To understand the phenomenon of flow oscillation in the dual-bell nozzle,a wind tunnel test was conducted to study the flow oscillation characteristics of dual-bell nozzles with different extension pressure distributions.Flow field oscillation character-istics under different flow states were obtained.The results show that,there are low-frequency fluctuation about 180 Hz caused by the acoustic resonance in the recirculation zone and the unsteady pulsation of shock wave in the flow field of dual-bell nozzles under both the separation state and the full-flow state.However,when the separation gradually increase,the shock system pulsation is sup-pressed and the oscillation phenomenon is significantly weakened.Specifically,when the separation shock suddenly changes from mo-tion to relatively static after the fast mode re-transition,the flow produces an impact effect,and the oscillation amplitude is amplified about 5 times,but then rapidly attenuated.In engineering practice,the structural design of dual-bell nozzles should ensure the char-acteristic frequency of the nozzle structure away from the characteristic frequency of flow oscillation to avoid resonance.Meanwhile,the impact load after fast mode transition should also be taken into consideration in the design of dual-bell nozzle structures.
dual-bell nozzleflow separationlow frequency oscillationshock wave pulsation
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