Journal of Mechanical Science and Technology 34 (1) 2020 DOI 10.1007/s12206-019-1221-1 Journal of Mechanical Science and Technology 34 (1) 2020 Original Article DOI 10.1007/s12206-019-1221-1 Keywords: · Boat tail · Sinusoidal boat tail · Drag reduction · Wind tunnel experiment · PIV measurement Correspondence to: Sang Joon Lee [email protected] Citation: Kim, J. J., Lee, E. J., Lee, S. J. (2020). Wind tunnel tests on drag reduction of heavy vehicles using sinusoidal boat tails. Journal of Mechanical Science and Technology 34 (1) (2020) 201-208. http://doi.org/10.1007/s12206-019-1221-1 Received July 26th, 2019 Revised Wind tunnel tests on drag reduction of heavy vehicles using sinusoidal boat tails Jeong Jae Kim1, Eui Jae Lee2 and Sang Joon Lee1 1 Biofluid and Biomimic Research Center, Department of Mechanical Engineering, Pohang University of Science and Technology (POSTECH), San31, Hyoja-dong, Nam-Gu, Pohang, Gyeongbuk 790-784, Korea, 2Hyundai Kia Motors Namyang Institute, Jangdeok-dong, Hwaseong-si, Gyeonggi-do 445-706, Korea Abstract A sinusoidal boat tail (SBT) is developed and investigated to greatly reduce drag, side force, and yawing moment under crosswind conditions. The drag and side forces of a heavy vehicle model with the SBT are reduced by 15.9 % and 22.6 %, respectively, relative to those of a reference model without the SBT at a yaw angle of 7°. Therefore, the SBT improves the aerodynamic performance and driving stability of the vehicle model. Flow characteristics around the vehicle model are measured using a particle image velocimetry technique. Mean velocity field results show that the SBT effectively suppresses the formation of longitudinal vortices by enhancing the streamwise velocity of the region behind the boat tail. In addition, the streamwise momentum deficit is reduced in the wake region owing to the development of a secondary recirculating flow region. November 7th, 2019 Accepted November 8th, 2019 † Recommended by Editor Yang Na © The Korean Society of Mechanical Engineers and Springer-Verlag GmbH Germany, part of Springer Nature 2020 1. Introduction Driving vehicles are frequently exposed to crosswinds. At a low driving speed, the effects of crosswinds on driving stability are negligible. However, when a vehicle is driving at a high speed, it experiences strong interference from crosswinds. Thus, the driving safety of the vehicle is greatly reduced. As heavy vehicles have large side surfaces, their exposure to crosswinds may lead to serious circumstances, including rollover [1]. In addition, the reduction of fuel consumption of transport vehicles has been an important global issue. The reduction of aerodynamic drag exertion on heavy vehicles has great significance because it is associated with vehicle mileage. The total aerodynamic drag involves the front part (45 %), trailer base (25 %), and underbody (30 %) of a heavy vehicle operating on a highway [2, 3]. Numerous flow control devices, such as cab deflector [4], front spoiler [5], vertical fence [6], cab side extender [7], and cab roof fairing [8], have been introduced to control the forebody flow of heavy vehicles. Side skirts consisting of straight panels curtaining the underspace between the front and rear wheels are commonly adopted to control underbody flow [9]. Boat tails and vortex generators have been proposed to reduce the aerodynamic drag of the rear part [10-12]. Most studies concerning boat tails have been carried out experimentally or numerically using the General Motors (GM) model and Ahmed body model [13-15]. The Ahmed model has been broadly adopted to analyze the effects of various additive devices on the reduction of aerodynamic drag [6, 16]. For example, flaps attached to the rear edge of a modified Ahmed body achieved 25 % drag reduction [17]. A 9 % drag reduction effect was obtained by attaching a flap to the front edge of a slanted surface [18]. A novel boat tail with a lower inclined air deflector (LIAD) attached at the rear end of a heavy vehicle was proposed as a passive flow control device [19]. The