다양한 보트 테일을 이용한 화물차의 항력 감소에 관한 실험적 연구

Abstract

Many attempts have been made to reduce the cost of transportation. Drag reduction of heavy vehicles, such as trucks or tractor-trailers, has significant influence on the reduction of fuel consumption and CO2 emission, because the transport of freight using heavy vehicles largely accounts for the cost of transportation. Thus, improvement of the fuel efficiency of heavy vehicles has been gaining considerable attention. Several aerodynamic flow-control devices, such as cab-roof fairing, side skirts, boat tail, and vortex generators, were introduced to reduce the aerodynamic drag force of heavy vehicles. However, it is still unclear how the unsteady flow around a heavy vehicle influence on the drag force and how to reduce this drag force effectively by adopting flow-control devices. In the present study, drag reduction of a real-shaped 15-ton heavy vehicle model was experimentally investigated by attaching various boat tails as a passive flow-control device attached on the near end of the vehicle. The drag force exerted on the vehicle model was measured by using a 7-component balance. The wind tunnel experiment was conducted at Re = 9.1 × 105, based on the height of the vehicle model. The drag coefficients of the standard vehicle model without flow-control devices were compared with those attached with various boat tails. Furthermore, particle image velocimetry (PIV) measurements were carried out to observe how the flow structures modified by the attachment of various boat tails. In this study, two different type boat tails were proposed to reduce drag and side forces. At first, the modified boat tail with a lower inclined air deflector (LIAD) was investigated as a passive flow-control device attached at the rear end of the heavy vehicle. The aerodynamic performance of the modified boat tail with LIAD was experimentally examined by measuring the drag force, side force exerting on the real-shaped heavy vehicle model with varying yawing angle. As a result, the maximum drag reduction effect of the boat tail with LIAD (Ɵ = 45°) is about 9.02%, compared to the standard vehicle without the boat tail, although the length of the bottom tail is half of that of four-way boat tail. This result indicates that the boat tail with LIAD effectively suppresses the formation of large-scale recirculating flow in the wake behind the rear body of the vehicle, due to effectively guide of the underbody flow. Next, a new boat tail was proposed for reducing drag and side forces. The sinusoidal- shaped of the proposed boat tail edge was bio-inspired by the feathers owl flying freely in a variety of environments to improve driving stability under crosswind conditions. The sinusoidal boat tail (SBT) was bio-inspired by the tail shape of the bird’s wings. The bio-inspired SBT was found to have a significant influence on the reducing of drag and sides force and yawing moment under crosswind conditions. At a yaw angle of 7°, the drag and side forces are reduced by 15.9% and 22.6%, respectively at maximum, compared to those of the standard vehicle without SBT. The yawing moment coefficient is noticeably decreased by the attachment of SBT. As a conclusive result, the bio-inspired SBT can effectively improve the aerodynamic performance and the driving stability of the vehicle. The PIV velocity field results show that the SBT effectively suppresses the formation of longitudinal vortices by enhancing the streamwise velocity of the wake behind the boat tail. In addition, due to the development of secondary swirling flow, the streamwise momentum deficit is also reduced in the wake region. The boat tails with LIAD and SBT proposed in the present study are found to effectively control the wake behind the vehicle by simple modification of tail shape or additional attachment of a flow-control device on the boat tail. The effective drag-reducing flow-control devices developed for heavy vehicles, would greatly contribute to the world-wide energy saving and reduction of environmental emission.