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How to Think About Car Aerodynamics: A Very, Very Basic Overview

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The study of aerodynamics is complicated. If anyone tries to tell you otherwise, run the other direction—it’s a sure sign they don’t know what they’re talking about.   Over the last two years especially, my thinking about aerodynamics and appreciation of its complexity has changed dramatically—a result of my going back to school to get another bachelor’s degree, this time in aerospace engineering where a good working knowledge of airflows is required and education in not just general fluid mechanics but also aircraft aerodynamic design forms a core part of the technical curriculum. I'm in the midst of my last semester now and to clarify my thinking at this point I decided to put some things in writing in the hopes they might help someone else as well as myself, specifically focused on car aerodynamics. A word of warning: I've tried to minimize the amount of math below, but some mathematical relations are unavoidable if you want to build an understanding of fluid flows. If anyth...
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Measuring Spoiler Performance

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In my previous spoiler testing, which was extensive, I made a lot of mistakes due to the fact that I only had a little education in engineering. Specifically, there were two major issues that I now want to correct: I did not normalize pressures (that is, turn them into dimensionless values for easier comparison), and I tested at only one Reynolds number. Because of these issues, the results aren’t really generalizable and are not easy to interpret. Let's fix that. Production spoilers are often art as much as—or more than, given how much styling sometimes dictates their shape—functional devices. This Toyota 86 has a very aggressive spoiler design for a production car. Recap   One of my most-read posts here has proven to be an article on spoilers I wrote early on, back when I didn't really know anything yet since I hadn't formally studied aerodynamics. I got the criticism of online theory right (yes, that weird theory that spoilers create "ideal" streamlines—w...
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Measuring and Improving Cooling System Performance – Part 4: Heat Exchanger

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A "heat exchanger" is, as the name suggests, a device for transferring energy from one working fluid to another. Here, we want to transfer internal energy from the liquid coolant to air; this transfer process is called "heat." If we input enough heat, we can use the increased energy of the air to generate thrust; this is basically what jet engines do. However, we're constrained in designing or modifying a road car by low mass flow. While a car cooling system can ingest a few pounds of air each second, a GEnx-2B67 engine on the 747-8, for example, swallows more than one ton per second at takeoff (that's per engine —747s have to move a lot of air to get off the ground!). Additionally, small temperature differences and tight engine packaging will make it difficult to get thrust/negative drag overall—even in the best case, the theoretical maximum thrust is very, very small. A better goal is simply to minimize drag from the cooling system, which we'll look a...
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Measuring and Improving Cooling System Performance – Part 3: Diffuser

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"Diffuser" is one of those terms that lots of car modifiers use but very few understand. In popular car culture, diffusers are rear bumper garnishes, grills, or add-ons that somehow magically create downforce, and you will find lots of these faux diffusers available for purchase at online retailers.   You might be surprised to learn that nearly every car manufactured in the last 20 years has at least one actual diffuser on it (most have open rear diffusers now too—"open" because they lack sides and thus are not true diffusers. The ground forms one wall of these diffusers while the opposite wall is a plastic panel under the trunk or, quite often, the muffler). The incorporation of this real diffuser into basic car design has had important effects in improving cooling system capacity and reducing cooling drag in modern cars. It isn't there for downforce, yo ; it isn't just for race cars; and it isn't found at the rear of the car. The most important diff...
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