boat tail with the LIAD reduces approximately 9 % of the drag coefficient at maximum. The biomimetic approach, which suggests solutions to practical problems from a number of morphological features of living creatures in nature, has received increasing attention [20]. For example, a biomimetic flow control device called the automatic moving deflector (AMD) was 201 Journal of Mechanical Science and Technology 34 (1) 2020 DOI 10.1007/s12206-019-1221-1 Table 1. Specifications of heavy vehicle model. Model : 15-ton Hyundai Trago Xcient Size (m) : 12.7 L х 2.5 B х 4.0 H Fig. 1. Schematics of (a) NBT; (b) SBT (α: Slant angle, ω: Side tail angle, L: Side length; a and w: Amplitude and wavelength of the sinusoidal tail part, respectively). Force measurement PIV measurement Scale ratio 1/8 1/15 Blockage ratio 4.7 % 6.5 % inspired by the secondary feathers of birds [21]. The AMD introduces a large-scale separation bubble above its slanted surface with a delaying separation point toward the trailing edge. A maximum drag reduction of 19 % was achieved on the Ahmed body. We examine a boat tail with a sinusoidal edge pattern to improve the driving stability of heavy vehicles under crosswinds. The design concept of the proposed sinusoidal pattern is applied to heavy vehicles to enhance their driving stability. Therefore, a sinusoidal boat tail (SBT) is attached to the rear end of a realistic 3D vehicle model to reduce its aerodynamic drag substantially at various yaw angles. In addition, the particle image velocimetry (PIV) technique is used to analyze the modification of flow structure in the wake behind the vehicle model attached with the SBT. 2. Experimental apparatus and method 2.1 Vehicle model A vehicle model of a 15-ton truck was developed by scaling down the prototype of a commercial truck (Hyundai Trago Xcient) measuring 4.0 m high, 2.5 m wide, and 12.7 m long. The scaled-down vehicle model was made of compressed polyvinyl chloride, and some flow control devices were made of polylactic acid. The vehicle model was produced with a 3D printer. The selected vehicle was scaled down to a scale of 1/8 to ensure that the blockage ratio of the vehicle model to the cross-sectional area of the wind tunnel test section was less than 4.7 %. PIV measurements were carried out with a 1/15 scaleddown model. The specifications of the tested models are summarized in Table 1. The blockage ratios of the vehicle models were less than 8 % and exerted an insignificant effect on the pressure distribution and drag coefficient [22]. SBT devices were used. Fig. 1(b) shows the diagrams of the SBT mounted on the vehicle model. The tail length attached to the actual vehicle was L = 90 cm. The slant angle (α) between 202 the upper horizontal surface and the slanted surface was fixed at 12.5° [23, 24]. The side tail angle (ω) defined as the angle between the side surface of the boat tail and the end of the rear body was 80°. The wavelength (w) of the SBT was designed to satisfy the geometric condition of H/w = 5, a/L = 0.5, and a/w = 1. The effective drag reduction was measured when the wave steepness of the sinusoidal patterns was more than 0.06 [25]. A conventional normal boat tail (NBT) with a straight tail was also tested as the comparative reference under the same experimental conditions. 2.2 Force measurement experiment setup Wind tunnel tests were carried out in the POSTECH subsonic wind tunnel [19]. The aerodynamic force and moment acting on the vehicle model were measured using an external balance, which was manufactured by the German Aerospace Center (DLR). The measurement accuracy of the three orthogonal coordinates is approximately 0.2 %. The crosswind effect on the model was measured by a turntable installed at the bottom of the wind tunnel test section. The crosswind experiments were carried out with varying yaw angles in the range of φ = 0°–15°. Herein, the yaw angle referred to the turning angle of the heavy vehicle away from the flow direction. The schematics of the wind test section, measurement system, and turntable