New Microchanneled Material Reduces Aircraft Engine Noise

Noise from commercial and military jet aircraft causes environmental problems for communities near airports, obliging airplanes to follow often complex noise-abatement procedures on takeoff and landing. It can also make aircraft interiors excessively loud.

Engineers at the Georgia Tech Research Institute (GTRI) are turning to innovative materials that make possible a new approach to the physics of noise reduction. They have found that honeycomb-like structures composed of many tiny tubes or channels can reduce sound more effectively than conventional methods. "This approach dissipates acoustic waves by essentially wearing them out," said Jason Nadler, a GTRI research engineer. "It's a phenomenological shift, fundamentally different from traditional techniques that absorb sound using a more frequency-dependent resonance." The two-year project is sponsored by EADS North America, the U.S. operating entity of EADS.

Most sound-deadening materials – such as foams or other cellular materials comprising many small cavities – exploit the fact that acoustic waves resonate through the air on various frequencies, Nadler explains.

Just as air blowing into a bottle produces resonance at a particular tone, an acoustic wave hitting a cellular surface will resonate in certain-size cavities, thereby dissipating its energy. An automobile muffler, for example, uses a resonance-dependent technique to reduce exhaust noise.

The drawback with these traditional noise-reduction approaches is that they only work with some frequencies – those that can find cavities or other structures in which to resonate.

Nadler's research involves broadband acoustic absorption, a method of reducing sound that doesn't depend on frequencies or resonance. In this approach, tiny parallel tubes in porous media such as metal or ceramics create a honeycomb-like structure that traps sound regardless of frequency. Instead of resonating, sound waves plunge into the channels and dissipate through a process called viscous shear.

Viscous shear involves the interaction of a solid with a gas or other fluid. In this case, a gas – sound waves composed of compressed air – contacts a solid, the porous medium, and is weakened by the resulting friction.

"It's the equivalent of propelling a little metal sphere down a rubber hose when the sphere is just a hair bigger than the rubber hose," Nadler explained. "Eventually the friction and the compressive stresses of contact with the tube would stop the sphere."

This technique, Nadler adds, is derived from classical mechanical principles governing how porous media interact with gases – such as the air through which sound waves move. Noise abatement using micro-scale honeycomb structures represents a new application of these principles.

“To have the hole big enough to let the sound waves in, but you also need enough surface area inside to shear against the wave". "The result is acoustic waves don't resonate; they just dissipate."

In researching this approach, Nadler constructed an early prototype from off-the-shelf capillary tubes, which readily formed a low-density, honeycomb-like structure. Further research showed that the ideal material for broadband acoustic absorption would require micron-scale diameter tubes and a much lower structural density.

Creating such low-density structures presents an interesting challenge. It requires a material that's light, strong enough to enable the walls between the tubes to be very thin, and yet robust enough to function reliably amid the high-temperature, aggressive environments inside aircraft engines.

Among the likely candidates are super alloys, materials that employ unusual blends of metals to achieve desired qualities such as extreme strength, tolerance of high temperatures and corrosion resistance. Nadler has developed what could be the world's first super alloy micro honeycomb using a nickel-base super alloy. At around 30 percent density, the material is very light – a clear advantage for airborne applications – and also very strong and heat resistant.

They estimate this new approach could attenuate aircraft engine noise by up to 30 percent. Micro-honeycomb material could also provide another means to protect the aircraft in critical areas prone to impact from birds or other foreign objects by dissipating the energy of the collision.

A prototype microchanneled material composed of many metallic nanotubes was developed at the Georgia Tech Research Institute to validate acoustic absorption experiments designed to reduce noise in aircraft

Advanced manufacturing

Nanotechnology and the tendency to miniaturization in the manufacturing industry are familiar to most people employed in the microelectronics and computer industry. Nevertheless, nanotechnology has already been introduced into other scientific and technological areas as well, like robotics, biology and medicine, fiber optic communication networks, aerospace technology, advanced materials technology, chemical engineering and precision manufacturing. Two interlinked trends are involved, the trend towards miniaturization and the trend towards ultraprecision processing. Ultraprecision processes are already extensively applied in manufacturing industry, for example, in manufacturing of car and aircraft engines by improving their performance and in manufacturing of optical parts, such as lenses and mirrors, by obtaining high quality surfaces, replacing, therefore, “traditional” manufacturing methods. On the order hand, in order to observe and measure at the nanometer scale, new instruments and measuring techniques, that provide this ability, are developed. By employing principles of physics, various new methods are used, such as atomic force microscopy, laser position measurement and tunneling electron microscopy. These techniques also allow for manipulating objects on the atomic scale and putting an atom or a molecule on a designated location.
The original idea for the development of nanotechnology comes from the area of microelectronics and its applications in computer systems. According to this manufacturing concept, the smaller the particles are, the reduced is their manufacturing cost and the higher is their productivity. This has already been achieved in the chips industry as it can be seen by comparing the size and the speeds of computers of the 1980s and today. Nanotechnology is made possible through technological advances made in several disciplines, where the commercial opportunities, arisen from engineering research in this field, are usefully classified.
As already mentioned, the two main trends in the miniaturization of products are the ultraprecision and the nanotechnology processing. By ultraprecision processing, defined are these processes by which the highest possible dimensional accuracy has been achieved at a given time. It is carried out by machine tools with very high accuracy. Nanotechnology is defined as the fabrication of devices with atomic or molecular scale precision, but it also includes all devices with size less than 100 nm. It is perhaps today's most advanced manufacturing technology and is usually called “extreme technology” or “bottom-up” manufacturing because it reaches the theoretical limit of accuracy in machining, which is the size of an atom or molecule of a substance.

