In combination with the focus schlieren and dynamic pressure measurement technology

the flow field and nozzle wall pressure were experimentally measured

under the different nozzle pressure ratios (NPR) of the single expansion ramp nozzle (SERN). Time and frequency domain analysis exhibited unsteady flow characteristics within the nozzle. Results showed that the flow field structure inside SERN was obvious asymmetrical. The flow separation mode on the upper wall of the nozzle was at restricted shock separation (RSS)

and that on the lower wall at free shock separation (FSS); in RSS mode

the pressure on the wall near the outlet was sharply oscillated in relation to FSS mode. The pressure standard deviation of the upper and lower walls of the nozzle was peaked near the separation point

and the probability density function distribution was skewed to one side or showed double peaks. In RSS mode

the shock motion exhibited obvious low-requency characteristics. In FSS mode

the unsteady characteristics of the shock wave were not only affected by the pressure disturbance of the recirculation region

but also affected by the separated shear layer.