Experimental Assessment of the Effect of Temperature Gradient Across an Aeroacoustic Liner

Abstract

This study investigates the temperature effect on the impedance of conventional singledegree-of-freedom liners, both without and with grazing flow. Experiments are performed in a controlled environment, with a detailed monitoring of the temperature all along the liner sample. The liner impedance is either derived from the reflection coefficient measured in a normal impedance tube, or is educed with an inverse method from acoustic velocity or wall pressure fields measured in the ONERA grazing flow duct. The influence of the acoustic source level on the temperature of the sample is also addressed, which enlights strong multiphysics coupling between acoustics, flow and thermal phenomena.

LEONAR liner concept : multiphysics coupling in presence of grazing flow, thermal gradients and high sound levels

“…The main effect appears to be a slight shift of the maximum loss band towards higher frequencies, indicating a similar change in the resonance frequency. This result is coherent with Mery et al [17]: the shift is due to the variation of the temperature inside the cavity, which directly impacts the Helmholtz resonator characteristics.…”

“…Two samples are used in this study. One is a simple SDOF liner that has already been studied previously at ONERA [17] . The second one is a LEONAR, a type of liner where the perforations of the facesheet are connected to hollow tubes extending inside the cavities (see Fig.…”

“…5). It is partly based on the one developed for the study by Méry et al [17] and can withstand high temperatures while remaining sealed and without undergoing significant thermal expansion. The sample holder has been designed to include as much thermal insulation as possible, so that significant thermal exchanges can only take place between the upper and lower walls of the sample and not on the four sides.…”

“…The incident acoustic wave is a multi-sine set to 130 dB per tone. A two-microphone method [17,22] is used to measure the acoustic pressures and in the upstream and downstream section (see Fig. 9).…”

“…A numerical restitution of the previous experiment was proposed by Esnault [16]. Méry et al [17] conducted a study on liner samples heated from the back wall, both with and without the presence of a cooler grazing flow. The acoustic response of the samples was investigated both in the linear and nonlinear regimes with respect to the acoustic level.…”