Today, ultraprecision machining refers to the achievement of dimensional tolerances of the order of 10 nm and surface roughness of 1 nm. The dimensions of parts or elements of parts produced may be as small as 1 μm and the resolution and repeatability of the machines used are of the order of 10 nm. Note, that the accuracy targets for today's ultraprecision machining cannot be achieved by simple extension of conventional machining processes, but, also, new processes are required. In order to use ultraprecision machines in the nanometer regime, three-dimensional control of the position of the tool and the workpiece is necessary. This can be achieved with CNC ultraprecision machine systems, that can provide such control, and with the aid of new measuring methods as the scanning tunneling microscopy and the atomic force microscopy. Several cutting material removal processes, depending on the desired result, are suitable for ultraprecision machining, like drilling of microholes, milling of grooves, turning of mirror-like surfaces and micropins, whilst usually, ultraprecision turning is combined with other ultraprecision processes, such as grinding. Typical products of the above processes are miniaturized machine parts mirror-like surfaces or macro-components with ultraprecision finished smooth surfaces. Other processes are also used for performing ultraprecision cutting, like EDM, ECM, laser beam machining, ultrasonic machining, micro-punching and injection molding.

At this stage, the pioneering work of the Laboratory of Manufacturing Technology of the NTUA in this field may be acknowledged. The precision and ultraprecision cutting and grinding of metals, engineering ceramics and polymers, with the aim in mind of manufacturing high-technology industrial parts, constitutes the topic of a large European Union research project with cooperation between Hungary, Ukraine, France and Greece and of subsequent forthcoming research projects .

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Advancing now from the micrometer domain into the nanometer regime (fig2), it renders difficulties to use the ultraprecision processes discussed so far. For such products, new processes need to be employed.

Fig. 2. Size scale and examples of micro- and nanocomponents.

The techniques used for nanotechnology applications are the energy beam processes, based on the principle, that the energy carried on a beam can remove material by melting, vaporization or ablation. These processes have been developed during the last decade due to their application in the electronics industry and may be listed: photolithography, X-ray lithography, micro-EDM, electron beam machining, focused ion beam, laser beam machining, excimer and femtosecond lasers, scanning tunneling microscopy and atomic force microscopy

Probably, the most important application of nanotechnology is the manufacturing of microchips. By reducing the dimensions of the chip components their number on the chip can be increased, making them even faster because the electric signal has to travel less distance between them. The dimensions of chip components cannot be reduced in size any further with today's manufacturing methods, because photolithography does not permit the manufacturing of particles with dimensions of 100 nm or less. Therefore, new methods are investigated, based, either on the same physical principles as photolithography, like LIGA, which is a deep etching process based on lithography, electroplating and molding or on completely new ideas.

Electron beam machining, focused ion beam and laser beam machining are new processes, providing the energy for material removal in the form of heat. The main advantage of these methods, for both the tool and the workpiece, is the virtually zero machining force. Although the mechanical properties of the workpiece do not affect the machining process, its thermal properties, like the melting point, the boiling point and the heat capacity, greatly influence the machining characteristics, which may cause problems. Furthermore, due to the excessive heat generated on the machined surface, a heat-affected zone is developed, since the molten material remaining on the machined surface re-solidifies during cooling, whilst the structure of the layers underneath the surface changes, resulting in phase transformations in ferrous materials, which may induce residual stresses and changes in the surface hardness. A typical application of focused ion beam is the sharpening of diamond cutting tools to a cutting edge of 10 nm.

In excimer and femtosecond laser processing, very high quantum energy is induced into the workpiece, exceeding the energy among atoms, permitting each molecule to be decomposed into atoms, which are removed from the workpiece. The great advantage of this method, in contrast with the previous methods, is that the developed heat-affected layer is very small. This particular feature of these processes allows for performing very accurate cutting surfaces almost without defects. Their main disadvantages are the low machining speed and, thus, the low efficiency and the high cost of the required equipment.

Scanning tunneling microscopy is very important technology in nanotechnology, because it makes possible atom manipulation. The technique is based upon a phenomenon discovered in the 1960s, if a conducting surface is brought within 1 nm from the tip of the instrument and a potential difference is applied between the tip and the surface, then a “tunneling current” will flow. The relation of the current towards the distance is known and, thus, the machined surface can be depicted. The invention that greatly advanced nanotechnology used was the “scanning tunneling microscope”, developed in the laboratories of IBM in Zurich, making possible not only to measure and depict surfaces at molecular scale, but, also, to manipulate atoms and put them in designated positions. The main disadvantage of this method is that it can be only applied to conducting materials and, therefore, atomic force microscopy has been designed to overcome this disadvantage.