Inverse method to characterize 'local' and 'non-local' absorbing materials submitted to a shear grazing flow

Invited paper

Frank Simon

Office National d'Etudes et de Recherches Aerospatiales

Tuesday 2 june, 2015, 16:00 - 16:20

0.7 Lisbon (47)

As aircraft traffic is constantly increasing, serious efforts are made to find ways to reduce noise produced by the engines. Among them, the design of performing absorbing materials, called liners, placed on the nacelle's walls. ONERA has developed an "impedance" eduction method (code "Elvin") applied to materials with "local reaction" in the presence of shear grazing flow. The inverse process is based on wall acoustic pressure or velocity fields acquired by Laser Doppler Velocimetry (LDV) above the liner. Computations rely on the resolution of the 2D linearized Euler equations in the harmonic domain, spatially discretized by a discontinuous Galerkin scheme, which presents advantageous properties. First, it is weakly dispersive and dissipative. In addition, boundary conditions are imposed through fluxes, which is particularly robust and straightforward. The minimization of the objective function is achieved by the resolution, at each iteration on the liner impedance, of the direct and adjoint equations. After a description of the architecture and current features of Elvin code, configurations of "linear" and "non-linear" liners are tested with the corresponding impedance eduction method from data measured in Onera aeroacoustic bench (B2A) or NASA flow ducts. Values of objective function are analysed in the impedance map to evaluate standard deviation associated to identified impedance. Then, the procedure to extend the code to open-cell porous media instead of "local reaction" liners is shown. This implies the integration of a computation domain in which acoustic propagation equations are solved in the media. The objective is to extract the macroscopic parameters governing viscous dissipation of sound waves in porous media, from Biot theory or derived theories : open porosity, static flow resistivity, geometrical tortuosity, thermal and viscous characteristic dimensions... A first validation of direct equations is finally presented in impedance tube configuration (without flow) with implementation of Delany-Bazley's approach.

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