Amateur Aerodynamics

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

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

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

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