Investigation of Field Decomposition Methods for Simulations of Turbulent Flow Grazing an Acoustic Liner

The subject of this thesis focuses on the study, analysis, and current challenges of recently introduced methods for flow field analysis and decomposition, namely Spectral Proper Orthogonal Decomposition (SPOD) and Wavelet Transform (WT). Spectral POD is a decomposition technique designed to identify energy-ranked modes within a flow field, where each mode corresponds to a specific frequency, allowing for the distinctive separation of phenomena occurring at different frequencies and energy levels. Wavelet transform is another powerful method for analysing signals in both time and frequency domains, by decomposing a signal into different scales with the use of wavelet functions, it provides a localized representation of signal features, making it suitable for detecting transient events or localized variations, like identifying coherent structures and turbulence study. The main topic is the application of these promising techniques to a numerical simulation of a turbulent flow grazing acoustic liners, which purpose is damping acoustic energy travelling a duct or a pipe. Acoustic liners exhibit diverse behaviours in response to the different conditions, such as incident waves' sound pressure level, frequency, or presence of a mean flow. Bearing in mind that modal analysis or proper filtering procedures can be used to decompose the pressure signal into acoustic and hydrodynamic components, which present different peculiarities, SPOD and WT are supposed capable of identifying these components for a deeper understanding of the underlying physics. The forementioned techniques, being very computational power demanding, became available for practical situations only with recent technological achievements, for this reason, HPC computational resources offered by Politecnico di Torino are used to analyse acoustic liners functioning in different characterizing conditions. The results are used firstly to identify the various functioning regimes of single degree-of-freedom acoustic liners, and after adopted to investigate whether is possible a separation of acoustic and hydrodynamic components of the resulting flow field. After a brief introduction of the main topics, an appropriate description of the used methods, algorithms, and flow conditions analysed is reported, key parameters and experimental setup is described right after and finally main results and outcomes are presented.