Noise Mechanisms of an Axial Turbine Stage Based on Large Eddy Simulation

This paper aims at identifying the noise sources in an axial turbine stage and their relative importance. The Large eddy simulation (LES) has been carried out on a geometry containing single rotor and stator passages and the mesh of the rotor domain is circumferentially sliding. The proper orthogonal decomposition (POD) is applied to data matrices constructed with the pressure fields in order to distinctly extract the coherent structures responsible for noise generation. The results show that the rotor–stator interaction contributes up to 50% of the total sound energy, the flow fluctuations are influenced by the rotor–stator interaction even in the very upstream region of the stator passage due to the massive pressure wave reflections between the stator vane row and the rotor blade row. Therefore, the tonal noise at the blade passing frequency and its second harmonic frequency are the dominant noise of the turbine stage. An aerodynamic-acoustic feedback loop is observed in the stator passage and it is mainly due to the emission, reflection and interference of the pressure waves generated by the trailing edge vortex shedding. The surface pressure levels of the rotor blade surface are lower than those of the stator vane surface, thus the rotor blades have a smaller contribution to the overall noise level of the turbine stage than the stator vanes, since there is no aerodynamic-acoustic feedback loop in the rotor passage.