From the December 2016 issue
It can be as loud as standing alongside a Boeing 767 at takeoff. And it’s about as irritating as having someone thumping on a bass drum in the back seat. Whether you call it wind throb or buffeting or just plain annoying, it happens when someone in the car opens a single window at speed and it stops when a second window rolls down.
The phenomenon that produces this noise is the Helmholtz Resonance, the same principle that makes a bottle hum when you blow over its open top. It’s the interaction of the gas in a container with a single orifice and the other gases that are, um, passing over that orifice. In this case, the container is the car. The interaction between the two masses of air produces vortexes that compress and decompress the air, producing the throbbing effect. Hermann von Helmholtz, the German physician and physicist who described this interaction, died in 1894 and was thus unavailable for comment.
How intrusive buffeting can get depends on the car’s shape and the size of its windows as well as the speed the vehicle is traveling. Modern cars and trucks are more subject to air thump because they’re so aerodynamically efficient and well sealed against wind intrusion. Jim Zunich, GM’s global vehicle performance chief engineer for wind noise, explains: “We want nice, smooth attached air for aerodynamics, but that’s worse for buffeting.”
Gimme Buffet
See the top image for how the side-mirror shape can affect air movement around the front windows, and the middle image for how not much can influence flow around the rears. A pop-up deflector would shift airflow over the open sunroof rearward, mitigating those annoying vortexes.
When the car was designed with little concern for aero, the haphazard mass of air boiling around it only coincidentally adhered to the vehicle’s surface and only occasionally allowed the Helmholtz Resonance to generate its vortexes. And even when the problem did arise, the car’s other windows and doors were hardly airtight, so the air leakage around them relieved any pressure differences. But a modern cockpit is a well-sealed drum from which only minimal air pressure leaks. In this small, particular way, a modern vehicle is too good for its own good.
But why is buffeting so much worse when just a rear window is down? Two words: side mirrors. They’re one of the last things developed in a vehicle’s design, and they’re placed and shaped precisely to direct airflow in a way that minimizes buffeting at the front windows. But there are no easy tweaks to be made to tune the airflow around the rear windows.
Buffeting is likely to get worse as vehicles get more aerodynamic. “Obviously, if we could resolve it free of charge, it’s something we would do,” concludes Zunich. “But because it comes with styling, aero, and noise penalties, it’s a trade-off engineers have to make.” Well, whether or not we understand how buffeting works, most of us have figured out a solution: Crack another window.
Powering Up
Software supplier Exa developed PowerFLOW, the simulation software that generated the images on this page. PowerFLOW is a computational fluid dynamics (CFD) program. CFD generates a detailed mathematical model of a fluid flow, in this case allowing engineers to observe and tweak how air flows over, around, and through a car on a molecular level. It’s just one of the tools that allows automakers to design a complete car before a single part is stamped or cast. Suck on that, Instagram. —Jared Gall
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