## Acoustic characterization of "locally" and "non-locally reacting" porous liners in flow with LDV measurements

F. Simon*, E. Piot**, F. Micheli***

ONERA Centre de TOULOUSE, DMAE

2, av. Édouard Belin, BP4025, 31055 TOULOUSE cedex 4, France

* Frank.Simon@onecert.fr – ** Estelle.Piot@onera.fr - *** Francis.Micheli@onera.fr

The traditional measurement methods to caracterize an acoustic resonator in the presence of grazing flow (i.e. used in aeronautic domain) require the use of microphones, mounted to the upper / rear faces of the liner (for impedance) or upstream / downstream from the liner with microphones flush with the wall (Transmission loss). So, no information is known inside the flow. That is the reason why a new testing method using Laser Doppler Velocimetry technique (2D LDV) has been developed at ONERA to obtain acoustic quantities like pressure, impedance or intensity fields near the liner under flow condition and incident acoustic waves. These complementary quantities are very useful to understand how the liner absorbs acoustic waves. That method is based on measurement of in flow acoustic velocity perturbation, extracted by signal processing. Then Galbrun's theory (Eulerian-Lagrangian description of the perturbations) permits to accede to the pressure and intensity fields. In-duct flow LDV measurements are realised on a test-bench, called B2A: It is a 4 m long wind tunnel regulated in turbulent flow rate, with a maximum Mach number of 0.5. Two loudspeakers can generate plane waves on range 300-3000Hz, upstream from a square test cell where a small liner is mounted (30 x 150 mm² with a variable thickness). Acoustic velocities components are extracted from total velocity by cross-spectra with a reference signal (the loudspeakers’one), in order to reject turbulence.

Measurements of (micro-)LDV fields have been performed, with or without flow (up to Mach 0.2), above different types of liner (perforated or micro-perforated facing sheet, upper honeycomb, fibres or hollow spheres) in order to bring to the fore "linearity" or "non-linearity" effect and "locally" or "non-locally" reaction. The absorbed acoustic power, extracted from intensity fields, allows comparing the behaviour of materials.

This activity takes place as part of national project COMATEC.