Characterization of a hybrid active-passive absorber by means of Laser Doppler Velocimetry
Résumé
The LMFA has been developing hybrid absorbers for about 10 years now. Combining passive absorption and active control, these absorbers are well suited to applications that feature a grazing flow. The absorber consists of distinct cells covered by a thin porous layer, each cell containing a control microphone and a secondary source. Active control is used to reduce the acoustic pressure at the rear side of the porous layer at low frequencies. This results in a cancellation of the imaginary part of the surface impedance and permits to approach optimal impedance, i.e. the one that results in maximum noise reduction. The presence of a uniform grazing flow has little influence on optimal impedance. The control system being well protected against the flow, there are by the way no convergence or stability problems. However, the performance of the absorber decreases significantly in presence of grazing flow. In order to explore the origin of this behavior, an absorber composed of three hybrid cells has been tested in the “B2A” test bench at ONERA Toulouse. Laser Doppler Velocimetry (LDV) measurements are performed that permit to assess acoustic velocity in a plane above the hybrid liner in a non-intrusive way. These measurements confirm the good performance without flow. In particular, one observes that the three cells cannot be distinguished anymore but appear as a homogeneous impedance surface. The absorber has a rather global influence on the duct. In presence of a grazing flow of bulk Mach number 0.1, however, the influence of the absorber on the duct is reduced on the immediate vicinity of each hybrid cell.
Origine : Accord explicite pour ce dépôt
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