Sound Generating Flow Structures in a Thick Orifice Plate Jet

The aim of this work is to study sound generating flow structures in a thick circular orifice plate jet, placed in a circular duct. Large eddy simulations (LES) are performed for two jet Mach numbers, 0.4 and 0.9. Characteristic frequencies in the flow, and their corresponding flow structures, are identified with dynamic mode decomposition (DMD). The results show that a tonal noise is generated at frequencies where the jet displays strong ring vortices, in the plane wave range. The main sound generating mechanisms seems to be a fluctuating mass flow at the orifice opening and a fluctuating surface force at the plate sides, caused by the ring vortices. The frequencies are believed to be chosen, and strengthened, by a feedback mechanism between the orifice in- and outlet.