https://scholars.lib.ntu.edu.tw/handle/123456789/61750
Title: | 電雙層於微流道旁通過渡流之效應 Effects of Electric Double Layer on Bypass Transition in Microchannel Flow |
Authors: | 許桓瑞 Shiu, Huan-Ruei |
Keywords: | 微流體;電雙層;旁通過渡流;微流道;microfluidic;EDL;microchannel;bypass transition | Issue Date: | 2004 | Abstract: | 本論文藉由高解析之直接數值模擬探討電雙層(EDL) 於旁通過渡流線性與非線性發展之影響,並以三種不同型態之局部擾動加以分析,首先為成對的反向渦旋,第二種為噴流式之軸對稱擾動,第三種為波動包之形式,且在低雷諾數下之電雙層流場以系統性的比較次臨界過渡區之巨觀尺度Poiseuille流場來探究此擾動場之發展。在線性發展階段中v與w方向擾動速度之發展在質與量上是極為相似的,於EDL效應下,在初始u為零之成對形式之渦旋擾動,建立起兩倍大之徑向渦度ωy,並且兩流場皆發展出傾斜的強力剪力層,整體來說,其兩近似之擾動場所顯示在三維線性機制下EDL流場生成之結構與Poiseuille流場有相當之強度。對於較大振福之擾動,即非線性成長,因EDL流場存在不穩定之反曲速度剖面,則其總動能大於Poiseuille流場,而且軸對稱擾動引起之能量成長較大於反向旋轉渦流,在過渡過程中之非線性交互作用,假若其擾動足夠強烈或是/並且雷諾數足夠高以克服暫態成長階段,即旁通過渡過程中的Tollmien-Schliching不穩定機制,觸發流場裂解而形成紊流斑。因此,在相同的流場條件下,局部擾動振幅會在比Poiseuille流場小於一個級數下之EDL流場下觸發流場裂解。 The effect of the electric double layer (EDL) on the bypass transition mechanism in the linear and nonlinear evolution stage is explored through direct numerical simulations of high resolution. Three kinds of localized disturbances are analyzed. The first one is a pair of counterrotating vortices, the second is a wall jet-like axisymmetric perturbation, while the third is a wave-packet. The time-space evolution of the perturbed field is throughly investigated at low Reynolds numbers in EDL flow by systematically comparing the results in the subcritical transition region of macro-scale Poiseuille flow. The wall normal and spanwise perturbation velocities development are both quantitatively and qualitatively similar in macro and micro flows in the linear stage. The streamwise velocity, which is initially zero for the pair of vortices and is set up by the generation of the wall normal vorticity is twice larger under the EDL effect. Both flows develop inclined strong streamwise shear layers. Overall is the close similarity of the disturbance evolution showing that the three dimensional linear mechanism in EDL flow lead to the structures that are at least as strong as in Poiseuille flow. For large amplitude perturbations, i.e. in the non linear regime the total kinetic energy associated with the EDL flow is larger compared with the Poiseuille flow because of the inherently unstable EDL inflexionnal velocity profile. The energy growth associated with axisymmetric perturbation is always larger than that associated with the counterrotating vortices. The nonlinear interactions trigger the breakdown and lead to turbulent spots, bypassing the transitional Tollmien-Schlichting instability mechanism, providing that the disturbance is strong enough and/or the Reynolds number is sufficiently high to overcome the transient growth stage. Thus the amplitude of the localized disturbance that lead to breakdown is an order of magnitude smaller in EDL flow compared to the macro flow under some circumstances. |
URI: | http://ntur.lib.ntu.edu.tw//handle/246246/61288 | Other Identifiers: | en-US |
Appears in Collections: | 機械工程學系 |
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ntu-93-D87522027-1.pdf | 23.53 kB | Adobe PDF | View/Open |
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