Determination of acoustic transfer matrices via large eddy simulation and system identification

The transfer behavior of acoustic waves at a sudden change in cross section in a duct system is investigated. In particular, a method is presented which allows to determine the coefficients of transmission and reflection of plane acoustic waves by combining large eddy simulation (LES) of turbulent compressible flow with system identification. The complex aeroacoustic interactions between acoustic waves and free shear layers are captured in detail, such that the transfer coefficients can be determined accurately from first principles. The method works as follows: At first, an LES with external, broadband excitation of acoustic waves is carried out. Time series of acoustic data are extracted from the computed flow field and analyzed with system identification techniques in order to determine the acoustic transfer coefficients for a range of frequencies. The combination of the broadband excitation with highly parallelized LES makes the overall approach quite efficient, despite the difficulties associated with simulation of low-Mach number compressible flows. In order to demonstrate the reliability and accuracy of the method, the results for the transfer behavior and the acoustic impedance are presented and physically interpreted in combination with several analytical models and experimental data.