setting are shown in Figs. 2(a) and (b). The definitions of the aerodynamic forces acting on the vehicle and coordination systems are depicted in Fig. 2(c). The aerodynamic forces and moment were determined by statistically averaging the data acquired from three independent experiments. The vehicle model was affixed to the external balance through holes on the bottom surface of the wind tunnel test section by connecting four contact points under the fairs of the front and rear wheels (Fig. 2(d)). The aerodynamic drag FD measured along the flow direction, drag coefficient (CD), side force (Fy), side force coefficient (CS), and yawing moment coefficient (CM) were evaluated by respectively using the following equations: p p FD = Fx cos( j ) + Fy sin( j) , 180 180 Journal of Mechanical Science and Technology 34 (1) 2020 DOI 10.1007/s12206-019-1221-1 Table 2. Reference dimensions used for evaluating the mean aerodynamic force coefficients. A [m2] As [m2] h [m] ρ [kg/m3] U0 [m/s] 0.148 0.648 0.03 1.293 25 Fig. 3. Schematic of experimental setup for PIV measurement. Wind velocity and yaw angle are fixed at 10 m/s (Re = 1.8 × 105) and φ = 7°, respectively. Fig. 2. (a) Photograph of POSTECH subsonic wind tunnel; (b) schematic of the test section, external balance, and turntable; (c) aerodynamic forces acting on the vehicle model and the coordination system used in this study; (d) photograph of the 1/8 scale model of the 15-ton heavy vehicle model mounted on the wind tunnel test section. CD = 2 FD / rU 02 A , CS = 2 FS / rU 02 A , CM = 2 M z / rU 02 As h , where ρ is the fluid density, φ is the yaw angle, U0 is the wind speed, A is the frontal area of the vehicle model, As is the side area of the vehicle model, and h is the height of the reference vehicle model at the center position. The reference dimensions used to evaluate the aerodynamic force coefficients of the tested vehicle are presented in Table 2. tured flow images to obtain the vector field information. As a tracer particle, olive oil droplets with an average diameter of 1– 3 µm produced by a Laskin nozzle were used for the PIV experiment. Oil droplets are widely utilized as tracer particles for gaseous flows because they present an adequate aerodynamic response to velocity changes in wind tunnel tests [8]. The detailed PIV setup was described in our previous papers [8, 19]. The measurement accuracy was evaluated by calculating the root mean square (RMS) error with the velocities for the reference model measured; the velocity magnitude in the calculation was the same as that of the freestream velocity. A total of 600 velocities were used, and the RMS error was 1.5 %. The maximum RMS velocity fluctuation in the center region was approximately ±0.516 pixels. The corresponding uncertainty in the velocity measurement was approximately ±0.15 m/s for a time interval of 6 μs. Turbulent kinetic energy (TKE) was evaluated with 2D flow 1 2 2 2 assumption using the formula of TKE = (u ' + v ' ) . 3. Results and discussion 3.1 Drag reduction effect 2.3 PIV measurement The PIV setup and the corresponding coordinate system are presented in Fig. 3. PIV experiments were carried out in another closed-return type of a subsonic wind tunnel with a test section measuring 0.72 m (width) × 0.6 m (height) × 6.75 m (length). The wind speed (U0) was fixed at 10 m/s, and the corresponding Reynolds number (Re) was approximately 1.8 × 105. The yaw angle was fixed at φ = 7° to observe the crosswind effect. PIV measurements were conducted at two different sections in the y-z and x-z planes. The physical dimension of the PIV setup in the measurement sections for the y-z and x-z planes was 328 × 245 mm2 and 300 × 225 mm2, respectively. A 105 mm macro lens (Nikon Nikkor) was attached in front of the camera to magnify the flow images. The 64 × 64 pixel interrogation window was selected with half pixels overlapping. A twoframe cross-correlation PIV algorithm was applied to the cap- The vehicle model without a boat tail exhibited a nearly uniform drag coefficient at wind speeds ranging