DA which is horizontal and opposes the forward motion of the car” (Scibor-Rylski 12).

Drag is one component of the total aerodynamic force acting on a car—specifically, the component that aligns with its longitudinal axis. This is arbitrary; we could set our axes in any orientation we like so long as all three are normal (perpendicular) to each other. But it is most useful to align them with the direction the car travels in a straight line and…


Lift/Downforce: “(2) The aerodynamic lift force LA which is directed vertically and at right angles to the drag” (Scibor-Rylski 13).

…the vertical direction, acting along the same axis as gravity. It is important to point out here that one of the many misconceptions you will see online is the idea that “lift” and “downforce” are fundamentally different things; just this week I came across a forum post suggesting that a particular mod might increase lift and the next reply chiming in that additionally, it might even decrease downforce. In this context, these describe the same thing: an increase in lift!* “Downforce” is simply the terminology for negative lift i.e. “lift” that pushes down (negative along the z axis in the image above) rather than up (positive). In fact, some textbooks don’t even use the term “downforce,” and those that do tend to favor it only in discussions of high-performance cars where large negative lift is a priority.

(*Kind of—since “downforce” can be used correctly to refer to negative lift, it can vary from some positive number to zero, with its opposite direction described simply as lift. Just be careful with this word; in the example above, where we don't know if the car in question has negative lift/downforce in the first place, it can only be interpreted as the same thing as an increase in lift—which means the second poster said the same as the first without realizing it).

Sideforce: “(3) When the airflow is not symmetric to the car shape we have a sideways component of the aerodynamic force, YA, which acts horizontally but at right angles to both the drag and the lift” (Scibor-Rylski 13).

By virtue of setting our first two axes parallel to the longitudinal and vertical directions, the third must run sideways through the car (in mathematical terms, this third axis is the cross product of the other two—the vector normal to both of them). If the oncoming flow is yawed (that is, skewed in relation to the direction of the car), the airflow over the car will be asymmetric and the aerodynamic force will “point” to one side or the other.

Laminar: “A state of flow where the various fluid ‘sheets’ do not mix with each other, where all stream tubes keep essentially parallel to each other and where their velocities are steady” (Hoerner 1—4).

This is another big one because it is often misunderstood. Named for the Latin lamina (“plate” or “blade”), it denotes air moving in smooth “sheets” with no mixing between layers. Very little of the airflow over real cars is laminar because of their size and speed (and because they move through a turbulent atmosphere; see also Reynolds Number below), but laypeople often think that reducing turbulence is the same as transitioning the flow to a laminar state. It is not. Once the flow has become turbulent, which happens very near the front of the car in most conditions, you can’t go back. Ultra-low-drag vehicles like HPVs and solar cars may use smooth shaping to try and delay the transition to turbulence—and it might be possible to do the same on your car—but once it’s gone, it’s gone.

Turbulent: “Turbulence is defined as the transition of a flow from flowing in an orderly steady state referred to as laminar flow to an unstable state in which high-frequency chaotic fluctuations, due to the dynamics of small vortices, are superimposed on all flow parameters” (Dillman 118).

The important takeaway here is that flow isn’t just laminar or turbulent; the degree of turbulence can vary, and it is possible to “smooth” flow even if it isn’t laminar (which, I suspect, is what most people mean when they talk about “laminar” flow). Reducing the magnitude or the frequency of “chaotic fluctuations,” for example by fitting panels to smooth out the underside of your car, can reduce turbulence and, subsequently, lift and drag. When you tuft test your car, pay attention to how much and how fast the tufts move around; if you make a change and they move less, you’ve probably made the flow less turbulent.

The flow here is turbulent—note the slight movement of the tufts—but the magnitude of that turbulence is small and the flow is generally smooth. Don’t call this type of flow laminar—it isn’t!

Reynolds Number: “The Reynolds number is the ratio of inertial force to viscous force” (Obidi 87).

Reynolds number—abbreviated “Re”—determines at what distance on the body of a car its flow tends to transition from laminar to turbulent. This happens as the inertial force of the flow (a product of displacement, velocity, and density) becomes much larger than its viscous force. A low-viscosity fluid like air moving at high speeds over a large object such as a car has a high Reynolds number, on the order of 105 or 106
, and consequently it develops turbulence over most of the car’s surface. This can be a good thing (see Boundary Layer below).

Attached Flow: “In more scientific terms a streamlined shape can be conveniently defined as one where the air flow follows the contours. In this case, the flow is said to be attached” (Barnard 7, emphasis original).

Attached flow over the hood of my truck. The air follows the curve of the hood and windshield.

Separated Flow: “A separated flow does not follow the contours of the body” (Barnard 10).

Separated flow. The air no longer follows the shape of the hood, as indicated by tufts flapping around in different directions.

Boundary Layer: “Within a comparatively thin layer of air surrounding the moving body there [is] a sharp variation of velocity, from zero at the surface to the local airflow velocity outside this layer. Within this layer there is therefore a velocity gradient. The layer itself is referred to as the boundary layer” (Scibor-Rylski 16, emphasis original).

(Image source: NASA)

You can think of the boundary layer as an envelope of varying thickness that surrounds the car everywhere its airflow stays attached. It forms because on the car body there is no relative motion between air and car—the lowest “layer” of air is dragged along with the car, and the shear stress between parcels of air in the boundary layer is responsible for friction drag as the velocity changes from 0 at the body surface to whatever the velocity is at the upper edge of the boundary layer. One of the benefits of turbulence is the tendency of a turbulent boundary layer to stay attached to curves that a laminar boundary layer won’t.

Bibliography

Barnard, R.H. Road Vehicle Aerodynamic Design: An Introduction (3rd edition). St. Albans:
MechAero Publishing, 2009.

Dillman, Andreas. “The Physical Principles of Aerodynamics.” In Aerodynamics of Road Vehicles
(5th edition), ed. Thomas Sch├╝tz (Warrendale: SAE International, 2016), 75-149.

Hoerner, S.F. Fluid-Dynamic Drag. Bakersfield: Liselotte Hoerner, 1992.

Obidi, T. Yomi. Theory and Applications of Aerodynamics for Ground Vehicles. Warrendale:
SAE International, 2014.

Scibor-Rylski, A.J. Road Vehicle Aerodynamics (2nd edition), ed. D.M. Sykes. London: Pentech Press, 1984.

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