Experimental investigations in turbomachinery noise-source identification

Abstract

The exact contribution of each of the various core-noise sources within modern turboshaft engines to the noise radiated from the engine exhaust and into the surrounding environment is poorly understood. The objective of the E.U. FP-7 TEENI (Turboshaft Engine Exhaust Noise Identification) project was to better characterise the specific contributions of each core noise-source to this exhaust noise. Sources of core noise include direct combustor noise generated by the unsteady heat release at the combustor stage, broadband turbine noise, periodic turbine noise and flow-noise. Core noise also contributes significantly to the exhaust noise in modern turbofan engines during approach. One of the key avenues of investigation for the TEENI project was to assess the efficacy of various coherence-based noise-source identification techniques for application in a full-scale turboshaft engine. Such techniques require several acoustic measurements to be made at the noise-sources of interest. These technicpies were first tested using a small-scale experimental rig constructed at the Department of Engine Acoustics at the German Aerospace Centre (DLR) in Berlin, Germany. The objective of these tests was to examine the effectiveness of both pre-existing and novel methods of noise-source identification, using the small-scale experimental rig as a simplified representation of a real turboshaft engine. The relative strengths and weaknesses of several pre-existing techniques are discussed, before a novel method of noise-source identification is introduced which identifies the relative contributions of two noise-sources to the specific acoustic modal content measured at a given axial location. This novel method combines both the advanced techniques of modal decomposition and coherence analysis, and represents a significant advancement on pre-existing methods.