Thermo-acoustic instabilities in lean gas turbine combustors have been well reported over the past decade. One option by which the generation of potentially damaging, large scale, pressure amplitudes can be avoided is to increase the amount of damping within the combustion system using passive damping devices. Common to these devices is the absorption mechanism by which acoustic energy, associated with incident pressure fluctuations onto an orifice, generates an unsteady flow that cannot be converted back into acoustic energy. This paper is concerned with providing a greater understanding of this fundamental process. Experimental results are presented for a single orifice that is exposed to plane acoustic waves within a rectangular duct. Measurements of unsteady pressure enable the acoustic power absorbed by the orifice to be determined, whilst Particle Image Velocimetry (PIV) is used to measure the unsteady flow field. A method is outlined for identifying those features within the measured unsteady flow field that are responsible for absorption of the acoustic energy. This is based on a Proper Orthogonal Decomposition (POD) analysis of the velocity field and identification of the relevant modes. The method is validated for the non-linear and linear absorption regimes by comparing the energy of the relevant velocity field features with the energy absorbed from the acoustic field. The good agreement obtained indicates the success of the technique presented. The improved understanding of the mechanisms by which energy is transferred out of the acoustic field, and into the unsteady velocity field, explains many of the observed absorption characteristics. This improved understanding should lead to the design of optimized damping systems. The presented methodology is also thought to be the basis by which numerical, CFD based, predictions relating to the absorption of acoustic waves should be analyzed and validated.

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