The unsteady characteristics of a hypersonic mixed-compression inlet below the self-starting Mach number were numerically studied.The results show that the inlet works at un-starting mode

and oscillation phenomenon of the flow in the inlet appears with a frequency of about 250Hz.The flow oscillation occurs primarily in front of the throat and attenuates downstream the throat.The disturbance propagates downstream from the throat with the speed of the flow.The ratio of the length to height of the isolator has no effect on the flow oscillation

but the Mach number of the freestream offects the flow oscillation greatly.With the freestream Mach number approaching to the self-starting Mach number

the flow oscillation becomes more and more intense

but the flowfield retains steady when the freestream Mach number is too low.The pressure drag of the inlet also fluctuates periodically with the frequency of about 250Hz.