本實驗使用泰勒-庫頁提(Taylor-Couette，T-C)流場，來探討重粒子偏好集聚之現象與其下沈之問題。T-C流場乃由兩同軸旋轉圓柱來產生眾多不同的流場形態，是目前流體力學領域最被了解的流場之一。有關粒子集聚觀測，主要是以高速攝影機配合高功率雷射切面，在只旋轉內圓柱而外圓柱固定不動的條件下，採用三種不同重粒子(鉛、鎢、銅)分別在不同轉速下之流場，來觀測在不同史托克斯數 (Stokes number，St)，粒子於一反向旋轉渦對陣列中之濃度場偏好性集聚之現象。St為質點粒子的響應時間與紊流場中Kolmogorov時間尺度之比。實驗結果發現，在 時，粒子均勻分佈於渦對結構內；當 時，粒子會朝高應變率低渦度流場區域(渦外圍)集聚，使在高渦度低應變率區域(渦心)幾無粒子存在，粒子均集中於渦漩外圍層(層厚僅約為原渦寬度即兩圓柱間隙之10%)；當 ，雖然大體結構分佈相似，但較多粒子開始出現在渦漩內層和渦心處。此結果驗證了直接數值模擬結果(Wang & Maxey 1993)，更顯示泰勒-庫頁提渦對陣列為說明粒子偏好性集聚現象之最佳流場選擇。 在重粒子下沈速度研究方面，使用兩組高速攝影系統，透過由內外兩圓柱反向旋轉下所產生的無特徵結構紊流場，採用玻璃粒子以質點影像速度量測技術(PIV)來量測重粒子於此流場之下沈速度(Vs)，初步結果發現，粒子平均下沈速度受紊流影響都會比其終端速度(Vt)來得大，且在St等於1附近時，下沈率有最大增加量，本實驗可能是第一個驗證紊流可增加粒子下沉速度之實驗。 This thesis investigates the preferential concentration and the settling velocity of heavy particles using a well-known Taylor-Couette(T-C) flow apparatus. The T-C apparatus consists of two concentric rotating cylinders which can generate many different flow regimes. For visualizations of particle preferential concentration, we use a high-speed laser sheet imaging technique in T-C flow when only the inner cylinder is rotated. Three different particles, lead, tungsten and copper particles with different mean diameters are used, such that the corresponding values of Stokes number(St) can be varied from 0 to 3. Here St is a time ratio of particle response to the Kolmogorov scale of turbulence. It is formd that these particles are uniformly scattered around these counter-rotating vortices when . As values of St increase up to unity, almost all particles are accumulated and concentrated at the edges of counter-rotating vortices where the strain rate is highest and the vorticity is lowest. In other words, there are no particles in these vortices’ core. The thickness of the particles’ accumulated region is only about 10% of the width of counter-rotating vortices. This is the so-called particle preferential phenomenon which was suggested by direct numerical simulation (Wang & Maxey 1993). The present experiment reveals that the T-C flow with only inner cylinder rotation is probably the best flow field to demonstrate the particle preferential phenomenon. For , a few particles can be formed in the core of there vortices. Concerning the settling velocity of heavy particles in featureless turbulence which is generated by counter-rotating both cylinders, we employ glass particles and use the particle image velocimetry to measure mean particle settling velocities(Vs). Preliminary results show that values of Vs are greater than their corresponding terminal velocity (Vt) in still fluid.The increase of the settling rate reaches its maximum as .This experiment may be the first experiment to show that turbulence can increase the settling velocity of heavy particles.
