Acoustic Scattering through a Circular Orifice in Low Mach Number FlowS. Sack , M. Abom 
 KTH, the Royal Institute of Technology, Stockholm, Sweden
The acoustic scattering through a circular orifice plate in a duct with low Mach number flow (M=0.1) is computed using the Linearized Navier-Stokes physics interface of COMSOL Multiphysics®. The work by Kierkegaard et al. is extended to account for higher order acoustic modes, i.e., behind the cut-on frequency of the first radial duct mode. Orifice flows tend to create a sharp separation zone at that hydrodynamic energy is transformed into acoustic energy and vice versa. At characteristic Strouhal numbers, a recirculation zone inside the orifice creates an acoustic feed-back loop which courses very strong amplifications of distinct tones, so called "whistling" tones. A computation is conducted for frequencies behind the plane-wave range and compared to measurements. The data is post-processed with advanced procedures in order to map mode-shapes on the acoustic field.