Combustion noise consists of direct noise related to the unsteady combustion process itself and indirect noise. As known, indirect noise is produced when entropy fluctuations originating from the combustor are accelerated through the turbine. According to the characterisation of the flow by pressure, entropy and vorticity perturbations accelerated vorticity fluctuations are likewise expected to generate indirect noise. In a model experiment the sound generation through the acceleration of vorticity fluctuations was studied. Within a swirl free and a swirling tube flow, vorticity fluctuations were generated artificially by injecting temporarily additional air into the mean flow. The spatial and temporal changes of the velocity field were determined with Hot-Wire Anemometry measurements. During the acceleration of the vorticity fluctuations in a choked convergent-divergent nozzle pressure disturbances are generated. The produced acoustic waves were detected downstream of the nozzle. Direct and vortex sound was identified and separated by varying the distance between the air-injection inlet and the nozzle in case of a swirling flow. The number of inlet ports for the air-injection was modified for a constant injected mass flow rate yielding to amplitude differences in the generated vortex sound. In addition, the intensity of the swirling flow and of the vorticity fluctuation was varied. Increasing the air-injection into the mean flow augments the generated indirect noise.
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