Liner Impedance Determination from PIV Acoustic Measurements

Regular paper

LAUM

Wednesday 3 june, 2015, 10:00 - 10:20

0.3 Copenhagen (49)

Abstract:

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.