Aircraft engines, typically turbofans, use acoustic liners to damp engine noise. Liners are applied on the internal walls of the engine nacelle, both in the intake and by-pass ducts, and use Helmholtz resonance principle for the dissipation of incident acoustic energy.
An acoustic liner is a sandwich panel made by:
The lower half of a liner features dedicated internal slots to allow liquid drainage in order to prevent ice formation or fire hazards. From an acoustic perspective, this implies that the upper acoustic panel is a locally reacting liner, whereas the lower one is a non-locally reacting liner. [1]
Acoustic liners can be distinguished by their internal configuration on the base of the number of honeycomb cell layers:
Porous layers (e.g. the septum and the face-sheet) can be for example a perforate plate, a wire mesh or a felt-metal. The honeycomb can be of aluminum or glass fiber and the cells' size is selected small enough to ensure an acoustic plane wave in the cell for the whole frequency of interest. The face-sheet and back-skin can be either metallic or in carbon fiber. In order to have a cylindrical barrel, panel parts are structurally joined together leading to a partial loss of the acoustic area known as splice. Current state-of-the-art of acoustic are made without splices and are known as zero-splice liners. For example, the evolution of the splices width on the Airbus family ranges from three splices of about 15 cm for the Airbus A320 to a zero-splice liner for the Airbus A380. [2]
The acoustic liners' performance can be verified in dedicated experimental test rigs, [3] by means of virtual prototypes or by means of ground tests on full-scale engines. [4] Both these type of tests and simulations allow to determine the acoustic attenuation in the far-field. Moreover, the acoustic performance is linked to the acoustic impedance which can be measured with one of the following techniques:
All these methodologies measure the liner's normal impedance. However, only flow duct facilities and the in-situ method allow the measurement of the impedance in presence of the grazing flow which can affect the impedance itself. [5] Moreover, the in-situ method is the only one able to measure the impedance directly on full scale acoustic liners. [4] [7]
The turbofan or fanjet is a type of airbreathing jet engine that is widely used in aircraft propulsion. The word "turbofan" is a portmanteau of "turbine" and "fan": the turbo portion refers to a gas turbine engine which achieves mechanical energy from combustion, and the fan, a ducted fan that uses the mechanical energy from the gas turbine to force air rearwards. Thus, whereas all the air taken in by a turbojet passes through the combustion chamber and turbines, in a turbofan some of that air bypasses these components. A turbofan thus can be thought of as a turbojet being used to drive a ducted fan, with both of these contributing to the thrust.
Computational fluid dynamics (CFD) is a branch of fluid mechanics that uses numerical analysis and data structures to analyze and solve problems that involve fluid flows. Computers are used to perform the calculations required to simulate the free-stream flow of the fluid, and the interaction of the fluid with surfaces defined by boundary conditions. With high-speed supercomputers, better solutions can be achieved, and are often required to solve the largest and most complex problems. Ongoing research yields software that improves the accuracy and speed of complex simulation scenarios such as transonic or turbulent flows. Initial validation of such software is typically performed using experimental apparatus such as wind tunnels. In addition, previously performed analytical or empirical analysis of a particular problem can be used for comparison. A final validation is often performed using full-scale testing, such as flight tests.
Soundproofing is any means of reducing the sound pressure with respect to a specified sound source and receptor. There are several basic approaches to reducing sound: increasing the distance between source and receiver, using noise barriers to reflect or absorb the energy of the sound waves, using damping structures such as sound baffles, or using active antinoise sound generators.
The CFM International CFM56 series is a Franco-American family of high-bypass turbofan aircraft engines made by CFM International (CFMI), with a thrust range of 18,500 to 34,000 lbf. CFMI is a 50–50 joint-owned company of Safran Aircraft Engines of France, and GE Aviation (GE) of the United States. Both companies are responsible for producing components and each has its own final assembly line. GE produces the high-pressure compressor, combustor, and high-pressure turbine, Safran manufactures the fan, gearbox, exhaust and the low-pressure turbine, and some components are made by Avio of Italy and Honeywell from the US. The engines are assembled by GE in Evendale, Ohio, and by Safran in Villaroche, France. The completed engines are marketed by CFMI. Despite initial export restrictions, it is the most used turbofan aircraft engine in the world, in four major variants.
Glass wool is an insulating material made from fibres of glass arranged using a binder into a texture similar to wool. The process traps many small pockets of air between the glass, and these small air pockets result in high thermal insulation properties. Glass wool is produced in rolls or in slabs, with different thermal and mechanical properties. It may also be produced as a material that can be sprayed or applied in place, on the surface to be insulated. The modern method for producing glass wool was invented by Games Slayter while he was working at the Owens-Illinois Glass Co.. He first applied for a patent for a new process to make glass wool in 1933.
