Flying with the Fourth State of Matter
A scaled-down gale blows over a flat plate set inside the tabletop wind tunnel. Despite the low lighting and hazy Plexiglas view portals, we can clearly see the frenzied fluttering of streamer ribbons, called telltales, in the field of little wind vanes that carpets the exposed test surface inside.
At first, each unruly telltale flies every which way, clear evidence of unsteady air flows gusting within. “OK, that’s off,” the researcher says.
“Here’s on…” Almost like a sorcerer’s spell, an otherworldly, blue-violet halo emerges at the front of the plate and hovers corona-like, casting peculiar purple shadows onto the walls. The telltales meanwhile become suddenly and strangely obedient, instantly swinging round in near unison, aligned by an insistent new wind.
“Off,” she says. The ribbons flap arbitrarily as the eerie electric flame fades. “On.” More purple haze and parallel ribbons.
“Off, off…and on.” The odd glow, curious order, and incessant roar of the fan drop away. By the time the lights come on, a whiff of ozone hangs in the air and everyone in the room is grinning uncontrollably.
And for good reason. We just watched moving air being controlled by plasma, the lesser-known, fourth state of matter which also exists in the blistering core of our sun. And while such lab demonstrations are both uncanny and awe-inspiring, these so-called plasma actuators could produce far more impressive benefits in the real world, especially for the aviation and wind power industries, and maybe even the trucking business.
On airplane wings, for example, tiny plasma actuators could help planes fly more safely, more efficiently, and with greater stability and control. They can speed, slow or divert air flows in ways that can cut drag, fuel use, and CO2 emissions by as much as 25%, researchers estimate. Some experts even think that these devices might someday replace conventional flight control surfaces such as flaps and ailerons. Imagine witnessing the ghoulish purple glow of the lab demo from the window seat of a transcontinental flight.
More immediately, aerodynamicists are looking to place the same technology on the huge, vulnerable, and costly blades of wind turbines to improve their efficiency, extend their lifetimes, and even help them more effectively cope with gusting winds.
Surprisingly, this unearthly technology is, in fact, most commonly found hiding inside people’s homes.
A Plasma Wind
One of the more common uses of plasma wind devices is in the glowing blue-purple-violet “fan” elements of household ionic air purifiers. Electric fields emitted by surface electrodes cause the adjacent air to become energized, allowing its component molecules to break down into ions and electrons to form plasmas—glowing clouds of electrons and charged air molecules, mostly nitrogen and oxygen ions. A natural version of this “cold plasma” phenomenon is sometimes seen ghosting off ships’ masts and superstructures, and aircraft wings. There, it’s known as the legendary St. Elmo’s fire.
Embedding high-voltage versions of these wind generators—flat, thin-film electrodes—in an airplane’s wings can not only summon artificial St. Elmo’s fire instantly and on command, but it can also send the charged plasma clouds—and air they carry along with them—moving in one direction or another. That is, these plasma actuators can readily accelerate, or “blow,” air where and when it’s needed with no moving parts, boosting lift or cutting drag depending on where and how they’re used.
The field of plasma aerodynamics itself has an intriguing history. After some early conceptual work for spacecraft reentry in the 1950s to the 1970s, the field reawakened when word arrived in the West that toward the end of the Cold War Soviet researchers had incorporated plasma-based technology into their scramjet-powered Ayaks hypersonic vehicle concept to enhance combustion and aerodynamic performance. Russian engineers later detailed in a magazine article “how plasmas could weaken the shock waves generated during hypersonic flight,” says Jon Poggie of Purdue University, who at the U.S. Air Force Research Laboratory Command in Ohio previously studied plasma control of supersonic flows.
Some people called it “plasma magic,” but western researchers wanted to know more as speculation grew and initial experimentation began, he says. “If plasmas really could reduce drag, it was thought that it might be possible to delay the onset of sonic booms, steer aircraft by applying plasmas selectively to different parts of the vehicle, and even reduce heating around hot spots on an airframe.” In time, the Soviet Union collapsed and émigré Russian researchers arrived in the west, spurring fruitful research on plasma aerodynamics among R&D institutions in Europe, the U.S., and Russia, an extraordinary international collaboration that lasted almost 20 years.
