Aircraft engine fan tone noise due to back-pressure distortion caused by a downstream pylon under high-speed conditions.

• Pylon–rotor interaction noise in low transonic speed of fan is elucidated. • CFD and duct acoustics are used to investigate the generation and propagation. • Periodic shock motion gives a prominent concentrated dipole source on a blade surface. • Advent of extra cut-on modes with low circumferential wave numbers contributes to the noise increase. Experimental, computational fluid dynamics (CFD), and duct-acoustics theoretical studies were conducted to investigate the mechanism of upstream noise propagation from fan stages with and without a downstream cylinder which simulates the presence of an engine pylon or bifurcation. Model fan experimental results obtained using an anechoic test facility show that the fan tone noise level increases sharply when the rotational speed exceeds a critical speed point, which is much lower when a downstream cylinder is present than when there is no cylinder. Distinctive features of the sound source due to pylon–rotor interaction (PRI), i.e. the unsteady aerodynamic blade loading resulting from interaction with the back-pressure distortion, are found by CFD. Characteristic modal structures of PRI noise were made clear by modal analysis of CFD results. The pressure fields upstream of the fan rotor blades and the rotor blade surface pressures obtained from three-dimensional unsteady CFD indicate that shock wave position and strength fluctuations caused by interaction with back-pressure distortion bring about highly peaked unsteady pressure amplitudes at the shock foot on the blade suction surface. The sharp increase in the PRI tone noise level in the low transonic speed range is ascribable to the advent of a large number of cut-on duct acoustic modes with high modal pressure amplitudes, which are absent when no cylinder is present. Comprehensive experimental, CFD, and duct-acoustics theoretical studies indicate that CFD reasonably simulates the experimental sound field. [ABSTRACT FROM AUTHOR]