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Aircraft noise reduction technologies
Aircraft noise is attributed to airframe structure as well as to powerplant contributions. INASCO is actively involved in developing strategies for reducing noise levels emerging from both sources in order to ensure that the gains potentially obtained at powerplant level are not limited by a noise floor at airframe level.

By adopting a whole aircraft approach based on the latest developments in active / adaptive technologies, flow control techniques and advances in computational aero-acoustics applied to the major causes of noise at source, INASCO aims to continuously developing breakthrough noise reduction technologies to meet the ACARE goals.
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This will be achieved by addressing the most significant noise sources of the aircraft, namely turbo-machinery, combustor, jet and airframe noise, across all relevant system components within an integrated, multidisciplinary approach aimed at managing the technical risk involved in developing breakthrough-oriented solutions such as active noise and flow control as well as adaptive and variable concepts, in addition to exploiting the developed advanced computational aero-acoustics methods.
 
i. Structure-borne noise
An extensive research work into structure-borne noise transmission over the last years has resulted in the development of novel theoretical and predictive tools for complex structures, such as double wall composite shells, representative of advanced airframe concepts. These can be used for tailoring the design to meet both the acoustic and structural specifications. In the case of composite structures the fiber orientation may contribute to additional noise reduction and minimize the weight of the structure.
 
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Theoretical developments include the development of models that predict noise levels for various fuselage concepts currently under investigation for aircrafts made of advanced composite materials, including double wall configurations.
 
ii. Powerplant-borne noise
Turbofan engines
The engine is wrapped in a nacelle and fan noise has two paths to propagate; the inlet and bypass ducts. The ducts have acoustic treatment to damp and absorb noise generated by the fan. Sound, which is not absorbed, propagates out the front and back of the nacelle and can be heard at the ground. In addition, liners are used inside the exhaust duct to minimize turbine and combustor noise.

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INASCO is actively involved in developing noise attenuation technologies for:
  • The exhaust duct – development of high-temperature multi-segmented broadband liners have been designed and implemented in the CFM56-B engine centerbody (plug) with minimum weight penalty resulting in very promising results in terms of noise attenuation potential.
 
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Plug as delivered and after testing
 
  • The by-pass duct – a non-locally reacting multi-segmented liner has been developed currently undergoing experimental characterizations. This type of liner consists of a perforate sheet and a cavity split into two parallel sub-cavities by a “perforated” metal foam segmented bulk reacting (axially and circumferentially) partition. Complimentary to these noise attenuation efforts will be new active stator designs to be initiated in 2010.
 

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Hot and cold stream liners placed inside an a/c engine
 
  • Breakthrough technologies for noise suppression exhaust ducts development - Variable Area Morphing Nozzles with V Shaped Prismatic Changing Chevrons to enhance mixing of the jet plume through (a) Geometric changes in the nozzle shape + (b) V shape prismatic changing chevrons at the nozzle exit + (c) Active metal foam based inner nozzle surfaces and thus inducing instability modes to which the jet flow is most susceptible thereby achieving a noise reduction.

Future Supersonic Aircrafts
In high-speed (Supersonic) aircrafts noise issues are very important for their acceptance and adoption. INASCO is currently working in reducing the noise of a mixer-ejector engine configuration by adopting materials that withstand high temperatures and a special variable impedance design. By adjusting the liners ’ geometrical features, the perforation patterns and the bias flow of each segment we can aim for optimal sound attenuation.

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Mixer - ejector engine concept

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 Novel liner concepts:

Variable impedance hot stream liner designs

 

 

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Insertion Loss (IL) Measurements: 30-40% improved performance compared to state of the art solutions

 

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Cyclic mechanical loading INASCO scale 1 hot stream liner and assessment of acoustic performance:

 

Within the context of HISAC project the effect of the acoustic performance of specially designed hot stream liner concepts under conditions representative of a real engine (temperature, vibration, oil and sand contamination) has been assessed. A series of tests have been performed from small scale coupons to scale 1 parts. INASCO hot stream liner fulfilled all design constraints and performed as expected even under severe conditions.

   
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