Numerical modeling and experimental validation of the acoustic transmission of aircraft's double-wall structures including sound package .

摘要

In the field of aeronautics, reducing the harmful effects of acoustics constitutes a major concern at the international level and justifies the call for further research, particularly in Canada where aeronautics is a key economic sector, which operates in a context of global competition. Aircraft sidewall structure is usually of a double wall construction with a curved ribbed metallic skin and a lightweight composite or sandwich trim separated by a cavity filled with a noise control treatment. The latter is of a great importance in the transport industry, and continues to be of interest in many engineering applications. However, the insertion loss noise control treatment depends on the excitation of the supporting structure. In particular, Turbulent Boundary Layer is of interest to several industries. This excitation is difficult to simulate in laboratory conditions, given the prohibiting costs and difficulties associated with wind tunnel and in-flight tests. Numerical simulation is the only practical way to predict the response to such excitations and to analyze effects of design changes to the response to such excitation. Another kinds of excitations encountered in industrial are monopole, rain on the Roof and diffuse acoustic field. Deterministic methods can calculate in each point the spectral response of the system. Most known are numerical methods such as finite elements and boundary elements methods. These methods generally apply to the low frequency where modal behavior of the structure dominates. However, the high limit of calculation in frequency of these methods cannot be defined in a strict way because it is related to the capacity of data processing and to the nature of the studied mechanical system. With these challenges in mind, and with limitations of the main numerical codes on the market, the manufacturers have expressed the need for simple models immediately available as early as the stage of preliminary drafts. This thesis represents an attempt to address this need. A numerical tool based on two approaches (Wave and Modal) is developed. It allows a fast computation of the vibroacoustic response for multilayer structures over full frequency spectrum and for various kinds of excitations (monople, rain on the roof, diffuse acoustic filed, turbulent boundary layer) . A comparison between results obtained by the developed model, experimental tests and the finite element method is given and discussed. The results are very promising with respect to the potential of such a model for industrial use as a prediction tool, and even for design. The code can be also integrated within an SEA (Statistical Energy Analysis) strategy in order to model a full vehicle by computing in particular the insertion loss and the equivalent damping added by the sound package.;Keywords: Transfer Matrix Method, Wave Approach,Turbulent Boundary Layer, Rain on the Roof, Monopole, Insertion loss, Double-wall, Sound Package.