Mohamed Ali Hamdi

Development of an analytical solution of modified Biot’s equations for the optimization of lightweight acoustic protection

During lift-off, space launchers are submitted to high-level of acoustic loads, which may damage sensitive equipments. A special acoustic absorber has been previously integrated inside the fairing of space launchers to protect the payload. A new research project has been launched to develop a low cost fairing acoustic protection system using optimized layers of porous materials covered by a thin layer of fabric. An analytical model is used for the analysis of acoustic wave propagation within the multilayer porous media. Results have been validated by impedance tube measurements. A parametric study has been conducted to determine optimal mechanical and acoustical properties of the acoustic protection under dimensional thickness constraints. The effect of the mounting conditions has been studied. Results reveal the importance of the lateral constraints on the absorption coefficient particularly in the low frequency range. A transmission study has been carried out, where the fairing structure has been simulated by a limp mass layer. The transmission loss and noise reduction factors have been computed using Biots theory and the local acoustic impedance approximation to represent the porous layer effect. Comparisons between the two models show the frequency domains for which the local impedance model is valid.

Variational Formulation by Integral Equations for the Sound Radiation in a Non Uniform Flow

We present in this work a new variational formulation by integral equations which enables the calculation of acoustic fields radiated in non uniform compressible flows. Indeed, this formulation reduces the integral domain without modifying the Sommerfeld’s radiation condition. Moreover it avoids computation of the “finite part” of singular integrals and leads to a compact symetrical linear system after discretisation by finite elements. This system is then solved by an iterative method.