Boundary Layer Considerations for Optimization of Acoustic Liners for Aircraft Engine Ducts

This paper presents a study of the effects of shear flow on the sound attenuation in lined ducts. The main emphasis is on proper selection of liner impedance for optimum attenuation performance of the acoustic liner in the presence of realistic duct boundary layer flows. Results are given for arbitrary spinning acoustic propagation modes, and combinations of modes in treated inlet and exhaust ducts for a variety of simple boundary layer velocity profiles and boundary layer thicknesses. It is shown that boundary layer effects are significantly reduced when all the propagating modes are combined to give total attenuation.