from 5 m/s to 55 m/s (Fig. 4). Re ranged from 1.8 × 105 to 2.0 × 106 depending on the wind speed. The drag coefficient became nearly constant at Re ≥ 9.1 × 105 (U0 = 25 m/s) because the flow around the vehicle was fully turbulent and the flow separation points were fixed at the end of the heavy vehicle. Therefore, the drag coefficient became independent of Re when the wind velocity was greater than U0 = 25 m/s. Therefore, we conducted a drag measurement experiment at a fixed Re = 9.1 × 105 for comparison on the basis of the SAE guidelines for trucks [26]. Drag coefficients were reduced by 5.54 % and 5.65 % when the NBT with a flat straight tail and the SBT were respectively applied to the vehicle model. In the previous result, the drag coefficient was reduced by 5.4 % after attaching a three-way boat tail to the reference model [27]. Such a slight discrepancy 203 Journal of Mechanical Science and Technology 34 (1) 2020 DOI 10.1007/s12206-019-1221-1 Fig. 4. Variations of drag coefficient (CD) of vehicle model without boat tail, with NBT, and with SBT according to Reynolds number. Fig. 6. Variations of side force coefficient (CS) of the vehicle model without boat tail, with NBT, and with SBT under varying yaw angles (U0 = 25 m/s). Fig. 5. Variations of drag coefficient (CD) of the heavy vehicle without boat tail, with NBT, and with SBT under varying yaw angles (U0 = 25 m/s). Fig. 7. Variations of yawing moment coefficient (CM) of the vehicle without boat tail, with NBT, and with SBT under varying yaw angles (U0 = 25 m/s). in drag reduction rates may be attributed to the difference in the external shapes of the test vehicles. The ground effect on the flow around the Ahmed model at Re = 2 × 105 was investigated using large eddy simulation (LES) [28]. As a result, the aerodynamic drag of the Ahmed model with moving ground was 8 % lower than that with fixed ground. We mainly focused on the effects of SBT on drag reduction and modified flow structure behind the vehicle under stationary ground conditions. The comparisons of the drag coefficients of the vehicle model attached with either the NBT or the SBT in accordance with the yaw angle are shown in Fig. 5. When the NBT was attached to the vehicle, the drag coefficient decreased in all yaw angle conditions relative to the reference model without the boat tail. The drag reduction effect of the SBT was much distinctive under crosswind conditions. The drag coefficient decreased by approximately 15.9 % at the yaw angle of φ = 7°. When the yaw angle was beyond 10° and the NBT was attached to the test vehicle, the drag reduction rate was only 1.5 %. By contrast, the drag reduction rate for the SBT was maintained at least 8 % for high yaw angles of φ ≥ 10°. The merit of the SBT was clearly elucidated in the reduction of the side force coefficient (CS), as shown in Fig. 6. The side force coefficient was reduced by 6.7 % and 22 % at the yaw angle of φ = 7° when the NBT and SBT, respectively, were attached to the vehicle. These results were obtained from the modification in the flow structure due to the presence of the sinusoidal-shaped side tail. Furthermore, the reduction effect of the side force coefficient at high yaw angles implied that the SBT significantly influenced the drag reduction under crosswind conditions. Crosswind conditions can play a major role in the drag reduction and driving stability of heavy vehicles driving on expressways because their side surfaces exposed to crosswinds are much larger than those of passenger vehicles [29]. In this point of view, these results indicate that the SBT effectively reduces the drag and side force exerted on a vehicle. Fig. 7 shows the variations of the yawing moment coefficient (CM) in accordance with the yaw angle. As the yaw angle increased, the yawing moment coefficient increased up to φ = 12° and then decreased. As the yaw angle increased, the action point of the side force moved from the front part of the vehicle to the rear container, consequently producing a negative yawing moment [1]. When the NBT and SBT were installed at the vehicle model, the yawing coefficient significantly reduced at all yaw angle ranges. The reduction effect of the SBT was most effective at small yaw angles in the range of φ = 0° – 3°. These results imply that the probability of a rollover accident is decreased for the vehicle equipped with the SBT relative to the reference model. Therefore, the installation of the SBT greatly contributes to the improvement of the driving stability of vehicles. 