Theory

How to Think About Car Aerodynamics
A very, very basic overview
Physical explanation, measurement devices, probes, and how to use them
Learn what yarn taped to your car can tell you about the airflow over it
A problematic method for measuring drag--with my suggestions for improvement
Coefficients in Aerodynamics Engineering
Just what is a "drag coefficient" anyway?
The difference between models and reality
Things I Wish I Had Learned Sooner
Lessons from a Top-10 engineering degree program

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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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What is Entrainment?

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Lots of technical terms in aerodynamics are thrown around by amateurs without a real, working knowledge of what they mean  and look like in practice, and consequently, without clarity of thought (I was certainly guilty of this for a long time). I’ve started to try and address this with a glossary, but here I’ll focus on one term in particular: entrainment . What is entrainment, and what is it good for?   Entrainment   To “entrain” is to cause something to be drawn along or to follow, and this works in fluids due to pressure differences. If I  “push” a fluid somewhere—say, out of a tube or nozzle—so that it moves at some speed and has a lower pressure than the fluid surrounding it, more of that surrounding fluid will want to come along for the ride, moved by the pressure differential. This is entrainment , and it’s a way of getting flow for “free” since you get more flow out than you put in.   Jet Pump   Now, the fun part. You can build a simple device at...
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Optimizing Aerodynamics of a Truck: Part 5

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After my last round of testing , I made a permanent version of the air dam extensions I tested: This brings my truck’s aerodynamic drag down to around 84-85% of what it was before I started this project:   Modification Percent Change mirrors removed, grill blocked -4.0% 9” air dam -8.6% air dam extensions -3.8% Total -16.4%   I didn’t expect to get that much with these changes, especially because I haven’t even tested anything on or around the bed, rear undercarriage, tailgate, rear wheel housings, or rear bumper.   Tailgate   Many websites claim that lowering a truck’s tailgate increases its drag. This was even explored on an episode of the popular television show Mythbusters ; the show’s hosts put a model truck in a water tunnel to illustrate the recirculation that happens in the bed (and ran some fuel economy tests with the tailgate down, removed,...
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