Numerical aeroacoustics investigation of the effect of axial gap length between the rotor and stator of a transonic compressor stage

Modern civil aviation regulations stipulates the commercial airline companies not to exceed certain noise levels in the neighbourhood of airports which are generally located in the city. Strict noise regulations requires detecting the noise generations of the aircraft engine and applying an appropriate design or treatment of the engine component accordingly. The main noise generation mechanisms of the aircraft engines are inlet turbofans, compressor stages, turbine stages, combustion and jet flow. Each component contributes their own effects on the noise spectrum of an aircraft; however, the primary noise sources in modern high by-pass pressure ratio aircraft engine is the turbofan among them. Turbofan noise, which constitutes the largest part in the engine noise spectrum, radiated in both upstream and downstream on the engine. However, the other sources, which are compressor stages, turbine stages, combustion and jet noise, radiate only in one direction. The turbofan, compressor stages and turbine stages are axial turbomachinery applications. That is why, the largest component of the aircraft engine noise sources is turbomachinery originated. In order to keep the engine noise emission under the required level the flow physics of the turbomachinery should be profoundly understanded and new designs should be formed accordingly. Two main components of the turbomachinery noise are discrete (tonal) and broadband noise. The discrete noise appears at certain frequencies (e.g. balde passing frequency), while broadband noise covers the whole noise spectrum. The discrete noise is generally originated from the interactions of the turbomachinery components like rotor and stator. It stems from the stator potential field on the rotor blades and rotor wake interaction with the stator vanes. However, the broadband noise arises from the turbulent content in the flow domain. In this study, discrete (tonal) noise generation of the transonic axial compressor turbomachiney application was studied. NASA Stage 37 test case was selected as the transonic axial compressor stage. The most significant tonal noise generation mechanism of the axial turbomachinery which is the rotor-stator interaction was investigated. The magnitude of the axial gap between the compressor rotors blades and stator vanes are the primary factor of the rotor-stator interaction. The smaller the gap is, the intenser the rotor-stator interaction becomes. That is why, in order to observe the effect of the axial gap length on noise generation, the flow field of the compressor with three different axial gap length was investigated. Computational fluid domain model of the transonic compressor stage with three different axial gap between the rotor and stator was formed. Unsteady Reynolds Averaged Navier Stokes Equations (URANS) analysis of three models were carried out. For the nearfield acoustic propagation unsteady pressure profiles were directly extracted from the URANS simulations. For the farfield acoustic propagation the usteady rotor and stator blade pressure values were imported an acoustic solver. The computational fluid dynamics results shows that the potential field between the rotor blades and stator vanes and interaction of the rotor wake and stator vanes strongly decayed as the axial gap between the rotor and stator blade incerases. This resulted in an observable reduction in the tonal noise generation at blade passing frequencies (BPF) and its higher harmonics.