## 1 Introduction  
The three aerodynamic forces acting on a high-speed driving vehicle, namely, the drag, lift (or downforce), and side force, have a significant impact on the driving stability, power performance and energy efficiency of the vehicle. The aerodynamic drag acts in the opposite direction of the driving direction of the vehicle and can considerably influence the maximum speed, fuel economy, and energy efficiency of the vehicle. Therefore, various devices for aerodynamic drag reduction, such as front grill shutters [1; 2], wheel deflectors [3], and underbody fairings [4; 5; 6] have been widely applied. The downforce acts in the direction opposite to that of the lift and tends to push the vehicle toward the ground. Therefore, the downforce increases the road grip of the tire, thereby increasing the vehicle stability during turning and braking. As a result, most high-performance vehicles are equipped with rear spoilers to increase the downforce [7; 8]. In general, with any increase in the downforce, the aerodynamic drag tends to increase as well; the increased downforce and aerodynamic drag lead to an increase in the driving resistance, which degrades the power performance and reduces the fuel economy [9]. Therefore, most automakers develop vehicles by finding a tradeoff between the aerodynamic drag and downforce through passive aerodynamic optimization [10]. Recently, various researchers have attempted to improve the driving stability by employing moving aerodynamic surfaces [11]. One of the typical research topics is the use of an active rear spoiler, which can control the downforce by changing the deployment height or tilting angle of the rear spoiler. Such variable spoilers can adjust the downforce in accordance with the driving velocity; however, they have a large weight, undesirable package layout, and complicated operating mechanism. In addition, owing to the inertia of the spoiler itself, it takes several seconds to change the spoiler position; thus, it is difficult to achieve a real-time response to rapid braking and turning. Some researchers have also focused on employing aerodynamic control via air blowing or suction. [PERSON] [12] performed numerical studies pertaining to the aerodynamic drag reduction by using a realistic vehicle model [13]. The authors conducted numerical experiments involving a change in the blowing position and strength, and a drag reduction of 7.5% could be realized. [PERSON] [14] investigated the technique of active flow control by using harmonic vortex generator jets for laminar boundary layer separation. [PERSON] et al. [15] carried out experimental studies on active flow control via high-speed blowing, using a 1:4 scaled model vehicle. These authors demonstrated a reduction of 5% in the rear axle lift with a negligible increase of 1% in the drag. The improvement of vehicle performance by controlling the downforce and aerodynamic drag is of particular interest in the domain of motor sports aerodynamics, such as for Formula-1 (F-1). In 2010, McLaren applied the RW80 duct, known as the F-duct, to an F-1 vehicle, and it was reported that the F-duct could increase the top speed to approximately 3-7 km/h by controlling the downforce and aerodynamic drag [16]. The F-duct could control the vehicle aerodynamic performance without excessive energy consumption because it employed high-speed ram air; however, its use was prohibited owing to FIA (Federation Internationale de L'Automobile) regulations. Recently, Lamborghini introduced the active aerodynamic control technology, termed active Lamborghini aerodynamics (ALA), in which the F-duct concept is adopted to one of their premium vehicles. In the ALA, ram air with a high total pressure is guided to the slit at the bottom surface of the rear spoiler and discharged to realize flow separation. Without flow discharge, no flow separation occurs at the rear spoiler, and the downforce is maximized. Thus, the grip force of the vehicle is increased, and superior dynamic stability can be obtained.