The Effect of High Temperatures and Grazing Flow on the Acoustic Properties of Liners
Regular paper
KTH
Wednesday 3 june, 2015, 14:00 - 14:20
0.3 Copenhagen (49)
Abstract:
Acoustic liners have traditionally been used to reduce fan noise from the
aircraft engine intake. To increase noise reduction there are now plans to also
put liners in hot stream parts of the engine. In order to test liners under as
realistic conditions as possible there has been a large development in inverse
techniques for determination of liner impedance under grazing flow conditions,
so called impedance eduction techniques. Testing under hot stream conditions has
received smaller attention. This paper discusses techniques for measuring liner
impedance under hot stream conditions and present some results obtained for
single degree of freedom Helmholtz resonator liners with different
configurations. These types of liners consist of a perforate top sheet backed by
a honeycomb cavity to give a locally reacting wall treatment which can be
characterized by an acoustic impedance. In the present case a number of
different perforate sheet and cavity depth geometries were tested under varying
grazing flow and temperature conditions. In some cases the liner test samples
also included a thin layer of metallic foam. These types of liners are used for
aircraft engine applications but are also of interest for IC-engine
applications. It could be argued that the main effect of high temperatures is a
change of medium properties such as: density, viscosity and speed of sound. If
this is true the high temperature impedance could be predicted by scaling from
the result at cold conditions. This is investigated in the paper by comparing
measured results from liner impedance models available in the literature.