Thursday, March 28, 2019

How to Cut a Car's Air Drag: More Air?

When you think of an aerodynamic car, what comes to mind? Smooth curves, sharp points, images of smoke streaming over surfaces in a wind tunnel—but what if we didn’t have to change something’s shape to help it cut through the air more efficiently? That’s the idea behind a new experiment reported in Physical Review Fluids, in which scientists showed that adding nozzles which shoot precisely timed jets of air can substantially cut a vehicle’s drag profile, potentially improving efficiency and gas mileage.

Aerodynamically speaking, the back-end of a semi trailer is a nightmare.

Generally, you want smooth lines, rather than whatever the heck is going on there.
Image Credit: Richard Smith, via Symscape

One of the main principles of streamlined design is that a shape with a curved, angled front experiences less air resistance than a flat shape. Giving a vehicle a curved front lets it nudge aside the air molecules it encounters as it moves, rather than wasting energy pushing them forward. However, the air’s behavior at the vehicle’s rear can be almost as important—and it’s generally given a lot less consideration.

When a truck like the one pictured above moves at high speeds, the corners of its boxy back-end create a low pressure zone; a slight vacuum in the space where the truck just was. As the truck travels, air currents passing over the sides can get sucked into that zone, pulled along toward the rear of the truck. These parcels of air accelerate briefly along with the vehicle before curling around to form vortexes that can break off, sapping the truck’s kinetic energy. Now, rather than wasting energy pushing the air ahead of it, it’s towing an invisible extra load.

One way to minimize this effect is with a design that gradually slopes to a point at its tail, to avoid those sudden pressure changes. But that comes with its own issues, such as carrying capacity, collision safety, and aesthetics—a car with a teardrop profile would be a huge departure from current trends in auto design (although that’s not stopping some people from trying). Still, just about anything would be better than the hard ninety degree angles of a tractor trailer. That’s why you’ll often see trucks on the highway equipped with deployable flaps that angle inward, bringing their shape closer to the aerodynamic ideal.

But rather than using solid flaps to manipulate airflow, new research suggests that using the air itself could produce the same results. To put that possibility to the test, scientists attached controllable nozzles to the back corners of a model truck, and put it in wind tunnel conditions that mimicked a road vehicle making a gradual turn at high speed.

Ordinarily, the asymmetrical airflow in this situation creates a lopsided wake that hurts the vehicle’s efficiency. But by firing short bursts of compressed air from the nozzles, the researchers found that they could partially negate this effect, reducing the drag on the vehicle by up to 7%.

The research involved some trial and error to figure out the optimal frequency for this system, but the settings that worked best for the particular wind speed and angle tested here aren’t likely to be a one-size-fits-all solution. In other words, to scale this up and make it useful in an actual highway scenario, the air jets will have to be controlled by a computer system that includes sensors to detect changes in relative wind speed and angle. Fortunately, the computing power that you’d find in the average new car is more than sophisticated enough to handle an operation like this, so adding air-based adaptive airfoil tech wouldn’t be too much of a stretch. For an automaker like Tesla, which focuses on squeezing every last mile out of a given-size battery, developing this concept into an optional feature seems like it could be well worth the R&D money—especially given the recent announcement that they’re trying to break into the shipping & hauling space.

Regardless of whether or not it finds commercial applications anytime soon, it’s a cool development—although maybe not quite as cool as you’d hope when you hear “jets on the back of a car”.

—Stephen Skolnick

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