The Effect of High Temperatures and Grazing Flow on the Acoustic Properties of Liners
Wednesday 3 june, 2015, 14:00 - 14:20
0.3 Copenhagen (49)
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.
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