204 Journal of Mechanical Science and Technology 34 (1) 2020 DOI 10.1007/s12206-019-1221-1 The effect of boundary layer formation was nearly negligible because the aerodynamic drag exerted on the vehicle model was mainly governed by the drag caused by the pressure difference induced by the massive flow separation instead of the drag from skin friction. Compared with those of a passenger vehicle with an aerodynamic profile, the separation points of the present heavy vehicle model were geometrically well determined, and the effect of the boundary layer that developed along the wind tunnel test section on the aerodynamic drag of the vehicle was insignificant. 3.2 Flow structure modification The velocity fields of the wake behind the vehicle model were measured using a PIV measurement technique. The flow characteristics of the wakes with and without the SBT under crosswind conditions were analyzed to elucidate the reduction mechanisms of the drag and side forces due to the attachment of the SBT. The large-scale flow structures that formed behind the vehicle with fixed flow separation points were not highly sensitive to the changes of Re under the present experimental conditions. Fig. 8 presents the spatial distributions of the in-plane velocities v and w and their magnitude and velocity vectors in the wake region behind the rear body of the vehicle in the vertical cross-sectional plane at the X/D = 1.5 downstream location (Fig. 8(a)). Unless specifically denoted, the yaw angle was fixed at φ = 7° for all cases. A large-scale longitudinal clockwise-rotating vortex was predominant in the cross-sectional plane, and it corresponded to the trailing vortices moving downstream. Under crosswind conditions, the NBT and SBT significantly influenced the longitudinal vortices that developed from the lateral sides of the rear body. The circulation of the longitudinal vortices decreased with the downstream. However, the longitudinal vortices existed even beyond the location of x/D = 1.5. Thus, the attachment of the two boat tails contributed to the reduction of drag force. When the SBT was installed, the size of the longitudinal vortex and the drag were effectively reduced (CD = 0.903, CS = 0.131 at φ = 7°). On the contrary, the reference vehicle had strong vortices, which corresponded to a large drag (CD = 1.07, CS = 0.167 at φ = 7°). These results indicated that the sinusoidal tip pattern of the SBT suppressed the formation of longitudinal vortices by enhancing the streamwise velocity in the wake behind the boat tail. The longitudinal vortices made up the recirculation flow region and induced a large drag on the vehicle model. The strength of the longitudinal vortex was reduced in the near wake region, leading to a noticeable reduction in the drag coefficient [16]. The TKE levels in the wake region were clearly distinguished between the two boat tails. The reference vehicle exhibited a high TKE value in the upper side of the longitudinal vortex (Fig. 10(a)). The two boat tails (Figs. 10(b) and (c)) decreased the TKE values near the boat tail and effectively reduced the TKE value in the wake region. When the SBT was applied to the Fig. 8. Comparison of spatial distributions of in-plane velocity vectors v and w and their magnitude in the cross section of the y-z plane behind the rear body of the vehicle model: (a) PIV measurement plane at x/D = 1.5; (b) reference vehicle without boat tail; (c) vehicle with NBT; (d) vehicle with SBT. Yaw angle is φ = 7° for all cases. Fig. 9. Contours of the TKE values behind the vehicle model in the crosssectional y-z plane at