Alexis JAMOIS : Development of meta-materials for aero-acoustic applications

Summary : 
 

The reduction of the low frequency acoustic is a current issue, especially in the transport fields. The conventional absorbing materials used are not that efficient in this frequency range or solely for a very narrow frequency band. The emergence of new materials offering abnormal properties an acoustical characteristics may offer an interesting alternative : meta- materials. They have shown way more interesting efficiency compared to the conventional materials with broadband low frequency range absorption characteristics. This new materials are for some of them based on a inner periodic geometrical definition, and can thus be designed by computational method and 3D printed.
The objectif of the thesis is to design such a material efficient for reducing the low frequency noise propagating in ducts with or without an air flow.
The LMFA develops since many years models and experimental benches to understand and optimize the wall treatment performances in such situation, mainly related to the aeronautics industry. The aircraft engines recently developed to reduce the energy consumption are actually accountable for a shift in low frequencies of the noise which is more difficult to mitigate. In addition this evolution also tends to reduce the nacelle length and consequently the potential treated region surfaces where it is possible to use acoustic treatment. The LMFA has worked on several European projects related to the subject, active-passive materials modeling and testing, eductive methods to characterize absorbing materials, numerical model using linear Euler equations in time domain. These works have allow better understanding of absorbing material behavior with an air flow ([1] to[7]).
The proposed thesis will pursue along these works, and also the works driven in collaboration with the LTDS concerning the development of vibroacoustic low-frequency noise mitigation ([8],[9]. The aim of the thesis is to develop a complete process to design and optimize materials from microscopic scale to 50 cm length sample that will be tested on LMFA test bench CAIMAN. If many currently meta-materials developments relay on that approach, the present thesis will try to target the reduction in flow duct from the conception stage to tests, targeting an optimization of the structure.
The study will be composed of the following steps :
-Modeling of a meta-material at microscopic scale taking the visco-thermic effects into account. Prediction of the behavior at a macroscopic scale.
-Selection of the most efficient materials and production of samples using a 3Dprinter. Experimental validation with impedance measurements in normal incidence and identification of the characteristic functions of the material.
-Modeling and test of the material in a duct with an air flow.

The studied materials will in a first step present a rigid structure and an extended equivalent fluid behavior. Following developments will include elasticity, membrane or so.
The modeling and tests will mainly be performed in linear acoustic context. Targeted applications for these materials -aeronautic field - is however subject to non-linearity due to the high intensity of the acoustic phenomena. Non-linear effects could also be investigated.