In order to mitigate or eliminate side force

the mechanism of symmetry breaking of flow field induced by flow separation was studied in detail. The finite volume second-order upwind interpolation scheme and k -ε turbulence model were used to simulate the working process of a supersonic nozzle on the ground. The flow structures inside the nozzle were studied

and the transition process of shock separation mode from freedom shock separation to restricted shock separation near the nozzle wall was analyzed in detail. In order to reduce the side force of over-expansion nozzle under the condition of low altitude and high back pressure

the flow field characteristics and flow separation mode under different length-diameter ratios and expansion ratios of nozzle were studied numerically. The results showed that a great side force can be induced in the process of shock mode transition

and the flow patterns or structures can be optimized by changing the nozzle configuration. And the side force can be effectively reduced by properly shortening the length-diameter ratio and expansion ratio. As the length-diameter ratio was 105

the side force was more than 4 000 N. But as the length-diameter ratio was 095 or 115

the side force didn’t exceed 20 N in the whole stage. And as the expansion ratio was 539

the peak value of side force reached more than 4 000 N

while as the expansion ratio was reduced to 45

the side force decreased obviously.