Impact of swirl-coupled BLI distortion on a transonic fan across varying rotational speeds

Boundary layer ingestion (BLI) can raise propulsive efficiency in civil aviation. Rear-mounted engines, however, introduce complex inlet distortions that include both total-pressure and swirl components, which together challenge transonic ducted fan performance. While recent studies have increasingly recognized the presence of swirl in BLI distortion, its specific coupling mechanisms with compressible flow features—particularly shock wave topology—and its thermodynamic dissipation breakdown under varying rotational speeds remain underexplored. This work aims to isolate and quantify these swirl-driven compressible penalties. Entropy peaks occur at the blade tip and hub. In these regions, local Mach number amplification reshapes the shock topology, which consequently intensifies both shock-boundary-layer interactions and tip leakage. Furthermore, analysis using the Boussinesq number reveals that viscous dissipation dominates the core flow, while thermal dissipation becomes appreciable only in the tip region when swirl is minimal. Reducing the rotational speed to 80% broadens the distortion-affected zones but weakens their local intensity. Incidence-angle analysis further shows that the upstream inlet distortion is not transmitted unchanged to the blade passage: away from the leading-edge influence region it produces strong low-order circumferential incidence peaks, near the leading edge its shape is reorganized, and immediately upstream of the leading edge it is largely smoothed into blade-passing-scale oscillations. Even after this pre-passage attenuation, residual high-incidence sectors can overlap with shock-sensitive spans, destabilizing the tip-region flow and causing substantial additional loss. These findings show that BLI fan penalties are governed not only by the magnitude of inlet swirl distortion, but also by its axial reshaping, spanwise phase shift, and residual alignment with passage shock structures.