Flow Visualization on the Steady and Unsteady Flow Structures around Ahmed Model

Ahmed模型的穩態和非穩態尾流結構的流動顯示研究

張英朝(Ying-Chao Zhang) ； 朱會(Hui Zhu) ； 周裕(Yu Zhou)

《中國機械工程學刊》 40卷6期 (2019/12) Pp. 727-731

摘要

Ahmed model is a simplified vehicle model for aerodynamic characteristic investigation. To understand the steady and unsteady flow structures, some flow visualization experiments have been carried out to show the flow structures under wake. Ahmed model will with high air drag or low air drag according to the slant surface angle. In low drag regime, the flow above the slant surface is shown by the smoke flow. The flow begins to separate at the upper edge of slant surface and forms a shear vortex of K-H instability due to the velocity difference in wake separation regions and forms a backwards flow and recirculation bubble at the slant surface. In the high drag regime, the presence of the C-pillar vortex is shown by the smoke flow visualizations. Based on the study of K-H instability of low drag model, it is speculated that the formation of C-pillar vortex is accompanied by the existence of shear vortex. Further PIV experiments show that the structure of the C-pillar vortex is studied on a smaller scale, and the vortex of the C-pillar is found. The C-pillar is a steady flow structure, and its sub vortices are unsteady shear vortices. This sub vortex is first report in automotive aerodynamics.

關鍵字

flow structures ； Ahmed ； steady and unsteady ； sub-vortex

並列摘要

Ahmed模型是一個用於汽車空氣動力學研究的簡化汽車模型。為了深入的研究Ahmed模型的穩態和非穩態的尾流流動結構，本研究進行了大量的可視化試驗。Ahmed模型根據尾部傾斜角度，分為高阻模型和低阻模型。對於低阻模型，通過煙流顯示了氣流在尾部傾斜面的流動。流動在尾部傾斜面上楞處就開始分離，並且在所有尾部分離區域由於速度差形成K-H不穩定性的剪切渦，並在傾斜面下楞處形成回流渦。對於高阻的模型，通過煙流顯示顯示了C柱拖拽渦的存在。通過對低阻模型K-H不穩定性的研究，推測C柱渦的形成也伴隨了剪切渦的存在。進一步的PIV流動顯示，對C柱渦的結構進行更小尺度的深入研究，C柱渦的子渦被發現。C柱渦是一種穩定的拖拽渦，而其子渦是非穩態的剪切渦。這個流動對於研究汽車空氣動力學是一個新的發現。