On a Mode-Matching Technique for Acoustic Scattering by an Annular Row of Airfoil-Shaped Pylons

Kurzfassung

This study concerns the propagation of sound waves in the bypass duct of turbofan engines in presence of bifurcations. A test bench was built to study the sound propagation in a simplified bypass duct, where the duct radius remains constant and structural bifurcations are made identical and periodic. An analytical model based on the mode-matching technique and slowly-varying approach is proposed to predict the scattering of incident annular modes by the periodic arrangement of large 'pylons' present in the experiment. To cope with the assumption of slow geometrical variations, the pylon rounded leading edge is simplified into a flat wall. With this geometry, the predicted reflected sound power is significantly improved compared to models relying on equivalent flat plates; a variance of around 3 dB remains for incident modes of radial order n=0, and a consistent underestimation of up to 5 dB is observed when n=1. The frequency peaks in the reflected power spectrum, due to resonances, are correctly estimated, which is important for use in the preliminary stage of a design. An interesting physical insight also shows how resonances take place in these rows of airfoil-shaped pylons, as they seem more likely to occur over a limited portion of the pylon, between a turning point and the trailing edge.