Journal of Jilin University(Engineering and Technology Edition) ›› 2020, Vol. 50 ›› Issue (5): 1635-1644.doi: 10.13229/j.cnki.jdxbgxb20190401

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Drag reduction of a square⁃back Ahmed model based on bi⁃stable wake

Zhi-gang YANG1,2,3(),Ya-jun FAN1,2,Chao XIA1,2(),Shi-jun CHU1,2,Xi-zhuang SHAN1,2   

  1. 1.School of Automotive Studies, Tongji University, Shanghai 201804, China
    2.Shanghai Automotive Wind Tunnel Center, Tongji University, Shanghai 201804, China
    3.Beijing Aeronautical Science & Technology Research Institute, Beijing 102211, China
  • Received:2019-04-26 Online:2020-09-01 Published:2020-09-16
  • Contact: Chao XIA E-mail:zhigangyang@tongji.edu.cn;chao.xia@tongji.edu.cn

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Abstract:

The wake of 1/4 scale square back Ahmed body with or without boat-tail structure was investigated at ReH=9.6×104 by pressure sensor and particle image velocimetry(PIV). The statistical analysis of the instantaneous pressure on the back shows that the wake presents a bi-stable behavior in the horizontal direction, characterized by alternating occurrence of two symmetry-breaking stable states, each can maintain a relatively long-time duration. During the two stable states, a lower pressure on the back presents a high probability, while in the switch state the pressure on the back is higher. The bi-stable behavior was also observed by instantaneous and conditional average PIV results of flow field in the horizontal plane. With boat-tail structure added at the model base, the bi-stable behavior of the wake is gradually suppressed with the increase in the boat-tail length, and the wake vortex structure tends to be symmetrical, the wake width becomes narrow, the vortex shedding strength weakens, leading to higher back pressure and lower drag coefficient.

Key words: vehicle engineering, square-back Ahmed model, bi-stable wake, boat-tail structure, particle image velocimetry(PIV)

CLC Number: 

  • U461.1

Fig.1

Experimental setup and coordinate system"

Fig.2

Boat-tail structure"

Fig.3

Cdvaries with length of boat-tail"

Fig.4

Averaging Cp of model base"

Fig.5

Instantaneous CP of two symmetrical points on back"

Fig.6

Pressure tap"

Fig.7

Phase diagram of pressure gradient"

Fig.8

Time-space diagram of horizontal back pressure distribution of boat-tail with different lengths"

Fig.9

Instantaneous vorticity field and streamline of cross section z/H=0.67 of wake"

Fig.10

Conditional mean velocity field of cross section z/H=0.67 of wake"

Fig.11

Velocity field of cross section z/H=0.67 of wake"

Fig.12

Instantaneous vorticity field and streamline of longitudinal section y/H=0 of wake"

Fig.13

Time-mean velocity field of y/H=0 plane"

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