Plasma wind is created by applying an alternating or pulsed current to electrodes to produce “a dynamically changing electric field,” says Steve Wilkinson, an aerodynamicist at NASA Langley, one of a cadre of researchers at government, industry and university labs in the U.S., Europe, Russia and elsewhere who investigate what’s called plasma aerodynamics. The applied voltage difference causes the charged ions in the plasma to shift and “some ions collide with nearby air molecules, which rebound in the same direction, creating wind,” he says.
“But these are not thrusters,” says Thomas Corke, a leading plasma aerodynamicist at the University of Notre Dame. “They blow like weak table fans, only enough to move notebook pages around.” But, paradoxically, if aimed and timed correctly, the breezes blown by these plasma actuator devices can be strong enough to significantly alter how wind flows over wings in beneficial ways, he adds.
“With growing computer power and practice, we can now model the detailed behavior of the airflows over the wings, the fluid dynamics, as well as behavior of the plasmas themselves with increasing precision,” Corke says. “This lets us identify natural instabilities in air flows that we can exploit with great effect because just a tiny push at the right place and time can excite a much larger, and often positive, result.”
These subtle nudges could radically change the way airplanes are designed and flown. “This technology can provide the leverage that airplane designers need to implement a range of potentially significant performance improvements,” Poggie says. Small interventions in the airflows over wings and tail surfaces, he says, could reduce drag, augment lift, enhance efficiency, muffle noise and vibration, or boost flight control.
Conceptually, Poggie says, researchers envision tiling wings and tail surfaces with arrays of flat plasma actuators that could be individually activated to tweak airflow to improve aerodynamics. The wall-to-wall electrodes “could even heat up for deicing in cold weather.”
Theoretically, plasma actuators can be effective at all speed ranges, from slow drones to fast jets and even hypersonic vehicles, but so far the technology has been tested mostly under ideal conditions in labs, wind tunnels, or on model aircraft. But as long as fluid mechanicians and aerospace engineers have correctly simulated the plasma behavior and the airflow over an airplane in most flight conditions—no straightforward task—an effective strategy could be developed to fine-tune the flows. “You could dial-in and modulate the flows, tuning them to do what you need to do,” Poggie says.
The power of such a fundamental approach was highlighted by a recent NASA-funded demonstration project in which Corke’s team at Notre Dame succeeded in deploying plasma actuators to suppress what he describes as “an important near-surface flow instability” that aerodynamicists know presages the onset of deleterious turbulence and drag.
Corke and his team reported that the wind tunnel test item, which used “new actuators that developed 20 times more thrust while consuming 100 times less power, produced a 65% drag reduction.” The Notre Dame researchers have found that introducing a small oscillation whose waves move perpendicular to the air flow path can halt the onset of the so-called near-surface flow instabilities that lead to turbulence.
The results, when scaled up, could be impressive. “I ran the numbers for an aircraft design the size of a Predator UAV on a computer,” Corke says. “If you put plasma actuators on one, it could extend its range by 300% and its endurance from 24 to 36 hours.” NASA has awarded the group a Phase 2 R&D contract, and now DARPA is said to be interested in the novel technology, which has since been patented and licensed to a start-up company.
In the near term, plasma actuators offer an attractive alternative to competing aviation flow-control technologies, says Rasool Erfani, a lecturer of mechanical engineering at the Manchester Metropolitan University in the U.K. Plasma-based devices have a very low profile and can provide endless, exact puffs over short time scales.
The technology could even replace traditional wing flaps, he says. Although the plasma-actuated “flapless” or “hingeless” wings are currently only suitable for smaller aircraft such as drones or UAVs, Erfani believes that further development could make the technology viable on larger, faster planes.
More Subtle Adjustments
But that will take better plasma actuators. The most common systems, known as dielectric barrier discharge (DBD) devices, pair high-voltage electrodes separated by an insulator, typically a temperature-resistant polymer, glass, quartz, or ceramic. When activated ionized air spreads from the front electrode to the insulated rear one, creating a plasma trail. The rear insulator doesn’t readily conduct the current so it prevents arcing between the electrodes, which allows the electric fields to break the air down into plasmas.
