Investigation of the influence of liner hard-splices on duct radiation/propagation and mode scattering

In a turbofan engine, usually, the acoustically lined region consists of several liner segments separated by longitudinal hard-splices due to manufacturing. The non-uniform impedance in the circumferential direction will excite other circumferential modes besides the incident modes. Consequently, the acoustic propagation in and radiation from a turbofan is considerably different from that of an uniform impedance. To investigate the effect of liner hard-splices on acoustic radiation and propagation, a boundary integral equation method (BIEM) in two-dimensional (2D) is expanded into a three-dimensional form. In this model, an axially uniform inflow passes a cylindrical finite duct and the liner inside the duct may be circumferentially or axially non-uniform. The verifications are presented and the influence of hard-splices is investigated. Then an infinite duct model is developed to investigate the mechanism of mode scattering excited by a circumferentially non-uniform boundary. BIEM is combined with the mode-matching method. The model is validated by comparing with the analytical result in an infinite circular duct with a hard wall. Then a variety of liner configurations containing periodic/non-periodic hard-splices are studied and the mode scattering mechanism is discussed.