Laser optical measurements of the acoustic particle velocity at bias flow liners with grazing flow

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For a stable and low-noise operation of a modern jet engine, especially in case of lean combustion, highly efficient sound attenuators with bias flow (bias flow liners) are needed, since on the one hand, the combustion chamber of such an engine tends to suffer from combustion oscillations, on the other hand less air mass flow is available for the damping. However, the optimization of the damping efficiency of these bias flow liners requires a deeper understanding of the physical damping mechanisms. The damping performance of such a liner is determined by complex phenomena of interaction between the incident sound field and the flow field. Therefore, the goal of the research project is to reach a better understanding of these phenomena. For this end, an energy balance of the transfer from the sound field to the flow field is aspired. On the one hand, this necessitates three-dimensional measurements of the acoustic particle velocity and the flow velocity field. To achieve this, the Doppler global velocimetry with sinusoidal laser frequency modulation (FM-DGV) will be applied, which has been proven suitable already during the first funding period. However, the current FM-DGV system has to be extended for volumetric measurements at first. In doing so, it is planned to integrate a high speed camera to increase the efficiency, which represents an alternative to the detector array used so far. Moreover, it is intended to accelerate the signal processing by parallelization. On the other hand, the challenge concerning the evaluation of the velocity fields measured above the bias flow liner is to separate the acoustically induced hydrodynamic oscillations from the primary sound and flow field. For this purpose, different analysis and separation methods (e.g. Helmholtz Hodge decomposition, spectral analysis and wavenumber filtering) have to be tested and to be applied. Here, the aim is to develop the energy transfer from the acoustic to the flow field based on the amount of energy of the separated fields. This transfer will be proven by a comparison with the sound energy dissipation of the liner obtained by microphone measurements. As a last step of this second funding period, the qualified measurement techniques and analysis methodology are applied at a simplified bias flow liner at the specifically built DUCT-R (duct acoustic test rig - rectangular cross section). Using the measured velocity field data, the energy transfer from the sound field to the flow field can be balanced which enables gaining new insights concerning the physical damping mechanisms at bias flow liners.