Eigenfrequency Studies
Both the linearized Euler and the linearized Navier-Stokes interfaces have the Eigenfrequency study as a predefined study type. It should be noted that this analysis type is challenging when a nonzero background mean flow is present. Mathematically it can be very difficult to identify the pure acoustic eigenvalues.
In order to identify acoustic modes in the eigenvalue spectrum it can be a good idea to use the Region option in the Eigenfrequency search method when setting up an eigenfrequency study step. In this way the nonacoustic modes that consist of entropy and vorticity waves (eigenfrequencies with large imaginary components) can be filtered out. The pure acoustic modes typically have eigenfrequencies with a small complex component can then more easily be identified. The finals identification needs to be done with a visual inspection of the mode shape in postprocessing. Set the search region for the real part as desired and then set a small interval for the imaginary part, for example, from −0.02 to 0.02.
For more details see the Eigenfrequency section in the COMSOL Multiphysics Reference Manual