Implementation and non-intrusive characterization of a hybrid active–passive liner with grazing flow

Different hybrid active–passive absorbers have been developed at the LMFA that seem well suited for the use as turboengine nacelle liners. The basic version is made of a resistive screen backed by cells that contain one microphone and one secondary source each. At low frequencies, acoustic pressure behind the screen is cancelled actively in order to obtain a purely real and constant surface impedance. At higher frequencies, active control is turned off and the liner acts as a classical SDOF resonator. An advanced version (the complex hybrid absorber) has been developed recently, featuring two microphones per cell. With the new system, the surface impedance of the cell can be measured and adjusted to a given (possibly complex and frequency dependent) target impedance. The present paper reports on two measurement campaigns that aimed at a characterization of these absorbers by the use of Laser Doppler Velocimetry (LDV). LDV allows the assessment of acoustic velocity in vicinity of the absorber in a non intrusive way. The measurements confirm the good performance of both absorbers without flow. In particular, one observes that in active mode the different hybrid cells appear as a homogeneous liner with a global influence on the duct. In presence of grazing flow, the influence of the absorber on the duct is limited to the immediate vicinity of each hybrid cell, which explains the reduced performance.