x/D = 1.5: (a) Reference vehicle without a boat tail; (b) vehicle with NBT; (c) vehicle with SBT. reference model, the TKE value significantly decreased in the region behind the sinusoidal tail of the SBT as the flow passed through the SBT (Fig. 9(c)). The four locations of the small TKE values at z/H = −0.35 matched well with the hollow wave shape of the SBT. This result implied that the SBT effectively transmitted the streamwise flow without considerable loss of flow momentum. Thus, the installation of the SBT reduced the drag force with the decreasing size of the longitudinal vortices and TKE values in the rear region of the vehicle model under crosswind conditions. The streamwise flow passing through the boat tail was evidently enhanced in the mean velocity field of the flow behind the rear body of the vehicle in the horizontal x-z plane at a yaw angle of φ = 7° (Fig. 10). Without a boat tail, the flow separated from both sides of the rear body and formed a large-scale recirculation flow. The pressure on the left side of the wake flow was reduced because the pressure had low values inside the recirculation flow, resulting in a large drag coefficient (CD = 1.07, 205 Journal of Mechanical Science and Technology 34 (1) 2020 DOI 10.1007/s12206-019-1221-1 tum in the wake region. Thus, the vehicle attached with the SBT had the minimum side force coefficient (CS = 0.131 at φ = 7°). On the contrary, the side force coefficient in the case of the NBT was high (CS = 0.156 at φ = 7°). 4. Conclusions Fig. 10. Comparison of mean velocity fields (U/U0) of the region behind the vehicle model in the horizontal center (x-z) plane: (a) PIV measurement plane at y/H = 0; (b) reference vehicle without boat tail; (c) vehicle with NBT; (d) vehicle with SBT. Fig. 11. Contours of TKE values of the region behind the vehicle model in the horizontal center (x-z) plane: (a) Vehicle with NBT; (b) vehicle with SBT. CS = 0.167 at φ = 7°). However, for the vehicle attached with a boat tail (Figs. 11(c) and (d)), the flow became more or less symmetric with respect to the horizontal center line of the wake. This finding implied that the boat tails effectively guided and enhanced the streamwise flow on the left side. Therefore, the drag reduction caused by the attachment of boat tails was mainly attributed to the decrease in the recirculation flow region. When the SBT was installed at the reference model, the sinusoidal tail of the SBT was more helpful than the NBT in enhancing the streamwise flow passing through the boat tail. As a result, the core region of the recirculation flow decreased, and a secondary circulation flow region formed on the left side of the rear body. The recirculating flow decreased the streamwise momentum deficit in the rear body region. Fig. 11 compares the TKE values in the wake behind the rear body of the vehicle model attached with the two different boat tails in the horizontal center plane. The sinusoidal tail tip of the SBT was more effective than that of the NBT in reducing the TKE values in the region near the left side of the boat tail. In addition, the TKE values in the region behind the boat tail clearly exhibited the pattern of the sinusoidal tail tip, as shown in Fig. 11(b). This finding implied that the streamwise flow passed through the SBT without a large loss of flow momen206 The SBT inspired by the wing tips of birds is developed to reduce the drag of heavy vehicles. The aerodynamic performance of the SBT is experimentally investigated by estimating the force measurement and flow field measurement around the rear body using PIV. The SBT significantly influences the reduction of drag force, side force, and yawing moment under crosswind conditions. At a yaw angle of φ = 0°, the reduction rates of the drag coefficient for the NBT and SBT are almost similar. However, at a yaw angle of φ = 7°, the maximum drag reduction of 15.9 % and maximum side force reduction of 22.6 % are achieved by attaching the SBT at the rear end of the vehicle. As heavy vehicles are easily exposed to crosswind conditions