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.