To study the phenomenon of unsteady flow in unstart variable geometry inlets,a two-dimension-al unsteady numerical study is performed to simulate the internal flow of a hypersonic inlet with a mobile cowl.The effect of this variable geometry on the unsteady flow is investigated.The inlet flow oscillation scenarios with high and low Mach number positions of the cowl are compared and the flow oscillation characteristics are demon-strated.The results show that the amplitude of the flow oscillations increases significantly with the increase of Mach number when the cowl is located at the high Mach number position,and the amplitude of flow oscillations increases from 0.8 kg/s at Ma=4.0 to 2.7 kg/s at Ma=4.6,while the amplitude of flow oscillations decreases with the increase of Mach number when the cowl is located at the low Mach number position.The small separation bub-bles and the shock train in the inlet lead to the pressure surge,which triggers a fractional linear change in the pressure.At Ma=4.6,the peak pressure at the wall rises sharply from 20 to more than 50 in a short time,while the peak pressure only fluctuates in a small range between 12.4 and 15.74 at Ma=3.3,where the change of the flow field is more moderate.At Ma=4.6,the flow oscillation pattern of the inlet is basically consistent with that of the monitoring points on walls,whereas the flow oscillation pattern at Ma=3.3 shows a large discrepancy between that of the monitoring points at the cowl and throat.The flow velocity in the inlet increases above the speed of sound after the cowl is moved forward,and the propagation of disturbance and oscillatory coupling between the upper and lower walls is blocked,which weakens the flow oscillation phenomenon in the inlet.