Characterization of Passive Acoustic Dampers with Orifices using Linear and Nonlinear Numerical Models

Passive acoustic dampers in the form of Helmholtz resonators, perforated plates and liners, are commonly used to suppress noise propagation or to control the acoustic feedback that can lead to thermo-acoustic instabilities in combustion engines. Their robustness, acoustic properties and easy integration into existing systems make them an appealing solution for noise control in many applications, from air conditioning systems in buildings to automotive mufflers or aircraft engine liners.

When designed properly, such silencers dissipate the energy contained in the passing acoustic waves within a specific frequency range. The damping mechanisms involved can differ significantly depending on the geometry of the perforations, the amplitude of the acoustic excitation, the local flow conditions and the structural response of the system. The large variety of operating conditions encountered by these silencers can be difficult to reproduce and to monitor in an experimental setup. Moreover, the applicability of many of the semi-empirical models developed to describe the acoustic behavior of perforates is severely limited by their restrictive assumptions or because important design parameters that could be used to enhance the acoustic damping are neglected. In order to better understand the physical phenomena influencing the flow-acoustic interaction at perforates and to improve the design of future passive sound absorbers, a clear need for efficient numerical prediction schemes and characterization methods comes to the fore.

For these reasons, the first part of this dissertation investigates linear acoustic operators combined with efficient high-order numerical methods to model the wave propagation through non-homogeneous medium and non-uniform flow regions, accounting for the various convective and dissipative phenomena. These linear methods are further applied to characterize the influence of local sheared flows, temperature profiles, and flow turbulence on the acoustic behavior of perforates.

In the second part, a methodology based on the computational fluid dynamics solution of the incompressible flow equations is presented and assessed for the determination of the impedance of Helmholtz resonators. This approach is used to analyze the nonlinearities occurring at the resonator opening due to flow separation under high amplitude excitations.

The vibro-acoustic behavior of perforated plates is the focus of the last part of this work. A numerical design and optimization tool for passive noise control devices based on flexible micro-perforated panels is proposed. This technique, which couples a potential acoustic solver and structural shell elements, is validated and applied to a cylindrical resonator containing a micro-perforated plate with circular square-edged orifices.

The developed numerical methods provide novel modeling capabilities for the flow-acoustic interactions, nonlinearities, and the vibro-acoustic coupling affecting the acoustic behavior of perforates in passive acoustic dampers. As such, these techniques can provide valuable insights in the physical behavior, paving the path for the development of novel noise control solutions.