AUTONEWS

Tailored surface vibrations to improve aerodynamic performance in passenger jets

A passenger jet soars across the sky at 640 mph, its wings pummeled with wind and boundary-layer turbulence, challenging the plane’s operation and efficiency.

Advancing research from Mahmoud I. Hussein aims to prevent that turbulence with engineered microscopic vibrations from synthetic, subsurface materials that can dramatically improve fuel efficiency.

Cutting down the fuel usage...Ever since the dawn of flight, making a plane faster and cleaner meant reinventing its silhouette or changing its shape. But the new study suggests that it is not entirely the case, and there are other new ways also. In this work, aerospace engineers are instead modifying the material under the aircraft’s skin to neutralize turbulence at the source before it ever rattles the wing.

The key is using synthetic, subsurface materials to generate microscopic vibrations that stop turbulence in its tracks and slash aerodynamic drag. 

At 640 mph, air pummels an aircraft’s surface to create a turbulent boundary layer that acts like an invisible anchor, forcing engines to burn more fuel. This new technology could dampen chaotic friction to cut carbon emissions and save commercial airlines billions of dollars on flights that consume over 10,000 gallons of fuel per cross-country trip. 

“The prevailing paradigm since the beginning of aviation is to control drag by only shaping the vehicle,” said Mahmoud I. Hussein, professor of aerospace engineering at CU Boulder. “Now we have a new concept to influence surface drag using materials that can dynamically interact with the airflow, enhancing the vehicle performance in an unprecedented manner,” the lead researcher added.

Decade-long work...Basically, researchers are looking to phonons, which are tiny, subatomic-scale vibrations trapped inside the physical structure of a material. 

Hussein started working on the concept of “phononic subsurfaces” (PSubs) a decade ago. These tiny subsurface structures absorb energy from passing wind and flex ever so slightly, sending out-of-phase vibrations back to the surface. The resulting microscopic ripple smooths the flow passively without heavy motors or complex moving parts. 

Early phononic subsurface prototypes were limited to targeting a single, precise vibration frequency, making them ineffective against the chaotic spectrum of real-world turbulence. 

In two papers published in Physical Review X and Proceedings of the Royal Society A, the team resolved these legacy issues by introducing super-resonance and scatterless interference. 

Commercial planes consume over 10,000 gallons of jet fuel on a single cross-country trip, so improvements in fuel economy could lead to big savings for airlines.

In new papers published in Physical Review X and Proceedings of the Royal Society A, Hussein and his team report dual discoveries that bring the research closer to reality: super-resonance and scatterless interference.

Visualization of super resonance on a coiled phononic structure(image above) Credit: Hussein et al./ CU Boulder

“The prevailing paradigm since the beginning of aviation is to control drag by only shaping the vehicle. Now we have a new concept to influence surface drag using materials that can dynamically interact with the airflow, enhancing the vehicle performance in an unprecedented manner,” said Hussein, a professor in the Ann and H.J. Smead Department of Aerospace Engineering Sciences at the University of Colorado Boulder.

Hussein also has a courtesy appointment in the Department of Physics and an affiliation with the Materials Science and Engineering Program.

His research focuses on phonons—tiny vibrations within the material itself, rather than the conventional vibrations of an entire structure. Although such movements are incredibly tiny, their effect is not.

Harnessing and controlling these internal vibrations is the basis of the emerging field of phononics, which Hussein has helped develop since its early stages more than two decades ago. In 2011, he co-founded the Phononics 20xx conference series, which has grown into a leading international forum for the field.

In 2015, he introduced the concept phononic subsurfaces (PSubs), materials that can passively manipulate vibrations on surfaces interacting with a fluid flow. Since then, PSubs have been designed―by his group and other researchers around the world―to operate at a single frequency.

Now, Hussein has shown that by coiling PSubs, it is possible to manipulate vibrations across a range of frequencies, enabling a new phenomenon called super-resonance, which significantly expands the technology’s potential.

“We started with one frequency and aspired to eventually cover a broad range of frequencies, which is the way turbulence is generated in the real world. Now we’re there. A coiled phononic structure overcomes a long-standing limitation in laminar flow control strategies,” Hussein said.

Scatterless interference takes the technology further. Instead of a single PSub at one location, groups of them can be arranged in a grid or as a lattice to delay turbulence across a large surface, like the wing of a plane or the body of a hypersonic vehicle.

“This allows effective downstream control,” Hussein said. “These two problems, downstream control and broadband control, have been the key limitations of the technology since its introduction over a decade ago. We’ve resolved both.”

Adam Harris is a materials science and engineering PhD student in Hussein’s lab and co-author on both papers. He said these advances take the research to the next level.

“These two new milestones provide complementary solutions towards the puzzle that is the effectiveness of PSubs for actual flight conditions,” Harris said. “Scatterless interference gives us a way to attenuate the spatial behavior of the instability field downstream of the PSub, while super-resonance gives us a way to broaden the range of frequencies over which the control can operate. Together, they bring the original PSub concept closer to the level of versatility needed for real-world flow environments.”

While the current research is still computational, PSubs are more than just theoretical. Several groups around the world currently have built functional physical prototypes and are working towards demonstrating their effectiveness in wind tunnels.

“Our goal is to move beyond the traditional paradigm that flow control must come from solely changing the shape of the exposed surface or, more recently, using active actuators,” Hussein said. “With phononic subsurfaces, a wing or a fuselage can retain its shape and smoothness and remain passive, while the material beneath it is engineered to allow interaction with the flow in a highly targeted way.”

Although this research is focused on aerospace structures, both super resonance and scatterless interference may have even greater applications.

“In addition to aircraft, this could be important for marine vessels, pipelines, turbomachinery, anywhere turbulence is an issue. In fact, both ideas may have application beyond flow control altogether” Hussein said.

The ongoing research also tackles hypersonic flows and is supported by a $7.5 million, five-year Department of Defense Office of Naval Research (ONR) Multidisciplinary University Research Initiative (MURI).

source: CU Boulder