Amateur Aerodynamics

When I first started this blog, I wrote a post on tuft testing which turned out to be one of the more popular articles here. I covered the basics of tuft testing but recently thought I could write a little more about how to interpret tuft behavior and what this looks like in the real world. This article will demonstrate how to set up a tuft testing session, how to get usable results, and how to start analyzing them.   Setup   As I wrote before, tuft testing is cheap and easy; all you need is yarn, tape, and a camera.   Your tufts shouldn’t be too short (they won’t show up well on camera) or too long (long, flapping tufts will give you less resolution in the airflow patterns). I like to hold a length of string between my second and third fingers, stretch it across my hand, and then cut it near the thumb: This gives me pretty consistent 60-70mm tufts—long enough to be visible to a camera some distance away.   Once you have your tufts cut, tape them to your car. You...

In my first round of measuring pressures across radiators , I found that small amounts of grill blocking can significantly reduce cooling airflow through heat exchangers, and that the optimization of engine bay flow in more modern cars can have a major effect on the flow through radiators. My older truck had less than half the pressure differential across its radiator than my Prius had through two stacked radiators (one in front of the other), and the instantaneous pressure readings in the truck jumped around a lot while in the Prius they were stable.   Recently, I went out again to test radiator pressures on the Prius after making a modification to the airflow under the car. In stock form, the large central exhaust tunnel is open: One change from stock here is the addition of a catalytic converter shield, visible just at the back of the engine undertray. Generally, cars benefit from having smooth floors, which tend to reduce both lift and drag. With the EV transition under way, mo...

It’s that time of year again: opening weekend at Watkins Glen, which means last Friday nearly 40 people took to the track to drive (slowly) around it in the annual Green Grand Prix. Although, not as slowly as you might think. If you do it right, you won’t touch your brakes; my traction control was cutting in on the back sweeper on the Boot and Turn 1. I’ve been making the trip out to New York from Illinois for a few years now, and I always find myself thinking about the efficiency of my car not just during the competition but also during the long drive (I removed the stereo several years ago, so I have a lot of time to think!). This year I had also read something thought-provoking in Carlo Rovelli’s Helgoland , a book about the discovery of quantum mechanics: “Abandoning assumptions that seem self-evident can lead to greater understanding.” What assumptions have I made about energy efficiency that might be holding me back? Well, the only way to answer that is to abandon everything an...

Cicero, in his letter On Moral Duties , wrote, “The definition of terms must in fact form the basis of every scientific exposition if the scope of the argument is to be clearly understood.” Terminology in aerodynamics is often not well understood even by people who think they have a good grasp on it, let alone those mostly or wholly unfamiliar with the field. You will see people throw around words with apparently little or no understanding of what they actually mean all the time or use vague terminology in an attempt to impress or obfuscate (I was guilty of this for years, aping what I saw modeled in online forums). To help dispel some of this uncertainty, here’s a primer on words and phrases you’ll see used in forum discussions and online articles about aerodynamics, often incorrectly—with accurate definitions pulled straight out of car aerodynamics textbooks . Aerodynamics : “Aerodynamics is simply the study of the forces involved in the movement of an object through the air. Vari...