1.1 Research Significance

With the rapid development of China's automotive industry, car ownershipis increasing year by year, according to incomplete statistics, by 2020,China’s car ownership will exceed 270 million vehicles ^[1], and thenthe problems of energy shortage and environmental pollution are becoming moreand more serious. The state has continuously issued new mandatory fuelconsumption standards for automobiles to improve the fuel economy ^[2].Automotive aerodynamics is one of the most important characteristics of a car,which directly affects the car's power, fuel economy, handling stability,comfort and safety ^[3]. It is of great practical significance forenergy conservation by studying the aerodynamic of automobiles.

Aerodynamic drag is divided into internal resistance and externalresistance. The external resistance consists of differential pressureresistance and viscous resistance. The differential pressure resistance iscaused by the flow separation phenomenon on the surface of the vehicle body,resulting in a pressure difference between the front and rear surfaces of thevehicle body, and the rear separation area is the main separation area of theflow field around the vehicle body ^[4]. Therefore, the aerodynamicdrag of the vehicle can be reduced by using appropriate flow control measures.

Flow control has become increasingly prominent in the aerodynamic controlof aerospace vehicles, and it is also at the forefront of fluid mechanics research ^[5]. There are mainly two categories: active control and passive control.Passive control achieves the purpose of flow control by changing flow boundaryconditions, pressure gradients, etc. Its structure is simple and the controlmethod is determined in advance, such as shroud, spoiler and so on. It iscommon in practice, but its performance under variable working condition ispoor, and it cannot be adjusted according to the change of the mainstreamworking conditions. The active control introduces auxiliary energy into theflow. It has good performance for variable working conditions and can changeits own structure or flow parameters according to the change of working conditions.Then, it can achieve the optimal control ^[6].

Under the background of rapid development of computer technology,numerical simulation technology is more and more commonly used in the researchof automotive aerodynamics. For cost reducing and research convenience, proportionalmodel is usually used for research, and then the research results are appliedto real vehicles ^[7].

1.2Foreign research status

In the 1970s,under the influence of energy crisis, people realized the importance ofautomotive aerodynamics and actively developed automotive air drag reductiontechnology. The major automobile companies in the world have respectivelyestablished dedicated car wind tunnels. In 1984, Ahmed proposed a simplifiedmodel of the car in a SAE paper, the later Ahmed model, and obtained the basicflow pattern of the surrounding air through wind tunnel tests ^[8].Howell and his partners set the jet orifices in different regions of the 1/4SAE square-back model’s tail. Under different Reynolds numbers, they studiedthe influence of the jet-hole and distribution area ratio on the aerodynamicdrag of the model. They found that the tail jet can effectively reduce the model’saerodynamics drag coefficient up to 0.02^[9]. Aubrun and his partners set a jet orifice on the upper edge ofthe tail slope of the Ahmed model with a tail inclination of 25°. The jetmethod uses a constant jet. It is found that the aerodynamic characteristicscan be significantly improved, and the drag reduction rate can reach 9%~14% ^[10]. Aider et al. used pulsedmicro-injection to control the separation of the rear of the car and found thatthe resistance was reduced by less than 1%, but the lift was reduced by up to 9% ^[11].G. Minelli et al. studied the effects of synthetic jets on theaerodynamics of a general-purpose truck cab through wind tunnel tests and foundthat the flow separation was significantly improved when additional flowcontrol was added at the rear end of the model ^[12]. Eulalie et al.studied several active and passive flow control solutions through a series ofnumerical simulations, reducing drag by more than 10% by using a large eddysimulation of experimental data ^[13].

1.3 Domestic research status

Somedomestic scholars also focus on the application of jet drag reduction technologyin the automotive field. Professor Gu Zhengqi, from Hunan University, proposedto install jet device at the rear of the car to reduce drag by changing tailvortex. Through numerical simulation, he got the conclusion that when the jetvelocity is 0.56 times of the incoming flow, the aerodynamic drag coefficientcan be reduced by 4.09% ^[14]. Zhu Hui, from Tongji University, usedthe large eddy simulation method to study the effect of the tail jet andsuction on the aerodynamic drag of theAhmed model with a tilt angle of 25°. When the direction of the jet is at an angle of 45°to the incoming flow, the velocity of the jet is 0.6times of the incoming flow, the drag coefficient increased by 5.61% using jetcontrol, the drag coefficient reduced by 13.2% using suction control^[15].Professor Zhang Yingchao, from Jilin University, gradually optimized the dragreduction for different velocities and positions by simulating the 35°Ahmed model, and achieved more than 6% dragreduction effect^[16].