Comparing different aero-acoustic formulations in terms of results of an eigenvalue problem

Since the 1950's, the field of aero-acoustics is constantly developing. Based on the work of Lighthill, Curle, Ffowcs Williams and Hawkins, Lilley, different formulations have been developed using different methods of decomposing and linearizing the field variables. In this work, different formulations, namely the Linearized Navier-Stokes Equations, Linearized Euler Equations, the Acoustic Perturbation Equations and the Galbrun Equation are compared in terms of an eigenvalue problem of a 2D rectangular duct case in the scheme of a finite element analysis. Utilizing these results, limits of the applicability can be defined.