Optimisation de la vérification de l’équation vibroacoustique des plaques sur un échantillonnage spatial : application au bruit intérieur des avions

This PhD thesis deals with the use of damping patches on aircraft skins to reduce interior noise. Induced mass penalties are considered high today, hence the interest of damping size and placement optimizations. Three excitations are used by Airbus to assess a damping technology: mechanical (laboratory or ground tests), acoustic (laboratory or ground tests) and aerodynamic (light tests) excitations. The strategy adopted to study the acoustic benefit of such treatments is chosen analytical so as to cover the large frequency band associated with low computation time. An original approach is first of all proposed to compute sound radiation from locally damped plates. The advantages of this method are twofold: first avoid difficulties of radiation impedances calculation and then handle complex aircraft structures. The scientific outcome in term of modelisation lies in the two following points. First an approach based on the minimization of error on a sample of observation points has been developed to solve vibroacoustic equations. A method handling Dirac delta functions to model heterogeneities at localized damping patch boundaries is then proposed. The analytical model developed is applied to unstiffened finite flat plates where a localized damping treatment, a multilayer plate or a varying thickness can be considered. The principle of the method remains valid for curved or stiffened plates; only the equation of motion must be adapted. All studies presented in this manuscript are led at ambient temperature. Numerical validations for mechanical, acoustic and aerodynamic excitations and an experimental validation for acoustic excitation are led. Results prove a very good accuracy of the method developed: fluid-structure interactions so as localized damping are well modeled. The focus is also put on the choice of the aerodynamic excitation model. It is pointed out a strong influence of the model for absolute levels and a low influence for relative levels, in-flight validation appear to be necessary even if such flight tests are challenging. Experimental studies led on localized damping patches under acoustic excitation have pointed out some tendencies for damping patch size optimization. Using the method developed, a numerical study has then detailed this analysis and extended it to excitation and damping patch placement influences. Acoustic and aerodynamic excitation plays clearly different roles in damping impact assessment. Damping patch size and placement effects are finally analyzed under aerodynamic excitation in realistic conditions. A non-linear behavior of the size versus the added mass is observed allowing definition of an optimum. The placement of the damping treatment is presented to be critical only at low frequencies where modal phenomena dominate. This PhD work gives an accurate prediction tool adapted to the industrial needs of aircraft interior noise control. Extensions are of course necessary to consider more realistic structures but the present method allows already the optimization of damping treatments with regard to mass constraints