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

The noise originating from the core of an aero-engine is usually difficult to quantify and the knowledge about its generation and propagation is less advanced than that for other engine components. In order to overcome the difficulties associated with dynamic measurements in the crowded core region, dedicated experiments have been set up in order to investigate the processes associated with the generation of noise in the combustor, its propagation through the turbine and the interaction of these two components, which may produce additional - so-called indirect combustion - noise. In the current work, a transonic turbine stage installed at the Laboratorio di Fluidodinamica delle Macchine of the Politechnico di Milano was exposed to 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, latter one was found to be much lower than usually assumed. The modal decomposition of the acoustic field in the upstream and downstream section show beside the expected rotor-stator interaction modes additional modes. At the frequency of entropy or respectively vorticity excitation, a significant increase of the overall sound power level was observed.