## 1 Introduction
[18] applied this device at the top edge of the slant surface of the 25\({}^{\circ}\) [PERSON] model, and experimental results pointed out that a drag reduction of 10% can be achieved at optimal control condition. A LES model was performed to evaluate the effect of a synthetic jet on near-wake region and aerodynamic drag obtained on a bluff-body model [19]. In particular, [PERSON] et al. [20; 21; 22] performed experimental and numerical studies, using the loudspeakers as synthetic jet actuators located at the rear surface of the vehicle, to investigate its ability of aerodynamic drag reduction on a simple truck. The results showed that the aerodynamic drag coefficient is reduced by 3.1% when a synthetic jet is activated compared with the reference case. Although there is no requirement for large input energy as the steady blowing jet method, the above studies pointed out that the capability of the synthetic jet to reduce the aerodynamic drag is insufficient. This can be explained by the deficiency of the output jet velocity and hence the interaction with flow field around the vehicle is limited. In addition, a study conducted by [PERSON] et al. [23] also showed that the solely blowing or suction phase of the synthetic jet contributes to the aerodynamic drag reduction. It means that only half-cycle of the synthetic jet can be used for the purpose of reducing aerodynamic drag. To overcome the limitations of the steady blowing jet and synthetic jet methods mentioned above, [PERSON] et al. [24] proposed a new control device, which requires a smaller amount of power input while maintaining an acceptable ability of the aerodynamic drag reduction. This device generates an unsteady jet, derived from a synthetic jet, in which only a blowing or suction jet is produced in a control cycle. The numerical results performed on the 25\({}^{\circ}\) Ahmed model have demonstrated the effectiveness of this new control method. An unsteady jet with different jet angles and control frequencies, applied sequentially at the edges of the back surface of the model has displayed a maximum aerodynamic drag reduction of 13.6%. The above literature study has pointed out the advantages and weaknesses of different active flow control techniques, and their potential in the aerodynamic drag reduction in vehicles. However, these flow control methods were applied to different vehicle models with various operating conditions. In particular, studies on the application of synthetic and unsteady jets to the square-back Ahmed model, both numerically and experimentally, have not been carried out yet. Hence, a systematic evaluation of the effectiveness of different flow control methods in reducing aerodynamic drag on this model has not yet been performed. Therefore, the main purpose of this article is to provide a comparative study on a range of flow control strategies for a square-back [PERSON] model to choose the best techniques for aerodynamic drag reduction. The control methods applied in this study are steady blowing, synthetic and unsteady jets placed on the four edges of the rear surface of the model. The effect of the control jet at various jet angles and control frequencies on the near-wake structure behind the model was analyzed, and the ability of drag reduction with different flow control strategies was then clearly compared.