in expressways, such vehicles attached with the SBT can reduce fuel consumption effectively. In addition, the yawing moment coefficient noticeably decreases after attaching the SBT. Conclusively, the SBT can effectively contribute to the improvement of the driving stability of heavy vehicles. PIV measurement is conducted to analyze the modification of the flow characteristics around the vehicle model due to the installation of the SBT. Without the boat tail, a large-scale separation region is formed, thereby resulting in a significantly large drag. On the contrary, the sinusoidal tip pattern of the SBT suppresses the formation of the longitudinal vortex by enhancing the streamwise flow passing through the boat tail. In addition, because the SBT develops a secondary circulation flow zone, the streamwise momentum deficit is reduced in the wake region. These results imply that the SBT is effective in reducing drag force and side force under crosswind conditions. The proposed SBT is found to effectively control the wake flow behind the vehicle with simple modification in its tail shape. The proposed SBT can be applied to improve the aerodynamic performance of high-speed transporting vehicles. For example, the SBT can be applied to the design of gap faring rear end to improve driving stability. The clarity of all figures is extremely important. Acknowledgments This study was conducted as the third year research of the development of aerodynamic technologies for efficient road freight transport under the support of the KAIA in the Ministry of Land, Infrastructure, and Transport (NTIS 1615007940). Nomenclature----------------------------------------------------------------------------------U ω α : Upstream velocity : Side tail angle : Slant angle Journal of Mechanical Science and Technology 34 (1) 2020 φ a w A As h ρ U0 CD CS CM : Yaw angle : Amplitude of sinusoidal tail : Wavelength of sinusoidal tail : Frontal cross-sectional area of vehicle model : Side area of vehicle model : Height of reference vehicle model : Density of air : Fixed upstream velocity : Drag coefficient : Side force coefficient : Yawing moment coefficient References [1] F. Cheli, R. Corradi, E. Sabbioni and G. Tomasini, Wind tunnel tests on heavy road vehicles: Cross wind induced loads—part 1, Journal of Wind Engineering and Industrial Aerodynamics, 99 (2011) 1000-1010. [2] R. Bradley, Technology Roadmap for the 21st Century Truck Program, Energy Efficiency and Renewable Energy, US (2000). [3] R. M. Wood, A discussion of a heavy truck advanced aerodynamic trailer system, 9th International Symposium on Heavy Vehicle Weights and Dimensions, University Park, PA, USA (2006). [4] K. R. Cooper, Truck aerodynamics reborn: Lessons from the past, SAE Transactions, 112 (2003) 132-142. [5] D. G. Hyams, K. Sreenivas, R. Pankajakshan, D. S. Nichols, W. R. Briley and D. L. Whitfield, Computational simulation of model and full scale Class 8 trucks with drag reduction devices, Computers & Fluids, 41 (2011) 27-40. [6] J. Allan, Aerodynamic drag and pressure measurements on a simplified tractor-trailer model, Journal of Wind Engineering and Industrial Aerodynamics, 9 (1981) 125-136. [7] H. Martini, B. Bergqvist, L. Hjelm and L. Löfdahl, Aerodynamic effects of roof deflector and cab side extenders for truck-trailer combinations, SAE Technical Paper, No. 2011-01-2284 (2011). [8] J. J. Kim, S. Lee, M. Kim, D. You and S. J. Lee, Salient drag reduction of a heavy vehicle using modified cab-roof fairings, Journal of Wind Engineering and Industrial Aerodynamics, 164 (2017) 138-151. [9] B. G. Hwang, S. Lee, E. J. Lee, J. J. Kim, M. Kim, D. You and S. J. Lee, Reduction of drag in heavy vehicles with two different types of advanced side skirts, Journal of Wind Engineering and Industrial Aerodynamics, 155 (2016) 36-46. [10] W.