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A Practical Guide to Aerodynamic Modification

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

Three-Dimensional Flow Fields

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

Testing a Smooth Engine Undertray

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

Coefficients in Aerodynamic Engineering

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

Thinking About EV Efficiency

Bardeen Quad, UIUC, with the Mechanical Engineering Laboratory and Engineering Hall visible in the distance. When I came back to school and met with an advisor last summer before registering for my fall semester classes, I was gobsmacked to find that the university required me to take RHET 105, a 100-level freshman composition course. A perfect storm of stupidity happened to flail together: the Transfer Credit Office did not accept the freshman writing course I took in my first undergraduate degree program to satisfy this requirement (for whatever reason); SAT and ACT scores could not be submitted after admission to satisfy this requirement (my scores are more than twenty years old, but easily exceed the minimum for composition credit); none of my masters or doctoral coursework apparently satisfied this requirement, nor the fact that I have the master’s degree and have taught at 3 universities including this one . So, here I am—a victim of mindless university bureaucracy, stuck in a co

Common Misconceptions in Aerodynamics: Part 9

Aerodynamics Matter Regardless How Fast a Car Goes The claim: Drag only matters if you go above a certain speed. 45 mph/50 mph/55 mph/60 mph—take your pick, as the critical speed depends entirely on the person reporting this myth. The reality: People who repeat this claim are telling only part of the story—as is true of many, if not all, of these misconceptions. To see why, we need to go back (again) to some fundamentals of fluids in motion.   If you’ve ever taken a college physics class, you might remember something called the “Bernoulli equation.” This equation describes the relationship between internal energy, potential energy, and kinetic energy in a fluid—not just a fluid in motion, but any (incompressible) fluid (e.g. the “hydrostatic equation” is the Bernoulli equation simplified for a static fluid). By definition, the specific volume, v , of an incompressible fluid must remain constant, allowing us to divide out volume, V , and convert the energy terms into pressures. Because