Numerical simulations were carried out to investigate the effects of synthetic jet actuation frequency on the separated flow in a diffusing S-duct. The Reynolds number based on the entrance height was 9.78×105. At first

the numerical model was validated with experimental data

and then

the interaction between the separated flow and the synthetic jets at different frequencies was discussed. The results demonstrate that the control effect is significantly dependent on the momentum mixing enhancement between inside of the separated boundary layer and the outer flow. There exists a narrow range of actuation frequency

in which effective separation control can be achieved using synthetic jets. A dimensionless frequency F+=1.0 is identified as the optimal frequency

with a momentum coefficient of 1.62×10-3

the separation area is reduced about 46%

and the aerodynamic performance of the S-duct is also greatly improved compared to uncontrolled case. Further analysis reveals that the choice of actuation frequency is mainly determined by the momentum flux produced by a single ejection and the spacing between adjacent ejections

the optimal frequency case can be understood as a balance between the two factors. In addition

it is found that the synthetic jets can also suppress the secondary flows while decreasing the separation.