Journal of the Korea Academia-Industrial cooperation Society (한국산학기술학회논문지)

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Effects of Rear Diffuser Size on the Driving Performance of a Passenger Car

자동차의 주행 성능에 미치는 리어 디퓨저 크기의 영향

  • Lee, Gyo Woo (Division of Mechanical Design Engineering, Chonbuk National University)
  • 이교우 (전북대학교 기계설계공학부)
  • Received : 2018.11.21
  • Accepted : 2019.02.01
  • Published : 2019.02.28
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Abstract

This study examined the change in driving performance according to the starting position of the rear diffuser of a vehicle. To accomplish this, the CATIA 3D design program was used to model the vehicle with reference to a commercial SUV vehicle and design the rear diffuser to start from 300, 400, and 500 mm from the rear tire. The flow and drag change were analyzed and the change in air flow was confirmed using Fluent, a flow analysis program at a vehicle traveling speed of 60, 100, and 140 km/h. The rear diffuser reduced the lift and drag forces compared to no diffuser regardless of the starting position. This is because if there is a rear diffuser, it will reduce the vortex phenomenon by suppressing the flow separation that occurs when air is drawn out from the rear portion of the vehicle. In this study, the starting point SP 400 was determined to be the optimal condition because the lift force was the smallest at SP 400 and the lift reduction effect was the best.

본 연구는 차량용 리어 디퓨저의 시작 위치에 따른 주행 성능 변화를 분석하고자 하였다. 이를 위해 CATIA 3D 설계 프로그램을 이용하여 상용 SUV 차량을 참고하여 차량을 모델링하고 뒤 타이어를 기준으로 300, 400, 500 mm 떨어진 위치부터 리어 디퓨저가 시작되도록 설계했다. 그리고 유동 해석 프로그램인 Fluent를 이용해 차량 주행속도가 60km/h, 100km/h, 140km/h 일 경우를 조건으로 하여 유동해석 후 양력과 항력의 변화를 분석하고 공기의 유선 변화를 확인했다. 해석 결과, 리어 디퓨저는 시작 위치에 상관없이 디퓨저가 없는 경우에 비해 양력과 항력을 감소시켰다. 이는 리어 디퓨저가 있을 경우 공기가 차량 하면부를 지나 후면부로 빠져나올 때 발생하는 박리 현상을 억제하여 와류 현상을 감소시키기 때문이다. 또한 본 연구에서는 SP 400의 조건일 때 양력이 가장 작았고 양력 감소 효과도 가장 좋았기 때문에 주행 중 타이어의 접지력을 최대로 확보할 수 있어서 이 경우를 최적의 조건으로 결정하였다.

Keywords

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Fig. 1. A half shape of a modeling car

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Fig. 2. A schematic showing the rear diffuser

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Fig. 3. A schematic showing volume meshes

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Fig. 4. Decrements of lift forces according to velocities

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Fig. 5. Decrements of drag forces according to velocities

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Fig. 6. Streamlines for the case of without rear diffuser

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Fig. 7. Streamlines on the rear part of the car for the cases of a) without rear diffuser, b) with rear diffuser at SP 500, c) at SP 400, and d) at SP 300

Table 1. Dimensions of a real car and a modeling

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Table 2. Lift forces according to velocities and starting points of rear diffuser

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Table 3. Drag forces according to velocities and starting points of rear diffuser

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