-H. Hucho and G. Sovran, Aerodynamics of road vehicles, Annual Review of Fluid Mechanics, 25 (1993) 485-537. [11] J. Leuschen and K. R. Cooper, Full-scale wind tunnel tests of production and prototype, second-generation aerodynamic drag-reducing devices for tractor-trailers, SAE Technical Paper, No. 2006-2001-3456 (2006). [12] R. McCallen, R. Couch, J. Hsu, F. Browand, M. Hammache, A. Leonard, M. Brady, K. Salari, W. Rutledge and J. Ross, Progress in Reducing Aerodynamic Drag For Higher Efficiency of Heavy Duty Trucks (class 7-8), Society of Automotive Engineers, Incorporated (1999). DOI 10.1007/s12206-019-1221-1 [13] T. Han, V. Sumantran, C. Harris, T. Kuzmanov, M. Huebler and T. Zak, Flow-field simulations of three simplified vehicle shapes and comparisons with experimental measurements, SAE Technical Paper, No. 960678 (1996). [14] V. Malviya, R. Mishra and J. Fieldhouse, CFD investigation of a novel fuel-saving device for articulated tractor-trailer combinations, Engineering Applications of Computational Fluid Mechanics, 3 (2009) 587-607. [15] M. Minguez, R. Pasquetti and E. Serre, High-order large-eddy simulation of flow over the “Ahmed body” car model, Physics of Fluids, 20 (2008) 095101. [16] S. Ahmed, G. Ramm and G. Faitin, Some Salient Features of the Time-averaged Ground Vehicle Wake, Society of Automotive Engineers, Inc., Warrendale, PA (1984). [17] J.-F. Beaudoin and J.-L. Aider, Drag and lift reduction of a 3D bluff body using flaps, Experiments in Fluids, 44 (2008) 491501. [18] G. Fourrié, L. Keirsbulck, L. Labraga and P. Gilliéron, Bluffbody drag reduction using a deflector, Experiments in Fluids, 50 (2011) 385-395. [19] E. J. Lee and S. J. Lee, Drag reduction of a heavy vehicle using a modified boat tail with lower inclined air deflector, Journal of Visualization, 20 (2017) 743-752. [20] H. Choi, H. Park and W. Sagong, Biomimetic flow control based on morphological features of living creaturesa, Physics of Fluids, 24 (2012) 121302. [21] D. Kim, H. Lee, W. Yi and H. Choi, A bio-inspired device for drag reduction on a three-dimensional model vehicle, Bioinspiration and Biomimetics, 11 (2016) 026004. [22] G. West and C. Apelt, The effects of tunnel blockage and aspect ratio on the mean flow past a circular cylinder with Rey4 5 nolds numbers between 10 and 10 , Journal of Fluid Mechanics, 114 (1982) 361-377. [23] S. Ahmed, R. Gawthorpe and P.-A. Mackrodt, Aerodynamics of road-and rail vehicles, Vehicle System Dynamics, 14 (1985) 319-392. [24] H. Choi, J. Lee and H. Park, Aerodynamics of heavy vehicles, Annual Review of Fluid Mechanics, 46 (2014) 441-468. [25] P. W. Bearman and J. C. Owen, Reduction of bluff-body drag and suppression of vortex shedding by the introduction of wavy separation lines, Journal of Fluids and Structures, 12 (1998) 123-130. [26] SAE 2012, SAE wind tunnel test procedure for trucks and buses, SAE Recommended Practice J 1252. [27] J. Ortega, K. Salari, A. Brown and R. Schoon, Aerodynamic Drag Reduction of Class 8 Heavy Vehicles: A Full-scale Wind Tunnel Study, Lawrence Livermore National Laboratory Technical Report, LLNL-TR-628153 (2013). [28] S. Krajnović and L. Davidson, Influence of floor motions in wind tunnels on the aerodynamics of road vehicles, Journal of Wind Engineering and Industrial Aerodynamics, 93 (2005) 677696. [29] K. R. Cooper, The wind tunnel testing of heavy trucks to reduce fuel consumption, SAE Technical Paper, No. 821285 (1982). 207 Journal of Mechanical Science and Technology 34 (1) 2020 Sang Joon Lee is a Professor in the Department of Mechanical Engineering at POSTECH and a Director of the Biofluid and Biomimic Research Center. His research interests are biofluid flows, microfluidics, quantitative flow visualization, and experimental fluid mechanics. Jeong Jae Kim is a Postdoctoral Researcher in the Department of Mechanical Engineering at POSTECH and a member of the Biofluid and Biomimic Research Center. His research interests are flow visualization and wind tunnel experiments. 208 DOI 10.1007/s12206-019-1221-1 Eui Jae Lee is a Senior Research Engineer in the Aerodynamic Development Team Research & Development Division at the Hyundai Kia Motors Namyang Institute. His research interests are flow visualization, applications of PIV velocity field techniques, and wind tunnel experiments.
