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Stromingsakoestische karakterisatie van microperforaties: een studie naar de invloed van aerodynamische druk en perforatiegeometrie

Year: 2025
Type: dissertation
Abstract: Conventional sound mitigation solutions for flow ducts typically utilize macro-perforated structures combined with fibrous or porous materials. While effective, these approaches have the drawback of leading to bulky designs for low-frequency attenuation in which the used materials are often flammable and prone to degradation. These limitations contrast with the growing demand for lightweight, compact, and environmentally friendly designs. Micro-perforated panels (MPPs) present a promising alternative. Consisting of thin plates perforated with numerous sub-millimeter holes, MPPs enhance visco-thermal attenuation by dissipating acoustic energy through viscous forces inside the sub-millimeter openings. This allows them to achieve high acoustic resistance without additional absorptive materials, enabling the development of more compact silencers with clean airflow. In order to effectively integrate MPPs into sound mitigation designs, accurate prediction models are essential. These descriptions typically represent MPPs as boundary conditions in flow-acoustic propagation solvers, where the impedance of the perforations plays a critical role. While semi-empirical models accurately describe the impedance of circular perforations in the absence of flow, mass manufacturing techniques often produce irregular, non-circular geometries which influence the acoustic properties and must be accounted for. Moreover, the specific flow conditions are known to significantly impact the acoustic behavior of perforations. Although empirical models have been derived for macro-perforated geometries under different flow conditions, disagreement exists of which flow parameters are most suitable to characterize the acoustic impedance. Furthermore, even when the same flow parameter is used, there tend to be a large deviation on the empirically determined coefficients, making it challenging to scale these models towards sub-millimeter dimensions. The goal of this thesis is to evaluate the influence of the geometry and flow conditions on the acoustic performance of sub-millimeter perforations. The first objective of this thesis focuses on developing and validating a suitable measurement framework to characterize the acoustic impedance of micro-perforations in the wall of a flow duct. To measure the relevant acoustic pressures and acoustic particle velocities constituting this variable, an indirect measurement approach is employed, combining a multi-microphone side-branch set-up with a three-port scattering matrix formulation. In addition, a correction step is implemented to account for plate dynamic effects and a side-branch fan is installed to control the aerodynamic pressure drop over the perforations. The second objective uses the experimental methodology to identify the key flow-acoustic variables to describe sub-millimeter perforations in design guidelines. Reference measurements are conducted on well-defined circular perforations and are compared to commonly used empirical correlations and corrections from the literature. In pure grazing flow conditions, measurements for perforation diameters ranging from 0.5 to 3 mm demonstrate how models based on flow Mach number and skin friction velocity qualitatively predict the observed trends. However, the correlation coefficients derived for macro-perforations do not scale well to sub-millimeter dimensions, necessitating their re-evaluation. Furthermore, the influence of a non-zero aerodynamic pressure difference across the sub-millimeter perforations is assessed both with and without grazing flow. The results reveal a large sensitivity of the measured impedance to small pressure differences, emphasizing the need for precise characterization of this parameter. The third objective explores the use of numerical simulations to predict the experimentally observed trends for sub-millimeter orifices in order to gain a deeper understanding of the underlying physical phenomena. Using a two-step calculation procedure, the perturbed linearized Navier-Stokes equations are solved under an isentropic assumption while neglecting turbulent damping. In quiescent conditions, simulations show excellent agreement with experimental results. However, under grazing flow, only qualitative agreement is observed, with quantitative discrepancies becoming larger at low Mach numbers and higher frequencies. These deviations suggest an underestimation of the viscous scrubbing losses, likely due to the absence of thermal effects and the inability to fully capture the local flow field around the perforations. For cases with an additional bias flow, simulations are restricted to 2D geometries, limiting their use for quantitative predictions, but still capturing the experimentally observed trends. The final objective assesses the potential of the combined experimental-numerical methodology in characterizing the effects of perforation geometry by focusing on a single geometrical parameter: the perforation inclination angle. In the presence of grazing flow, the orientation of the inclination relative to the flow direction has a significant impact on the measured impedance, even for small inclination angles. Perforations inclined against the flow direction show a weaker dependence on frequency compared to those aligned with the flow direction. Using global flow variables such as the flow Mach number and aerodynamic pressure difference, numerical simulations qualitatively predict the trends observed for an orifice inclined in the flow direction. However, these variables prove insufficient in capturing the experimental trends when the perforations are inclined with the flow, where the impact of local flow phenomena on the perforation impedance appear to be more outspoken.
Source: Lirias
Author: Vandemaele, Simon
Language: English
Open Access status: green
OpenAlex ID: W7112686442
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