To investigate the physical mechanism of active control of flow separation by vortex generator jets(VGJs)

the flow field in a rectangle diffuser and the generation and development of vortical structures were analyzed on the basis of the large-eddy simulation(LES)method in this study. The results show that the streamwise vortices carry the high momentum into the separated region

leading to the increase in cross-stream mixing of streamwise momentum and then the suppression or delay of the separation in the diffuser. The vortical structure consists of shear-layer vortices

horseshoe vortices and wake vortices. The jet shear-layer vortices evolve from vortex loops to vortex rings with the time. For the pulsed VGJs

the streamwise vortices in the form of vortex loops control the flow separation effectively at low frequency. At high frequency

the shear-layer vortices develop into vortex rings intermittently and the flow control is suppressed. The influence of the pulsed frequency and the duty cycle on flow control was discussed. It seems that the pulsed VGJs at 20 Hz may control the flow separation better when the duty cycle is 0.5.