Liner Impedance Determination from PIV Acoustic Measurements
Wednesday 3 june, 2015, 10:00 - 10:20
0.3 Copenhagen (49)
An experimental investigation of the acoustic field in a lined channel is performed, using Particle Image Velocimetry (PIV). The liner is composed of parallel, hexagonal channels rigidly terminated and with constant depth, and is treated as locally reacting. It is therefore characterised by a single complex impedance. A single-frequeny acoustic field is imposed in the channel using a loudspeaker. Snapshots of the acoustic velocity field are taken at desired time instants, which are set to coincide with certain phases within a time period. The acoustic field is therefore obtained at a number of uniformly spaced phases, which span a full period. The Fourier components are then calculated on the measuring plane. In order to obtain a homogeneous distribution of light-reflecting particles, a slow flow is imposed at the channel inlet. This background convective flow shows an approximately parabolic profile, in accordance with Poiseuille viscous regime. In the present case of an approximately steady background flow, the entire unsteady component of the flow can be identified with the acoustic field. The main goal of this study is to determine the value of the liner impedance using information of the entire acoustic field. Minimizing an error function (that quantifies the global difference between the measured and the computed acoustic fields) does this. The numerical solutions are obtained using a wave equation solver, based on a mixed multimodal-finite difference approach. Secondly, we focus in the field at the vicinity of the upstream liner edge. According to the numerical solutions, in that region exist discontinuities in the field and or its gradient. The validity of the numerical solution will be addressed.
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