# Research on the Aerodynamic Characteristics of Tractor-Trailers with a Parametric Cab Design
## 1 Introduction  
In 2016, the total length of Chinese expressways exceeded 130,000 km, ranking first in the world, and another 5000 km of highways was added in 2017. Road transportation accounts for nearly 77% of the total volume of transported goods, and tractor-trailers played an important role in this. According to the auto industry production and marketing data released by CAMM (China association of automobile manufacturers), in 2016, the production and sales of commercial vehicles totaled 370 million and 365 million, respectively, and the numbers of tractor-trailers were 74.14 million and 73.29 million, which increased 38.29% and 33.08% year-by-year. The increase in vehicle ownership has resulted in greater energy consumption. For a tractor-trailer, the purchase price is fixed and disposable, while the operational cost is long term. In the course of its operation, the aerodynamic drag of the body directly affects performance and fuel economy, and the fuel consumption caused by the air resistance accounts for most of its transportation costs. Therefore, it is necessary to study methods of drag reduction for tractor-trailers to reduce fuel consumption. The aerodynamic characteristics of passenger vehicles have been extensively investigated in the past two decades [1; 2; 3]. In recent years, the aerodynamic performance and geometry optimization of tractor-trailers have received much more attention, and many studies have been conducted toreduce their aerodynamic drag [4; 5; 6; 7]. At present, by means of wind tunnel tests and numerical simulations, researchers have found that four critical parts of these vehicles make up the major parts of the aerodynamic drag of a tractor-trailer, as shown in Figure 1; these are the forebody of the tractor, the gap between the cab and the trailer, the underhood plus the wheels, and the rear of the trailer [8; 9]. For a tractor-trailer driving on a highway, the front part of the vehicle causes approximately 45% of the total drag force; the other contributing factors include the rear wake of the trailer (25%) and the underbody flow (30%) [10]. Corresponding research was carried out at different positions, and various kinds of drag-reducing devices have been introduced to effectively decrease the aerodynamic drag of tractor-trailers, including their cab roofs or gap fairings [11; 12; 13; 14], their side skirts [15], their base flaps [16], and their boat tails [17; 18]. Some studies focused on their aerodynamic characteristics, using a combination of various devices and interactions with the actual complex shape of tractor-trailers [19; 20]. Previous studies mostly focused on performance optimizations using aerodynamic drag reduction devices. Little analysis has been conducted on aerodynamic drag reduction effects based on the adjustment of the tractor-trailer shape itself, although this method will only involve a small additional cost, but could avoid the matching problem between the pneumatic accessories and the vehicle. [PERSON] (2013) designed a variable outer contour of the trailer to adjust the rear end; the curved top surface of the trailer was most obviously changed, and, compared to the initial vehicle the aerodynamic drag, was clearly reduced [21]. Furthermore, except for the existing drag reduction methods, commercial vehicle brands, such as SCANIA tractor-trailer (SCANIA, Malmo, Sweden), VOLVO tractor-trailer (Volvo Trucks, Goteborg, Sweden), or TESLA Semi electric tractor-trailer (TESLA, Palo Alto, CA, USA), have begun to explore new styling designs of the cab to reduce the aerodynamic drag force of tractor-trailers. The way to solve the engineering problems can be obtained from living creatures found in nature, because they possess optimal morphological structures that have been adapted to their environments. Marine and terrestrial animals have often been used as a reference for designers to study when developing new transportation styling design or methods for aerodynamic drag reduction.