<p>An experimental study was conducted to examine the performance of the isosceles triangular hole vortex generator jets (VGJs) on the active flow control of boundary layer separation over the NACA4421 airfoil under typical angle of attack around the stall angle. During the experiments, two 3D printed NACA 4421 airfoils, embedded with different vortex generator jets and same air cycling configuration, were used to examine the performance of each system on the flow control, especially near the stall angle. The flow field information was visualized and quantified by the particle image velocimetry (PIV) system during the experiments, which clearly illustrate the boundary layer flow development with the circular hole and triangular hole jets, compared with the baseline case without flow control. The results show that the operation of the VGJs, both circular and triangle hole jets, would contribute to the flow control, presenting a less separation region with moving back of the separation point. Furthermore, the PIV results indicate that, compared to the circular jet, the triangular jet leads to a noticeable increase in the peak TKE near the airfoil at 20° AOA. Under this condition, the separation point is displaced downstream by approximately 5.8% chord-length, and the separation region is reduced by about 30%. Similarly, at 22° AOA, the peak TKE near the airfoil increases significantly, with the separation point further delayed downstream by an additional 3.1% chord-length, and the separation region is further reduced by 8.9%. These results demonstrate that the triangular jet provides superior suppression of flow separation under both moderate and high AOA.</p>