層化異重流於等速段之實驗分析

Abstract

本論文之研究之目的為探討層化重流體流入均質環境輕流體之運動現象。擬透過50組不同初始層化條件之定界交換水槽(lock-exchange)試驗，進行分層異重流於等速區段之實驗觀察。層化實驗中的初始條件設置由兩個無因次參數所控制，分別為其對應的驅動流體所佔有之鹽份質量比例關係(B*)及初始層化重流體的密度差異比(ρ*)，並探討等速區段與慣性區段之關聯性。結果顯示，等速段分層異重流之流動形貌與其福祿數值(Fs)將隨此二參數而有所改變。其中，當分層之條件為下層重流體主導(0 <B*<0.6)，下層流動將領先上層且隨ρ*增加，福祿數將減少；反之，重流體之上層所佔鹽份較高時(0.6 <B*<1)，上層流動會超越(override)下層，福祿數將隨ρ^*增加而增加。此外，於B*=0.6的條件下為過渡區域，實驗結果顯示，上層流體幾乎伴隨於流動流體之後，而福祿數約為一定值(Fs≅0.46)並與ρ^*之改變無太大關係。然而在初始分層條件為弱層化時(0.4 <ρ*<1)，兩層將有顯著的混合現象產生，且福祿數將分佈於0.45~0.5此區間，此結果與Keulegan 1958及Barr 1967於均質異重流之研究有類似之處。相反的，若初始分層條件為非弱層化之情況下(0 <ρ*≤0.4)，混合現象不明顯，並於下層主導時之福祿數分佈將超過於均質異重流之範圍(Fs>0.5)，而在上層主導則有小於此區間(Fs<0.45)之結果。

The main purpose of this research is to investigate flow morphology caused by the penetration of density-stratified heavy current into light homogenous ambient fluid. For this reason, 50 different initial conditions of lock-exchange experiments are conducted to find out the influence of the density-stratified gravity current on slumping phase and the relation between slumping phase and inertial phase. These initial conditions depend on two controlling parameters, the density difference ratio (ρ*), and the distribution of driving buoyancy(B*). According to the experimental results, flow morphology and the Froude number of density-stratified gravity current will be changed by the two parameters in slumping phase. First, when currents are dominated by the lower-layer(0<B*<0.6), the lower-layer will lead the current front, and the Froude number will decrease as ρ* increases. Second, due to dominating the flows (0.6<B*<1), the upper-layer will override the lower-layer, and the Froude number will grow as ρ* increases. Third, no matter how ρ* changes in B*=0.6, the upper-layer follows the lower-layer. And the Froude number remains constant (Fs≅0.46). By comparing the results with Keulegan 1958 and Barr 1967, we can know that the Froude number’s distribution of gravity currents produced from the weakly stratified source(0.4<ρ*<1) will be similar with homogenous buoyancy source(Fs=0.45~0.5), and that the layers will mix immediately. In addition, if gravity currents are in the situation of the strong stratified source(0<ρ*≤0.4) and dominated by the lower-layer, the Froude number will be higher than homogenous buoyancy source(Fs>0.5). In contrast, when upper-layer dominates the current, the Froude number will be lower than 0.45 (Fs<0.45), and the mixing between two layers is limited.