Analysis of aerodynamic noise characteristics of high-speed ducted fans under distorted inlet conditions

Purpose This study aims to address intake distortion in high-speed ducted fans during actual flight. Using the hybrid SST k – ω + LES numerical method and Ffowcs Williams–Hawkings (FW-H) acoustic theory, it investigates flow field and aerodynamic noise characteristics under normal, 20% and 40% occlusion conditions, reveals the distortion-induced noise mechanism and provides a theoretical basis for low-noise fan design and intake system optimization. Design/methodology/approach This study uses a hybrid numerical method combining the SST k − ω + LES model and LES, coupled with the FW-H acoustic analogy theory. It investigates three operating conditions (normal intake, 20% and 40% inlet occlusion) and verifies simulation reliability via semi-anechoic chamber experiments on a consistent prototype. Findings Inlet occlusion distortion breaks the axisymmetry of incoming flow, induces large-scale flow separation on the blocked side, intensifies tip vortex fragmentation and turbulence enhancement and significantly amplifies unsteady pressure pulsation on blade surfaces and duct inner wall. Compared with the clean inlet condition, the average overall sound pressure level (OASPL) increases by 12.4 dB under 20% occlusion and 14.8 dB under 40% occlusion. The fundamental blade passing frequency (BPF) tonal noise experiences a globally averaged enhancement of 10–15 dB, while in specific lateral focusing directions, the localized peak amplification reaches up to 20–25 dB, becoming the dominant component of radiated noise. The noise directivity shifts from a symmetrical butterfly-shaped distribution to a highly asymmetric pattern, with the strongest radiation concentrated in the 105°–135° and 210°–240° lateral rear directions. Meanwhile, distortion leads to significant redistribution of noise source contributions: the contribution of duct casing noise to total noise increases significantly, while the relative contribution of blade noise decreases. Originality/value The originality of this study lies in focusing on the high inlet occlusion ratio (up to 40%) distortion scenario rarely involved in existing literature. It reveals the intrinsic coupling mechanism between severe inflow distortion, flow unsteadiness evolution and aerodynamic noise mutation, clarifies the redistribution law of blade/casing noise source contributions under distorted conditions and provides experimental-validated quantitative data for the noise effect of high occlusion ratio distortion. This study fills the quantitative research gap on the aeroacoustic characteristics of ducted fans under severe inflow distortion and can provide direct technical guidance for acoustic optimization of ducted fan propulsion systems for VTOL aircraft and unmanned aerial vehicles in complex flight conditions.