DBDs will, however, need to be more effective if they are to be more widely used, says Subrata Roy, a professor of mechanical and aerospace engineering at the University of Florida who with his colleagues has developed a new design that is more versatile than standard units. They use electrodes arranged in serpentine, or sine wave-like, patterns, rather than rectilinear rows. These new geometries can, for instance, blow in three directions at once or create three-dimensional aero effects such as tiny tornado-like vortices that can seed similar larger whirlpools in much larger airflows.
Roy’s team is also looking at using new materials, such as insulators made of aerogels, the lowest density, lightest material known. Other plasma actuator research focuses on improved pulsing, phasing, and timing of the electric currents and the fields they create.
The other big challenge for flight applications are the weighty power supplies for the high-voltage current. “The trouble is that making plasmas is not at all energy efficient,” NASA’s Wilkinson says. “It takes a lot of energy to dissociate the air molecules and move them about.” But progress elsewhere toward electric aircraft—equipped with hybrid systems and better batteries—could help overcome that limitation since electric actuators are a natural fit in electric aircraft.
The fact that DBDs and similar plasma actuators are relatively simple means that the technology poses a relatively low barrier to entry, which helps account for the continued popularity of plasma aerodynamics research. It’s the kind of thing that a graduate student can set up and demo, several researchers say, though truly understanding the devilishly complex air and plasma flows is another thing entirely.
Engineers are also looking to apply plasmas elsewhere on airplanes. GE, for example, is investigating using the technology in turbine engines: a first-ever full-scale compressor application has been demonstrated. GE researchers are also exploring the use of a related cold plasma technique in lean-burn combustors for aircraft engines to stabilize the flame and keep it lit as fuel input is reduced. Another potential application is to break down the fuel-air mix into ions to make it burn more readily in engines of all kinds.
Back to Earth
With Russia and the West now again at odds, the previously successful aerospace research collaborations have ended. Government support has ebbed, leaving many practitioners scratching for funding. It is perhaps unsurprising that the most promising near-term application for the technology is wind power, which has no problem with heavy power supplies and offers the possibility of a more immediate return on investment.
“Wind turbines could greatly benefit from plasma DBDs,” Purdue’s Poggie says. Turbine blades must cope with heavy gusts and constantly varying, unsteady airflows that rob efficiency and stress the composite blades over time, he says. Existing mechanical fixes such as feathering, or rotating, the blades to less-exposed angles have proved either problematic or unsatisfying depending on what they’re attempting to address.
“The concept has been around for awhile to tap off some of that generator power to run plasma actuators on the blades,” Poggie says. “That would allow the blade to respond to changes in wind speed and direction much faster to better maintain maximum power production.”
“Wind turbine blades constantly bend, bend, bend—and are built to withstand it,” he says. But over years of constant stress and strain, these high-cost components eventually fail from mechanical fatigue. Plasmas actuators could significantly reduce those loads to extend blade lifetimes considerably, saving wind farm operators money.
Another terrestrial technology that could benefit from plasma aerodynamics are big-rig trucks and other highway vehicles. Plasma DBDs could be installed on the surfaces of truck bodies where they could smooth turbulent airflows and cut drag and thus fuel consumption rates, particularly at higher speeds. Plasma Stream Technologies, a small Iowa firm that’s developing the concept, recently licensed the technology and design for a power supply from Notre Dame based on past research done with GM and NASA, according to news reports.
In time, it’s possible that expertise developed closer to Earth will allow engineers to wield plasma in the skies, where their research began. But whether it’s airplanes, wind turbines, or trucks, the promising efforts to master St. Elmo’s fire will likely someday bring an odd, new glow into the world.
— Steven Ashley, for Nova Next, PBS.org, August 2016
For more information about other Notre Dame technologies at work in the market or technologies available for licensing, visit the Notre Dame Office of Technology Transfer.