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The noise generation by accelerated vorticity waves in a nozzle flow was investigated in a model experiment. This noise generation mechanism belongs, besides entropy noise, to the indirect combustion noise phenomena. Vorticity as well as entropy fluctuations, originating from the highly turbulent combustion zone, are convected with the flow and produce noise during their acceleration in the outlet nozzle of the combustion chamber. In the model experiment, noise generation of accelerated vorticity fluctuations was achieved. The vorticity fluctuations in the tube flow were produced by injecting temporally additional air into the mean flow. As the next step, a parametric study was conducted to determine the major dependencies of the so called vortex noise. A quadratic dependency of the vortex noise on the injected air amount was found. In order to visualise and classify the artificially generated vorticity structures, planar velocity measurements have been conducted applying Particle Image Velocimetry (PIV).

Entropy noise caused by combustors increases rapidly with rising Mach number in the nozzle downstream of the combustion chamber. This is experimentally shown with a dedicated test facility, in which entropy waves are generated in a controlled way by unsteady electrical heating of fine platinum wires immersed in the flow. Downstream of the heating module called entropy wave generator (EWG), the pipe flow is accelerated through a convergent-divergent nozzle with a maximum Mach number of 1.2 downstream of the nozzle throat. Parameters like mass flux of the flow, nozzle Mach number, amount of heating energy, excitation mode (periodic, pulsed, or continuously), and propagation length between EWG and nozzle have been varied for the analysis of the generated entropy noise. The results are compared with the results of a one-dimensional theory found in early literature.

It is assumed by theory, that entropy noise emitted by combustion systems increases rapidly with rising Mach number in the nozzle downstream of the combustion chamber. Model experiments have been carried out to verify the existence of this sound generating mechanism. A dedicated test facility was built, in which entropy waves are generated in a controlled way by unsteady electrical heating of fine platinum wires immersed in the flow. Further experiments have been carried out in a model combustor test rig where a broadband noise phenomenon, presumably related to indirect noise generation mechanisms, was found.

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Combustion noise of aero engines originates from unsteady combustion processes which in turn lead to vortical and temperature fluctuations. These so-called vorticity and entropy waves are convected from the combustor into the turbine where their acceleration results in an additional sound release, namely the indirect noise. In the present study the noise generation by accelerated vorticity waves is investigated in a convergent-divergent nozzle representing the simplest model of the flow through the turbine. A hybrid CFD/CAAapproach is applied which consists of RANS mean flow simulations followed by frequency domain simulations of linear acoustic and vortical fluctuations based on the linearized Navier-Stokes equations (LNSEs). Vorticity waves are excited by a body force term which is deduced from an analytical solution of the linearized vorticity equation. By this means, the indirect noise released by the accelerated vorticity waves is computed and compared with experimental measurements. The numerical simulation captures in general the coupling mechanisms relevant to indirect noise.

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