微泡減阻方法應用於貨櫃船之數值模擬計算

Abstract

本研究主要利用計算流體力學中的二相流方法模擬微泡減阻的現象，其中在貨櫃船船體底部噴入空氣以觀察減阻的效果，將空氣噴口設置於船長方向五個不同位置，再針對各種不同設計方案的計算結果，加以比較分析，以做為微泡噴口位置選用之參考。貨櫃船船型採用台船公司於先前研發計畫中所設計的RD542_2船型，藉助於CFD計算軟體COMET利用在不同含氣率的條件下，計算減阻效果的變化；計算條件採用k-ω紊流模型，在計算時將空氣與水之混合流體以不同含氣率為10%到90%之間在球形艏後緣噴出，以探討含氣率對於減阻效果所產生的影響，其中發現含氣率為10%~20%時，有較佳的減阻效果。接著設計其他四種不同位置之開口，初步觀察在不同開口使用相同含氣流體時，含氣流體之流動軌跡與產生之減阻效果。最後綜合比較三種不同船速、三種不同噴氣速率、三種不同含氣率與四個不同開口彼此之間減阻效果與噴氣量的關係，其中發現開口之噴氣方向垂直向下所產生的減阻效果較開口之噴氣方向偏向船側為佳，且船速較高時可以容許較高含氣流體注入流場並產生較佳的減阻效果。

This study conducted simulations of injecting air and water into the flow field around a container ship by two-phase flow method of computational fluid dynamics (CFD). In order to understand behavior of air-water mixed fluid in different flow conditions, many conditions were investigated and analyzed. The influence parameters were taken into account , including injection velocities, void fractions of air , ship speed and injection positions . These results were helpful to determine which position and condition is better for drag reduction. MBSL k- turbulence model and CSBC RD542_2 model were used to proceed numerical computing . At first , the design condition of void fraction of air were set between 10% to 90% , and inject position were at the end of bulbous bow. It has been shown that the drag reduction rate will decrease when void fraction of air increase and void fraction between 10% to 20% has better reduction rate. Then we used constant air-water ratio fluid (10%) to inject into four other different positions and compared the track of bubble flow with friction reduction rate . At last , we want to observe how the other influence factors can effect friction reduction rate . So the conditions of three different Froude number , three different jetting velocities , three different void fractions of air and four different positions were desighed to compute and compare the results with friction reduction rate. Among these results , we found that the reduction capability was better when the jet direction faced downward ship hull rather then toward hull side. And when ship speed increase , the flow field can allow injecting more air quantity as well as producing higher friction reduction rate.