柱型異重流於層化行為下之流動形貌分析

AN EXPERIMENTAL INVESTIGATION OF DENSITY-STRATIFIED GRAVITY CURRENTS IN AN UNCONFINED TANK

異重流又名密度流，為流體中因密度差異所產生的水平流動現象，其成因多數源自於流體中溫度的變化或流體中挾帶懸浮介質的不同。倘若其流體密度分佈為縱向且具壓力梯度變化，稱之為層化現象。本研究擬藉由實驗觀察層化異重流的流動特性，依據不同的分層重流體釋放源，配合染劑顯影及影像分析來觀察其不同的流動型貌。由於該流動特性因無渠槽的侷限，便產生類扇形狀的運動特徵並往徑向擴散移動，稱之為柱型層化異重流。其中，產生不同流動形貌的控制因子分別為兩個無因次參數：分層釋放源的密度比（𝑅𝜌）以及重力比（𝑅𝐵）。實驗結果顯示，當分層異重流為下層主導時（𝑅𝐵＜0.3），此流況會因密度比不同而產生分離流動或部分混合的流動現象；於上層主導（𝑅𝐵≥0.3）且強層化條件下（𝑹𝝆→𝟎），會出現下層流體先後被混合及上層流體超越的二次覆蓋現象；於上層主導（𝑅𝐵≥0.3）且弱層化條件下（𝑹𝝆→𝟏），會因分層流體之間劇烈混合而形成與均質釋放源所產生的異重流相似的流動形貌。最後，將藉由福祿數量化不同層化釋放源條件下所產生的異重流運移軌跡。

Gravity Currents, also known as density currents, occur when fluid propagates into another fluid of a different density. In environments, the density difference can be attributed to temperature difference or dissolved materials. This kind of fluid motion often occur as sediment transportation in river channels and suspended sediments in dams, with vertical density difference of sediment, namely stratification, being the critical driving force. For this reason, Investigation of density-stratified gravity currents has significant benefits for predicting sediment transportation and deposition in natural channels and dams. In the literature, numbers of investigations have been focused on one-dimensional gravity currents in the lock-exchange systems. However, there are rare discussions for gravity currents propagating into ambient environment without lateral boundary. This kind of radial fluid movement with sector-liked flow morphology is named as cylindrical gravity currents. The purpose of present investigation is to deepen our understanding of cylindrical density-stratified gravity currents through laboratory experiments by means of qualitative and quantitative analysis.