Acoustic source two-dimensional localization was performed during flyover tests with an Airbus A340 in Tarbes, France, in 1997. The method combined a cross-shaped array of 29 microphones developed by ONERA to a D-GPS tracking system operated by Aerospatiale. These tests aimed to validate the method and evaluate its precision and performance. The signal processing used a 32-channel digital tape recorder, where both D-GPS synchronization tops and the acoustic signals were recorded on the ground while D-GPS information was recorded on board. The aircraft performed flyovers above the array at altitudes between 40 and 180 meters and for different flight configurations. An acoustic emitter producing tone noise placed on the aircraft nose has helped for the method calibration. The source is clearly localized with a bias on position that can be lower than 50 cm. The method interest for extended and multiple sources is also confirmed : a few examples of airframe noise source maps are presented for configurations with landing gears and high-lift devices extended.
The analysis of aeroacoustics propagation is required to solve many practical problems. As an alternative to Euler’s linearized equations, an equation was established by Galbrun in 1931. It assumes the flow verifies Euler’s equations and the perturbation is small and adiabatic. It is a linear second-order vectorial equation based on the displacement. Galbrun’s equation derives from a Lagrangian density and provides conservative expressions of the aeroacoustics intensity and energy density. A (CAA) method dealing with the numerical resolution of Galbrun’s equation using the finite-element method (FEM) is presented. The exact solution for the propagation of acoustic modes inside an axisymmetric straight-lined duct in the presence of a shear flow is known and compared with the FEM solution. Comparisons are found to be in good agreement and validate a first step in the development of a CAA method based on the FEM and Galbrun’s equation. The FEM is then applied to an axisymmetric duct including a varying cross section and a nonuniform flow with respect to both the axial and the radial coordinates. The expression of the aeroacoustics intensity implemented in the FEM provides an accurate in-duct power balance.
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