Experimental impedance assessment of innovative liner under shear grazing flow

International audienceOn modern aircrafts, passengers and crew breathe a mixture of fresh and recirculated air. This combination allows the regulation of temperature, pressure and humidity. The air is bled from the engines and supplied to air conditioning units. It is then ducted into the cabin, circulated and eventually drawn into the lower fuselage where it is sucked out by the pressurization outow valve for the cycle to begin again. Besides creating a safe and comfortable environment, the aircraft air conditioning systems generate noise. The noise radiated from the aircrafts' air conditioning systems is reduced thanks to acoustic liners. These liners present a major design challenge because of the need to address a wide range of conicting requirements. Acoustic liners must provide high levels of noise reduction over a wide range of operating conditions. They should also be light and exible to meet strict weight and tight space restrictions. Until now, acoustic liners for air conditioning systems are made of porous materials, very efficient for sound absorption in the high-frequency range. Locally-reacting liners made of a multiple layers of a honeycomb core topped by a perforated facesheet are classically used in turbofan engine nacelles for mitigating fan noise. These liners are denoted as SDOF (Single Degree of Freedom) or DDOF (Double Degree of Freedom) liners depending on the number of perforate-over-honeycomb layers. Mid-frequency noise attenuation (in the range of 1-5 kHz) can easily be obtained by tuning the liner acoustic impedance to a target value specific to the duct environment (geometry, ow Mach number, noise source modal content). The impedance achieved by a SDOF or DDOF liner depends on its geometry (porosity and holes diameter of the facesheets, honeycomb depth), on the grazing ow features, and on the noise source level. During the ALIAS project,5 a simulation-based design process was implemented to assess the liners concepts that were the best suited for attenuating the air pump noise in the mid-frequency range. A trade-off between acoustic efficiency, weight and cost manufacturing issues was made before selecting the liners. In the framework of the IDEAS Project funded within the European initiative CleanSky 2 SYSTEM- ITD, ONERA, the French Aerospace Lab, and the SMEs ATECA and Poly-Shape combine their research and technological capabilities to propose new ideas in the domain of acoustic liners and in-duct modal detection for air conditioning systems from Liebherr Aerospace. A compact innovative acoustic liner is designed in order to mitigate this jet pump noise source over all the frequency range, while meeting the strict weight, costs and tight space restrictions. This paper presents two concepts of liner for a large frequency range with high industrial constraints. These liner architectures need to be assessed regarding the ow effect and the incident sound pressure level. The acoustic behavior and the effect of a grazing ow on the impedance of each liner concept are assessed in this paper. This liner concept analysis enables to increase our experimental data base and to identify reliable liner models to optimize the best liner solution regarding industrial cases