Design and Flight Test of a Kevlar Acoustic Liner

This paper summarizes the design, fabrication, and flight evaluation of a Kevlar acoustic liner for a JT15D turbofan engine The liner was designed to suppress by a measurable amount a dominant (13,0) BPF tone This tone or spinning mode was produced for research purposes by installing 41 circumferentially distributed small diameter rods upstream of the 28 fan blades Duct liner attenuations calculated by a finite element procedure are compared to far field power (insertion) losses deduced from flight data The finite element program modeled the variable geometry of the JT15D inlet and used a uniform flow with a boundary layer roll off to model the inlet flowfield Measured far field power losses were generally greater than attenuations calculated by the finite element computer program Nomenclature outer radius at fan face speed of sound liner depth frequency a c0 d / k = free space wave number, a>/c0 Mf = Mach number at fan face ( m n ) = (circumferential, radial) mode numbers where (0,0) denotes plane wave SPL = sound pressure level t = time, e~j^ time convention x = axial coordinate /3 = acoustic admittance ratio, $R +j@f = f ~~ 1 y = wave number f = acoustic impedance ratio (normalized by characteristic impedance of air) 6 +jx £c = characteristic acoustic impedance ratio of bulk material