Further Investigation of Impedance Eduction in a Grazing Flow Duct Facility

Abstract

An established protocol for impedance eduction is based on a search procedure in which wall impedance in a duct propagation model is varied to obtain an acoustic field converging with one measured in a grazing flow duct. Here, an acoustic velocity potential formulation for the finite element model propagation model is extended to examine the relative effectiveness of driving the model with measured acoustic pressure at the source and termination and driving with acoustic pressure at the source and closing the computational domain with measured termination impedance. Also assessed is the appropriate location of measurement microphones to satisfy plane wave requirements for the computational model. Source acoustic pressure and termination impedance driving the propagation model produce superior eduction results with lower residual error, though the model driven with pressure at both the source and termination produces acceptable spectra. The microphone location in the gazing flow test facility is judged to be adequate for the range of grazing flow Mach number and frequency considered. Examination of impedance spectra for a range of lining samples has revealed a correlation of a decrease in spectrum quality, measured by observed data scatter, with a decreasing face sheet open area ratio and therefore with increasing lining nonlinearity.