## 1 Introduction  
Flow separation at the rear end of a square-back trailer contributes to approximately 25% of the overall drag that is encountered by an articulated Lorry [1]. A project called \(<\)Aerodynamics and Flexible Truck\(>\) under the Horizon 2020 framework was recently established by the European Commission attempting to reduce 5 to 10% of fuel consumption in articulated Lorries through aerodynamic efficiency improvement [2]. It is anticipated that significant drag reduction and fuel saving could be achieved if flow separation control could be effectively implemented to reduce the size and strength of the wake region downstream of the trailer rear end. Effective flow separation control over the rear end of square-back articulated Lorries could be achieved by installing boat tails [3; 4; 5; 6; 7; 8] or flaps [9] at the trailer rear end. It was shown that these passive flow control devices could reduce the base drag by approximately 6-11%. However, as the vehicles need to be lengthened in order to accommodate these flow control devices, daily operation of the vehicle becomes illegal if the current legislation regarding the maximum length and width of articulated Lorries remain unchanged [10]. Other than boat tails and flaps, flow separation control in articulated Lorries using vortex generators have been investigated in several studies [11; 12; 13; 14]. In general, no obvious effects in altering the wake size and drag reduction could be achieved using this passive flow control device on the trailer of an articulated Lorry. It is interesting to note that a recent study conducted by [PERSON] and [PERSON] [11] suggested that the size of the recirculating bubble might be reduced by installing a series of counter-rotating vane-type vortex generators on the roof near the front end of a square-back trailer. However, smoke visualisation images were used to measure the size of the recirculating bubble; therefore, the data shown in [11] should be treated qualitatively. Flow separation control over square-back articulated lorries using base bleeding has been proven to be effective in several studies [15; 16; 17; 18; 19]. Similar to the effects provided by base cavities and flaps, approximately 10% of base drag reduction could be achieved using this active flow control technique. However, implementing this flow control strategy to achieve flow control in square-back articulated lorries is also uncommon. This is because the lorry itself needs to be significantly modified in order to accommodate the bulky and heavy air pump to generate the air jets. In the last decade, a novel active flow control technique known as the Alternate Current Dielectric Barrier Discharge (AC-DBD) plasma actuation was proposed and developed [20; 21; 22; 23; 24; 25; 26; 27; 28; 29]. A linear single dielectric barrier discharge (SDBD) plasma actuator is composed of an embedded ground electrode and an exposed electrode separated by a dielectric material. A potential difference in the range between 10 and 40 kV AC (peak-to-peak) is maintained between the two electrodes which ionised the air adjacent to the exposed electrode. This results in plasma generation, and the flow control effect could be provided by the electromotive wind that is generated by the movement of the ionised air molecules around the exposed electrode. A promising flow control effect using the plasma induced jet has been achieved on inclined flat plates [20], aerofoils [21; 22; 23; 24; 25], circular cylinders [26; 27], and bluff bodies [28]. In contrast, the effect of flow separation control and drag reduction on the trailer of articulated lorries using the AC-DBD plasma actuation has not yet been well-studied. In fact, to date, only one study on this topic could be found in the literature [29]. The experimental study conducted by [PERSON] et al. [29] using a 1:60 scale articulated lorry model showed some interesting but contradictory results when applying the AC-DBD plasma actuation at the rear end of a square-back trailer model.