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.