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標題: 縱列雙菱柱之流場實驗探討
Experiments of Flow Past Two Diamond Prisms in a Tandem Arrangement
作者: 吳鑑峰
Wu, Chien-Feng
關鍵字: Tandem arrangement;縱列;Two diamond prisms;bistable;reattachment flow;jump flow;Time-history;雙菱柱;雙穩定現象;再回附流場;跳躍流場;時序列
出版社: 機械工程學系

Experiments of Flow Past Two Diamond Prisms in a Tandem Arrangement
Chien-Feng Wu
Department of Mechanical Engineering
National Chung Hsing University
Taichung Taiwan
The flow around two identical diamond prisms in a tandem arrangement has been studied experimentally. The experiments were performed in a wind tunnel and measurements of vortex shedding frequency, surface pressure and drag coefficient were made mainly for the various spacings between the two prisms. The Reynolds numbers were between 2800 and 11000. In addition to the pressure and frequency measurements, the flow pattern was examined using the flow visualization technique.
The flow around the two prisms depends strongly on the spacing between them. At small values of dimensionless spacing, the shear layer separating from the upstream prism reattaches to downstream prism and forms vortices behind, which is referred to as reattachment flow. As the spacing is increased to a certain value called critical spacing, vortices are observed between the two prisms, which is referred to as jump flow. In general, the Strouhal number decreases with increasing spacing, then at the critical spacing it jumps to a much higher value, and then continues to increase gradually to reach a constant level of the single prism Strouhal number. At critical spacing there are indeed two different flow patterns changing intermittently, which is referred to as the bistable phenomenon. The bistable phenomenon happens in a spacing range of about 10 mm nearing the critical value. Although the switch between two different flow patterns in the bistable range does not have a regular period, an increase in Reynolds Number causes the intermittent interval to become smaller, i.e. switching more quickly. The bistable phenomenon can be detected by recording velocity time-history at a measuring position between the two prisms. The velocity time-history shows small amplitude for the reattachment flow and large amplitude for the jump flow. In reattachment flow, the base pressure of upstream prism and the front pressure of downstream prism increase with increasing spacing. In jump flow, the base pressure of upstream prism shows indiscernible change with spacing, and the front pressure of the downstream prism is larger than its base pressure. In reattachment flow, the drag coefficient of upstream prism decreases with increasing spacing, but increases for the downstream prism. As the jump flow appears, the drag coefficients of both prisms increase discontinuously to reflect the switch of flow pattern. The switch of flow pattern also has a significant effect on the fluctuating pressures.
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