A combustor is a component or area of a gas turbine, ramjet, or scramjet engine where combustion takes place. It is also known as a burner, combustion chamber or flame holder. In a gas turbine engine, the combustor or combustion chamber is fed high-pressure air by the compression system. The combustor then heats this air at constant pressure as the fuel/air mix burns. As it burns the fuel/air mix heats and rapidly expands. The burned mix is exhausted from the combustor through the nozzle guide vanes to the turbine. In the case of a ramjet or scramjet engines, the exhaust is directly fed out through the nozzle.
A metal foam is a cellular structure consisting of a solid metal with gas-filled pores comprising a large portion of the volume. The pores can be sealed or interconnected. The defining characteristic of metal foams is a high porosity: typically only 5–25% of the volume is the base metal. The strength of the material is due to the square–cube law.
Helmholtz resonance or wind throb is the phenomenon of air resonance in a cavity, such as when one blows across the top of an empty bottle. The name comes from a device created in the 1850s by Hermann von Helmholtz, the Helmholtz resonator, which he used to identify the various frequencies or musical pitches present in music and other complex sounds.
Noise control or noise mitigation is a set of strategies to reduce noise pollution or to reduce the impact of that noise, whether outdoors or indoors.
Computational aeroacoustics is a branch of aeroacoustics that aims to analyze the generation of noise by turbulent flows through numerical methods.
A sandwich-structured composite is a special class of composite materials that is fabricated by attaching two thin but stiff skins to a lightweight but thick core. The core material is normally low strength material, but its higher thickness provides the sandwich composite with high bending stiffness with overall low density.
Ducts are conduits or passages used in heating, ventilation, and air conditioning (HVAC) to deliver and remove air. The needed airflows include, for example, supply air, return air, and exhaust air. Ducts commonly also deliver ventilation air as part of the supply air. As such, air ducts are one method of ensuring acceptable indoor air quality as well as thermal comfort.
A deep foundation is a type of foundation that transfers building loads to the earth farther down from the surface than a shallow foundation does to a subsurface layer or a range of depths. A pile or piling is a vertical structural element of a deep foundation, driven or drilled deep into the ground at the building site.
In supersonic aerodynamics, an unstart refers to a generally violent breakdown of the supersonic airflow. The phenomenon occurs when mass flow rate changes significantly within a duct. Avoiding unstarts is a key objective in the design of the engine air intakes of supersonic aircraft that cruise at speeds in excess of Mach 2.2.
Honeycomb structures are natural or man-made structures that have the geometry of a honeycomb to allow the minimization of the amount of used material to reach minimal weight and minimal material cost. The geometry of honeycomb structures can vary widely but the common feature of all such structures is an array of hollow cells formed between thin vertical walls. The cells are often columnar and hexagonal in shape. A honeycomb shaped structure provides a material with minimal density and relative high out-of-plane compression properties and out-of-plane shear properties.
The Clean Sky Joint Undertaking (CSJU) is a public-private partnership between the European Commission and the European aeronautics industry that coordinates and funds research activities to deliver significantly quieter and more environmentally friendly aircraft. The CSJU manages the Clean Sky Programme (CS) and the Clean Sky 2 Programme (CS2), making it Europe's foremost aeronautical research body.
Sweeping jet actuators are a type of active flow control technology based on fluidic oscillators used to produce sweeping jets. The first use of fluidic oscillators in the form of sweeping jets for flow control was demonstrated by Raman et al., 1999.<Cavity Resonance Suppression Using Miniature Fluidic Oscillators, G. Raman, S. Raghu and T.J. Bencic' AIAA-99-1900, 5th AIAA/CEAS Aeroacoustics Conference, Seattle, WA, May 10–12, 1999> and later by several authors working in the area of flow control. Many organizations have been working on the use of such actuators for flow control. Boeing, NASA and the University of Arizona Department of Aerospace and Mechanical Engineering, Illinois Institute of Technology, [Advanced Fluidics], Technical University of Berlin are a few of them. They are slots built into the control surface of an airfoil that build on the same principles as that of blown flaps; that by actively blowing air over the surface of an airfoil the effective lift produced by it is increased.
ACTRAN is a finite element-based computer aided engineering software modeling the acoustic behavior of mechanical systems and parts. Actran is being developed by Free Field Technologies, a Belgian software company founded in 1998 by Jean-Pierre Coyette and Jean-Louis Migeot. Free Field Technologies is a wholly owned subsidiary of the MSC Software Corporation since 2011. Free Field Technologies and MSC Software are part of Hexagon AB since 2017.
The ecoDemonstrator Program is a flight test research program of aircraft company Boeing, which has used a series of specially modified aircraft to develop and test aviation technologies designed to improve fuel economy and reduce the noise and ecological footprint of airliners.
A blade vortex interaction (BVI) is an unsteady phenomenon of three-dimensional nature, which occurs when a rotor blade passes within a close proximity of the shed tip vortices from a previous blade. The aerodynamic interactions represent an important topic of investigation in rotorcraft research field due to the adverse influence produced on rotor noise, particularly in low speed descending flight condition or maneuver, which generates high amplitude impulsive noise.
