Numerical investigation of the flow characteristics in a semicircular S-shape inlet duct with large offset for boundary layer ingestion propulsion system

The boundary layer ingestion (BLI) propulsion system is mostly partially or fully embedded into the aircraft fuselage. To adapt to the irregular geometrical shape of the blended wing body aircraft surface, a semicircular-to-circular S-shape inlet duct is preferably utilized as an intermediate to connect the fuselage and the BLI fan. With the intensive requirement of a compact inlet duct and fan propulsor, the designed offset-to-length ratio of an S-duct is continuously increased, which poses a great challenge to internal flow inside the S-duct. Moreover, our previous explorations have revealed that when the offset-to-length ratio of an S-duct reaches a certain value, a significantly distinct flow feature is noticed in a large offset-to-length ratio semicircular-to-circular S-duct compared with that in a conventional circular-to-circular S-duct. In this study, numerical simulations are conducted to investigate the influence of offset-to-length ratio on the internal flow characteristics in a semicircular-to-circular S-shape inlet duct. Results have shown that compared with the conventional circular-to-circular S-duct, another vortex pair occurs near the lower wall, except for the well-known vortex pair induced by the low-momentum wall-around flow. It is found that this new vortex pair originates mainly from the central pressure differential-driven flow and is located adjacent to the low-momentum wall-around flow-induced vortex pair. As the offset-to-length ratio is increased, the circumferential total pressure distortion and the swirl distortion are intensified at the aerodynamic interface plane, while the radial total pressure distortion intensity is decreased. The number of swirl pairs increases with the offset-to-length ratio. Moreover, the vortex formed by the central pressure differential-driven flow demonstrates greater stability than the vortex formed by the low-momentum wall-around flow, leading to a “siege” phenomenon. Both of the vortices generated by the low-momentum wall-around flow and the central pressure differential-driven flow are intensified as the offset-to-length ratio grows. Since the flow characteristics at the outlet of an S-duct could have great impacts on the aerodynamic performances of the downstream fan propulsion system, the results can provide hidden guidelines for high-performance design of a compact inlet duct and fan propulsor system.