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A Practical Guide to Aerodynamic Modification

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Updated August 15, 2023 Tuft testing shows the streamlines on a car as the yarn aligns itself with airflow while you drive. Gas prices have recently reached their highest level in nearly a decade. You may find yourself looking at your car, wondering if it’s possible to use less fuel on your long commute and keep some money in your pocket. You may have heard of people who modify their cars to get better fuel economy. You might have even seen cars like the Aerocivic, a weird-looking contraption that was reported on in mainstream media articles during the gas price spike of 2008-09. Would doing something like that work on your car? Can you modify the aerodynamics of your car at home? The good news is, you can! The better news is, you don’t have to (and shouldn’t) make your car look like the Aerocivic. Air drag has an influence on the fuel economy of cars, and that influence is greater the faster you typically drive. You can also do a lot more with airflow than just reduce drag. Many peo
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Optimizing a Tail for Low Drag: Part 2

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Formulating Requirements   Before I begin testing (and especially before I begin construction!), I want to identify the requirements my tail should meet. “Requirements” specify the objectives a design must fulfill and how it should achieve them. Requirements can be split into two categories: technical (or engineering) and stakeholder. Stakeholder requirements lay out the needs or desires of all stakeholders in a project (in this case, that’s just me). Technical requirements specify the performance objectives a design must fulfill in exact language with specific, measurable goals.   TECHNICAL REQUIREMENTS   Length   I wrote in my first post that tails are a good option for reducing drag if you don’t care about the length of your car—since a tail requires length to function, adding length to your car is unavoidable if you decide to build one.   But how much length you’re comfortable adding is something you’ll have to decide. My car still has to fit in my garage (first constraint), and s
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Optimizing a Tail for Low Drag: Part 1

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Now that the semester is almost over and I have no plans for the summer, I decided to revisit a modification I’ve cursorily stabbed at before, in a not-very-smart manner: the drag-reducing tail. I’ll elaborate on these poor attempts as I walk you through this project and how I’m approaching it differently this time. But before all that, first we need to visit some theory and look at how tails are (often incorrectly) characterized in online discussions, what their purpose is, and how best to design one.   Streamlining   First, some basic definitions. A lot of the simplified aerodynamic theory you’ll encounter online is based on analysis of airfoils, which is an important part of aeronautics but not always applicable to cars. One of the reasons for this is that cars are not streamlined shapes. Even the most perfectly shaped production car has a large area of separated flow at its back end: …which you can see here. My Prius has attached flow over most of its upper and side surface, bu
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Three-Dimensional Flow Fields

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Every day, it seems, I understand something new I had failed to grasp before—especially in aerodynamics. For example, take this statement:   "Once again, it is necessary to remember that road vehicles and their air flow patterns are highly three-dimensional " (Barnard 15, emphasis added).   This always bothered me a little. Of course the flow over cars is 3-dimensional, I thought; how on earth can something be more or less , let alone highly 3D? Isn’t flow just...3D or not 3D?   Well, a few weeks ago in an Incompressible Flow lecture I finally understood it. Now you can too. Investigating the veracity of Barnard's claim. I've done this before, and have yet to find him wrong. Flow Fields   A field is a region of space where properties vary as a function of position within that space. The flow field around a car is the 3-dimensional space where the seven properties needed to completely characterize a flow (pressure, density, temperature, viscosity, and three compo
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Testing a Smooth Engine Undertray

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When I investigated the effects of a splitter on my Prius , I discovered something unusual: gauge pressures on the stock engine undertray were a lot higher than I expected. Julian Edgar’s Vehicle Aerodynamics: Testing, Modification & Development includes several examples of engine undertrays with measured pressures much less than atmospheric. My test showed that the Prius undertray was developing pressures at atmospheric or higher. What was going on? Gauge pressure at 80 kph. Left: no splitter. Center: with splitter. Right: difference. Hypothesis and Testing   So, I’ve got a problem here I want to investigate: high pressures on the engine undertray where most examples I’ve read about have lower pressures. Where to begin?   When you investigate something like this, a good place to start is fundamental principles. I know that velocity and pressure are related, and that as pressure goes down, velocity goes up. So the velocity under my car’s engine undertray must be slower than on
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Coefficients in Aerodynamic Engineering

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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 ( p R ), reduced temperature ( T R ), and reduced specific volume ( v’ R ), where 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 a
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