Characterization of the Aeroacoustic Transfer Matrix of Narrow Ducts and Perforated Plates based on Compressible Computational Fluid Dynamics and System Identification
Creators: name: Orunnukaran Mani, Anantha Krishnan
Publisher: name: figshare
Year: 2022
Subjects: subject: 90101 Aerodynamics (excl. Hypersonic Aerodynamics)
subject: FOS: Mechanical engineering
subject: Aerospace Engineering
subject: Mechanical Engineering
subject: 20301 Acoustics and Acoustical Devices; Waves
subject: FOS: Physical sciences

description: Thermoacoustic instabilities induced by the coupling of acoustic modes and unsteady heat release are a matter of concern in a wide variety of high-performance systems, including rocket engines and gas turbine combustors. They can result in severe noise emissions, operational difficulties, and even catastrophic damage to the system. The Linear Stability Analysis (LSA) of low-order network models is a reliable method for identifying and predicting the thermoacoustic instability in a system by approximation of complex configuration as a network of acoustic elements. Mathematically these elements are described by an acoustic scattering or transfer matrix, defining the dynamic relation of acoustic elements travelling within the configuration. Deriving the transfer matrix coefficient enables quick evaluation of the system’s acoustic behaviour under various operating conditions. The present thesis describes a procedure for the characterization of the acoustic transfer matrix, which predicts the aeroacoustic properties of the narrow ducts and perforated plates in a burner configuration. The transfer matrix coefficients are determined by combining modern compressible computational fluid dynamics based on direct numerical simulation (DNS) and empirical modelling via system identification (SI). The work begins by computing a statistically stabilized non-excited DNS and then perturbing the mean flow by imposing a low-pass filtered, constant amplitude broadband excitation at the inlet or outlet of the target configuration asynchronously. The temporal data series are extracted from the acoustically excited DNS solutions, and the characteristic acoustic waves or Riemmen invariants are deduced for the linear acoustic model identification based on CFD/SI method. Henceforth, the acoustic system will be described as a Single-Input Single-Output Linear Time- Invariant (SISO-LTI) system. A causal matrix representation based on the acoustic scattering matrix, defined by upstream and downstream characteristic waves f and g, is obtained for model identification based on the SI method. The matrix coefficients describe the respective transmission and reflection of the acoustic waves within the configuration. The acoustic system dynamics are identified by solving Wiener-Hopf equations through correlation analysis, described by the system’s response as Finite Impulse Responses (FIRs). The scattering matrix coefficients will be algebraically transformed to yield the acoustic transfer matrix. The analytical solution and the highly-refined axisymmetric solution validate the method’s accuracy and reliability. <br> The combustors or burners are subjected to acoustic resonance with or with- out flames, causing thermoacoustic instabilities. The passive acoustic property of combustor components in the absence of a flame yields excessive noise propagation through acoustic wave transmission and reflection within the configuration. This research focuses on characterizing the transfer matrix of a non-reactive or cold case burner configuration. The cold case acoustic transfer matrix enables the prediction of the acoustic characterization of the hot or reactive burner. These results can be used to understand the acoustics and dynamics of large industrial systems such as gas turbine engines, low-NOx combustors, or injectors.