Abstract:
Researchers at the University of Colorado Boulder have proposed a new method that uses tiny vibrations inside materials to regulate airflow. In the future, it is expected to reduce turbulence and drag without changing the shape of the aircraft, thereby improving fuel efficiency.
Passenger aircraft can cruise at speeds of approximately 640 miles per hour. The air near the wing surface creates a turbulent boundary layer, which increases flight resistance, reduces flight efficiency, and results in higher fuel consumption. A commercial aircraft can consume more than 10,000 gallons of fuel on a cross-country flight, so even a small increase in efficiency could result in significant cost savings for airlines.
Mahmood I. Hussain, a professor in the Department of Aerospace Engineering Sciences at the University of Colorado Boulder, and his team are exploring another path: instead of just reshaping the outer contours of the aircraft to control drag, they can place engineered synthetic materials beneath the surface and use microscopic vibrations generated by the materials to affect airflow, thereby reducing turbulence and improving fuel economy.
Hussain's team recently published papers in "Physical Review
Hussein said that since the birth of the aviation industry, the mainstream idea of controlling drag has been to change the shape of the aircraft. Now, researchers have proposed a new concept: using materials that dynamically interact with airflow to affect surface drag, improving aircraft performance in an unprecedented way. In addition to his appointment in the Department of Aerospace Engineering Sciences, Hussein holds a concurrent appointment in the Department of Physics and is affiliated with the Materials Science and Engineering program.
The core of this research is "phonons". Phonons are not macroscopic movements of the entire structure, but extremely tiny vibrations occurring within the material. Although these movements are small in magnitude, if they can be controlled, it could change the way the material surface interacts with the flowing air.
The research surrounding these internal vibrations is called phononics. Hussain has been involved in promoting the development of this field for more than two decades and helped found the "Phononics 20xx" conference series in 2011, which has now become an important international exchange platform for researchers in this field.
In 2015, Hussein proposed the concept of "phonon subsurface", which is to arrange specially designed structures under the surface of materials so that they can passively control vibrations at the interface between the surface and the fluid. In the past, designs developed by his team and other researchers often only functioned at a single frequency.
Latest research shows that designing the phonon subsurface into a coiled structure can extend its effect to a range of frequencies. This phenomenon, known as "superresonance," solves a major limitation of earlier designs: It enables the structure to interact with a wider range of frequency components found in real turbulence.
Hussain said the team could initially only handle one frequency, but later wanted to cover a wider range of frequencies, because this is how turbulence in the real world is generated in this broadband manner. Today, this goal has been achieved. The coiled phonon structure breaks through long-standing limitations of laminar flow control strategies.
"Scattering-free interference" addresses another problem. Instead of just placing a phonon subsurface in a single location, researchers can arrange multiple structures in a grid or lattice, extending their influence downstream of the airflow over a larger area, such as an aircraft wing or the surface of a hypersonic vehicle fuselage.
Hussein said this approach enables effective downstream control. Downstream control and broadband control have been two of the technology's key limitations since its introduction more than a decade ago, and the team has now solved both of these issues.
The two advances complement each other, noted Adam Harris, a doctoral student in materials science and engineering in Hussein's lab and co-author of both papers. Scattering-free interference provides a way to attenuate the spatial behavior of unstable flow fields downstream of the phonon subsurface; superresonance broadens the frequency range over which control can be effective. The combination of the two brings the original phonon subsurface concept closer to the flexibility required in real flow environments.
At present, these results mainly come from computational studies, but phonon subsurfaces are no longer completely in the theoretical stage. Research teams around the world have created working physical prototypes and are moving towards wind tunnel testing.
Hussein said that the team hopes to break through the traditional thinking that "flow control can only rely on changing the shape of the exposed surface, or in recent years, relying on active actuators." Using phonon subsurfaces, a wing or fuselage retains its original shape, flatness, and passive properties, while the material underneath is designed to interact with airflow in a highly targeted manner.
Although current research is primarily geared toward the aerospace field, the potential uses of superresonance and scatter-free interference are not limited to aircraft. Hussein believes that ships, pipelines, turbine machinery and any system affected by turbulence could benefit; in fact, both ideas may even have applications beyond flow control.
Relevant research also involves hypersonic flow issues and is supported by a US$7.5 million, five-year multidisciplinary university research program from the Office of Naval Research of the U.S. Department of Defense.
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