Please use this identifier to cite or link to this item: http://hdl.handle.net/11455/2255
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
摘要: 
摘要
本實驗量測探討雙菱柱流場的特性,並與單菱柱流場相互比較。在實驗中主要參數為雙菱柱間的無因次化間距L/D。我們也變換不同的雷諾數,範圍從2800至11000之間,以瞭解雷諾數對雙菱柱與單菱柱的流場的影響及差異。實驗在開放式低速風洞中進行,菱柱的尺寸為攻角45度邊長20mm的方柱,垂直於自由流速方向的特徵長D=28.3mm。流場特性的量測包括尾流自然剝離頻率、表面壓力、阻力係數、雙穩定狀態、渦漩結構及可視化拍攝。
雙菱柱之間距對流場有明顯的影響,在臨界間距產生兩種不同型態的流場交替產生,稱為雙穩定現象。透過尾流頻譜的分析,低頻部分是再回附流場的剝離頻率,渦漩在下游柱體後方形成;高頻的部分則是跳躍流場的剝離頻率,渦漩在上下游柱體中間形成。我們從速度時序列結果得知,小振幅的時序列流場型態為再回附流場,大振幅的時序列流場型態為跳躍流場。於臨界間距時兩種型態的流場交替變化沒有一定週期可言,雙穩定現象發生在約10mm的範圍內,且隨著雷諾數的增加交替次數變得頻繁。再回附流場上游柱體背壓分佈隨L/D增加而增加,下游柱體在迎風面壓力分佈隨L/D增加而增加;在跳躍流場中渦漩中心靠近上游柱體背面導致平均壓力值下降,下游柱體受到上游柱體後方形成的渦漩影響,迎風面的壓力較背風面高。在再回附流場中上游柱體阻力值隨L/D增加而減少,下游柱體的阻力值為負值(為推力)隨L/D增加而增加。在跳躍流場中上下游柱體的阻力值產生躍升後隨L/D的增加約保持定值或微幅增加。雷諾數除了會影響單菱柱渦漩形成長度與雙菱柱的臨界間距外,對於其它的流場特性影響並不大。

Experiments of Flow Past Two Diamond Prisms in a Tandem Arrangement
Chien-Feng Wu
Department of Mechanical Engineering
National Chung Hsing University
Taichung Taiwan
Abstract
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.
URI: http://hdl.handle.net/11455/2255
Appears in Collections:機械工程學系所

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