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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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Explainer: Aerodynamic Pressure

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In aerodynamics engineering, we talk about pressure a lot . But what is pressure anyway? It's something we're all familiar with in our everyday lived experience: your ears "popping" when you drive up a mountain due to the pressure change with altitude; feeling drained or tired after a long flight in a cabin at a different pressure than what you're used to; sticking your hand out a car window and feeling the "push" of the air backward. You might even remember from a physics or chemistry course that pressure arises from the molecules in a gas zipping around, occasionally bouncing off a surface, and that the faster they move, the greater is the pressure the gas exerts. "Movement" implies velocity alone, but of course the number and mass of these molecules also matters: more mass, greater pressure. Mass and velocity multiplied give momentum , and pressure arises from the transfer of momentum between molecules in a gas and any surface in contact wit...
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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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