Low, Drag, Variable-Depth Acoustic Liner

This technology is an evolution of acoustic liners developed for engine noise abatement that are typically located inside nacelles. The acoustic liners described here can be outfitted on external surfaces and in tight spaces. Three initial areas of the aircraft have been considered as part of an aircraft configuration incorporating an open rotor propulsion system. The three areas where the liner configurations were applied were (1) under the rotor, (2) on the upper surface of the elevon, and (3) on the surface of a strut.

NASA researchers are extending an existing oxide/oxide CMC sandwich structure concept that provides mono-tonal noise reduction. That oxide/oxide CMC has a density of about 2.8 g/cc versus the 8.4 g/cc density of a metallic liner made of IN625, thus offering the potential for component weight reduction. The composites have good high-temperature strength and oxidation resistance, allowing them to perform as core liners at temperatures up to 1000°C (1832°F). NASA's innovation uses cells of different lengths or effective lengths within a compact CMC-based liner to achieve broadband noise reduction. NASA has been able to optimize the performance of the proposed acoustic liner by using improved design tools that help reduce noise over a specified frequency range. One such improvement stems from the enhanced understanding of variable-depth liners, including the benefits of alternate channel shapes/designs (curved, bent, etc.). These new designs have opened the door for CMC-based acoustic liners to offer core engine noise reduction in a lighter, more compact package. As a first step toward demonstrating advanced concepts, an oxide/oxide CMC acoustic testing article with different channel lengths was tested. Bulk absorbers could also be used, either in conjunction with or in place of the liners internal chambers, to reduce noise further if desired.

This NASA technology is ideally suited to absorb sounds below 1000 Hz (at the low end of human auditory range), which commercially available materials have struggled to absorb effectively. NASA innovators designed the acoustic liner to mimic the geometry and the low-frequency acoustic absorption of natural reeds. To provide excellent noise absorption that endures even in a variety of challenging conditions, researchers have created and tested prototypes of acoustic filters using thin and lightweight parallel-stacked tubes one-fourth to three-eights of an inch in diameter. The assembly can feature a porous or perforated face sheet positioned on one or more sides of the acoustic absorber layer to increase noise-reduction capability as needed. These filters have demonstrated exceptional acoustic absorption coefficients in the frequency range of 400 to 3000 Hz. Results indicate that these assemblies can be additively manufactured from synthetic materials, generally plastic; however, ceramics, metals, or other materials can also be used. The reeds can be narrow or wide, hollow or solid, straight or bent, etc., giving this acoustic liner remarkable flexibility and versatility to meet the needs of virtually any application. This technology effectively demonstrates that a new class of structures can now be considered for a wide range of environments and applications that need durable, lightweight, broadband acoustic absorption that is effective at various frequencies, particularly between 400 and 3000 Hz.

NASA Langley designed the shape memory alloy slat-cove filler to provide significant broad-band noise reduction to any aircraft wing structure that has a leading-edge, high-lift device and that is distinct from the main-wing element. The design can be retrofitted to existing aircraft structures and can be easily incorporated into the existing or future designs for aircraft wing structures. The concept involves very few components, requires no additional mechanical support from pneumatic or hydraulic systems, and makes use of existing slat-actuation systems for retraction. The design is autonomous, simple, and constitutes low-weight addition. The concept is also considered fail-safe because the lift would not be diminished in the event that the slat cove filler failed to deploy. Several advancements have been devised to accommodate complex features encountered in application to practical airframe structures. Graphics from a computational model of a 2D physical demonstration system show the configuration and strain in the slat-cove filler in the deployed and stowed conditions. Features enabling stowage of a large curvilinear length (sliding hinge) and maintenance of the optimized outer mold line (auxiliary component) are highlighted. Other advancements for application to 3D, flight airframes are visible in the image from a model for one entire section of a slat-cove-filler treatment for a wide-body, transport-class aircraft. NASA Langley also offers a design for a deformable structure that is deployed from the leading edge of the main-wing element, termed the slat-gap filler. It closes and covers the gap between the slat and the main-wing element, but can be readily and autonomously opened in emergency to regain the baseline high-lift configuration and its corresponding lift performance at high angles of attack. This approach has similar benefits as the slat-cove filler device.

This technique injects precisely defined stationary transient growth disturbances into the free air slipstream over a wing that develop into streamwise elongated "streaks." These streaks are created with an alternating pattern of low and high streamwise velocity in the boundary layer flow adjacent to the aerodynamic surface of interest. Judicious selection of streak wavelength, amplitude, and profile allows the first-mode instability waves responsible for transition via oblique mode breakdown to be damped while the remaining, uncontrolled waves are kept below an amplification threshold. A similar control concept is also applicable to second mode transition at hypersonic Mach numbers.