風砂躍移試驗與模擬研究

Abstract

風砂運動是指因大氣對流運動所產生的風力對地表土壤產生侵蝕的作用，進而帶動土壤顆粒產生懸移、躍移及地表蠕移等運動現象。風砂運動中又以躍移運動為主要形式，其強大的降落衝擊力是造成地表風蝕的主要原因，且亦是造成懸移運動及地表蠕移運動的主要因素。故本研究主要以風砂運動中的躍移運動做為研究主題，以風洞試驗結合「高速攝影技術」，在不同風速下量測躍移顆粒之運動軌跡、運動速度及旋轉速度等躍移特性，並探討其物理現象及統計之特性。本研究並建立一風砂單步躍移的物理模式，在考慮拖曳力、莎弗曼上舉力及馬格納斯上舉力等不同力的作用下，模擬躍移顆粒單步躍移的運動軌跡。並使用試驗之資料率定並驗證本模式。由驗證結果顯示本模式在給定剪力速度及相關顆粒條件下，可以良好的模擬顆粒躍移的軌跡及躍移歷程中諸項物理特性的沿程變化。並且發現在模擬顆粒的躍移運動時，上舉力為不可忽略的重要因素。

Aeolian sand motion can be distinguished into three types: saltation, suspension, and surface creep. In these three motions, saltation is the ruling component, and the large impact velocity of saltaion is the principal source force that causes wind erosion and the other two motions. This study combines wind tunnel and high-speed photographic technique to observe trajectory, velocity, and spinning rate of saltation motion, and discusses the physic and statistic characteristics. Furthermore, this study develops a mathematical model that considers Saffman and Magnus lift forces for aeolian particle single-step saltation. The calibration and verification are accomplished by the experimental data. The simulation results reveal that when the size of particle and the ejection velocity are known, this model can simulate the single-step trajectory precisely, and other simulated saltation characteristics that vary along longitudinal direction can be fitted well with the experimental data. Finally, this model revels that the lift forces are the important factor and can’t be neglected in the simulation of saltaion motion.