Experimental assessment of noise generation and transmission in a high-pressure transonic turbine stage

Noise originating from the core of an aero-engine is challenging to quantify since the understanding of its generation and propagation is less advanced than that for noise sources of other engine components. To overcome the difficulties associated with dynamic measurements in the crowded core region, dedicated experiments have been set up in order to investigate mainly two processes: the propagation of direct combustion noise through the subsequent turbine stage, and the generation of indirect combustion noise by the passage of inhomogeneities of entropy and vorticity through the turbine stage. In the current work, a transonic turbine stage was exposed to isolated and well-characterized acoustic, entropic, and vortical disturbances. The incoming and outgoing sound fields were analyzed in detail by two large arrays of microphones. The mean flow field and the disturbances were carefully mapped by several aerodynamic and thermal probes. The results include transmission and reflection characteristics of the turbine stage, the latter was found to be much lower than commonly assumed. The modal decomposition of the acoustic field in the upstream and downstream section shows additional modes besides the expected rotor-stator interaction modes. At the frequency of entropy or vorticity excitation, respectively, a significant increase of the overall sound power level was observed.