Plasma Actuators – Extremely complex physics explained with the help of semi trucks
Unfortunately, that’s not true. Some 66 percent of a semitruck’s gasoline goes to combating drag. Plasma Stream Technologies, a company using science developed at the University of Notre Dame, thinks it has a solution: Use a plasma actuator to alter airflow—turning a turkey into a hawk—without actually changing the shape of the truck. Sounds cool, but, uh, what’s a plasma actuator?
According to Notre Dame engineering professor Thomas Corke, when an insulating material such as Teflon is sandwiched between two staggered, electrically charged copper plates, one exposed to air and the other trapped behind the Teflon, the surrounding air transforms into the fourth state of matter, plasma, which means that it develops an electric charge. The exposed electrode repels similarly charged plasma, pushing it toward the electrode with the opposite charge. As the plasma moves, it sucks neutrally charged air along with it, creating an air current the same way a fin or a spoiler would.
– CAMERON JOHNSON Popular Mechanics
“Headlight white” and “stoplight red,” the characteristic colors of the highway at night, could soon get a companion: “plasma purple”—that is, if a new fuel-saving, aerodynamic technology catches on big with big-rig trucks. The electric plasmas emitted by the devices, which glow purple-violet in the dark, can smooth the turbulent wakes that sweep off the rear edges of truck trailers travelling at highway speeds. Simulations and wind tunnel tests indicate that these novel active-flow control systems could reduce the aerodynamic drag generated by semi-trailers by almost one-fifth, thus lowering fuel consumption by up to 10%.
Plasmas, the lesser-known “fourth state of matter,” are clouds of charged atomic ions created in the air by high-voltage electric fields. The glowing electric-flame phenomena are commonly seen in household air purifiers and more rarely in natural phenomena such as St. Elmo’s Fire, the ghostly purple haze sometimes seen hanging off airplane wings. For application to trucks, strong electric fields emitted by surface electrodes push the plasmas around, producing breezes that can redirect critical airflows at the trailing edges of “bluff-bodied” vehicles such as Class 8 cargo trucks, helping to smooth out the turbulent, high-drag wakes.
Somewhere around 65% of the fuel that a truck burns is wasted in overcoming highway drag, researchers say. And with about 133 million American trucks consuming 60 billion gallons of fuel annually, if only a fraction were to adopt this promising new slipstreaming technology, the effect would soon start to become significant.
That’s the ultimate target for Plasma Stream Technologies, the small start-up firm based in Bettendorf, Iowa, that is developing the “dielectric-barrier plasma actuator” devices for trucks. Up to now its concept has been tested solely in supercomputer simulations and sub-scale wind tunnels, but real-world confirmation should come from road tests planned for this March, according to Pranay Bajjuri, a cofounder and investor. Favorable results would mean that the company could proceed toward full commercialization in the hope that retrofit products could hit the market in early 2018, he noted.
(Go to https://www.youtube.com/watch?v=8HAa-bI6170 for a company video on the technology and its development.)
Bajjuri, whose day job is as an electrical engineer at a heavy-equipment maker, explained that a few years ago he and his partners had searched for promising technology to commercialize and discovered the potential benefits of plasma actuators in improving aerodynamics in applications such as airplane wings, wind turbine blades, and even compressor blades in turbine engines. They also learned that General Motors and other organizations had investigated using plasma-based active airflow control devices for highway vehicles and had achieved favorable results in the lab.
“Plasma actuators have been around for about 15 years, but there’s still no commercial product yet,” Bajjuri said. “We’re trying to be the first to bring them to the commercial market in volume.”
The Plasma Stream team realized that these plasma actuators could replace the many passive flow-control add-ons that trucking firms have employed over the years to reduce vehicle drag, including air deflectors, vortex generators, and aft boat tails.
Simple electrode strips
The simple retrofit device, called eTail, has no moving parts. It’s basically two long, high-voltage electrode strips made of copper separated by an insulator such as Teflon (polytetrafluoroethylene), he explained. The 4-inch-wide (102-mm) units are fitted to the top and side trailing edges of semi-trailers. When the juice switches on at speeds of 45 mph (72 km/h) and up, the surface electrodes energize the air above, producing charged ions that the field then sends in one direction as wind. The wind alters “the turbulent, high-energy airflow at the edges and directs it to a low-energy region” right behind the vehicle, minimizing drag losses, Bajjuri explained.
Even though the strap-on eTail system runs on high-voltage current, operations are quite safe because it requires low amperages. “The average power of the electrode strips is only about 1 watt per meter,” he stated. He added that the technology works in all weather conditions—rain, snow and dust—and with an added grid overlay, it might even be possible to heat up the electrodes for deicing in winter.
The company’s prototypes are based on licensed plasma actuator technology developed by a group of aerodynamics engineers and researchers at the University of Notre Dame, said Thomas Corke, who leads the team. “For a ground vehicle, the general approach is to shape the wake, to smooth out the airflow coming off the rear,” he said. “The optimum is to turn the airstream about 12 degrees from the horizontal.”
The flow-control devices are powered by a compact, lightweight power supply also of UND design that can be operated with an Apple iPad. Low power is achieved, Corke said, because “the DC power is pulsed, so the device is powered only for brief intervals,” giving it a duty cycle of 0.001%.
In contract research conducted in late 2015 at Notre Dame, Corke’s team ran CFD simulations to investigate the use of plasma actuators to cut drag on Class 8 truck semi-trailers. The virtual truck’s shape resembled a 1/12th-scale model that had been previously evaluated in wind tunnel tests at the university. The computer simulations sought to determine an optimum arrangement of an aft ramp geometry (the slope and width of the device) and plasma actuators that would in combination, yield a drag reduction comparable to the passive boat-tail retrofit devices currently used on semi-trailers. And unlike conventional boat-tail units, the eTail does not impede easy access via roll-up doors and swing doors.
Plasma Stream’s final design, which features an 11-degree angle, attains this goal, achieving a predicted 22.7% drag reduction at highway speeds that would yield an 11% fuel savings. The design requires only a 4-in extension around three sides of a trailer which makes it considerably smaller than boat-tails, which add about 4.5 ft (1.4 m) to the trailer length. The retrofit product is expected to cost around $2,000, Bajjuri said, and could save somewhere around $8,000 per year in diesel fuel.
The new technology can be installed at various locations around a semi-truck to streamline the aerodynamics including near the gap between tractor and trailer, on the rearview mirrors, and in the front grille area of tractors. The company thinks that the technology could also work on buses, locomotives and racecars.