Assessment of Total Pressure and Swirl Distortions in a Busemann Inlet at Mach 6

Flow distortions in high-speed inlet systems are complex, and high-performance air-breathing propulsion systems. In this paper, large eddy simulations are performed to study the total pressure and swirl distortions in a Busemann inlet at freestream Mach number 6. The on-design flow condition with both the Attack Angle and Sideslip Angle equal to zero and two off-design conditions (Attack Angle = 6 deg, Sideslip Angle = 0 deg and Attack Angle = 6 deg, Sideslip Angle = 6 deg) are considered to explore the flow characteristics inside the inlet duct as well as the distortions at the inlet exit plane. It is found that under the on-design flow condition, the shock structures and boundary layer development are nearly axisymmetric about the inlet axis. The captured freestream is compressed smoothly through inlet duct. The total pressure loss is limited primarily to within the boundary layer region, and nearly no swirling flow is introduced during the flow compression process. Under the off-design flow conditions, the shock structures inside the inlet duct become non-axisymmetric, and localized strong shock–boundary layer interactions occur. In the case of the off-design flow condition with Attack Angle = 6 deg, Sideslip Angle = 0 deg, a large flow separation zone appears owing to the incidence of a strong curved shock on the wall surface at the leeward side in the inlet duct, and the low-kinetic-energy flow contained in this flow separation zone leads to an obvious total-pressure reduction at the exit plane of inlet. Meanwhile, a large-scale swirling flow is formed at the exit plane of inlet owing to the appearance of a nonuniform transverse pressure gradient. Under the off-design conditions, a pair of vortex is observed at the exit plane of inlet. The shock wave–boundary layer interactions under the off-design conditions are stronger than those under the on-design condition, which results in more intense total pressure and swirl distortions. The averages of the fluctuating distortions are more evident than the temporal-averaged total-pressure and swirl distortions. These results show that turbulent flow fluctuations are important in determining the overall distortion level in a Busemann inlet.