On semi-empirical liner impedance modeling with grazing flow

Abstract

Noise is created at the front of turbofan engines where a large-diameter fan spins at high speed to continuously compress a large volume of air. The engine is wrapped in a nacelle that has acoustic treatment to damp and absorb noise generated by the fan. This acoustic treatment is commonly of the locally reacting type. The impedance of these liners is what determines their efficiency to absorb sound waves. A lot has been published on modeling the impedance of liners, but there are still some unanswered questions. There are different criteria for defining the end correction that is added due to the finite length of the orifices and the effect of the interaction between neighboring orifices. There are several models available for the effect of grazing flow. The aim of the current paper is to contribute to answering some questions through impedance measurements on a large number of liners with grazing flow. Empirical expressions for the liner impedance were deduced. The perforate model is also of interest to the car silencers applications. A new impedance measurement technique is also proposed in this paper, which is based on liner educing methods. © 2003 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved.