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    题名: 邊界條件與材料組成對顆粒流堆積型態及流動特性之效應;The effects of boundary conditions and material properties on the dynamic characteristics and deposition patterns for granular flows
    作者: 李璟芳;Ching-Fang Lee
    贡献者: 土木工程研究所
    关键词: 邊壁效應;坡角遞減;質點追蹤速度計;顆粒流;粒徑分離;重力崩落;particle tracking velocimetry;granular flows
    日期: 2010-07-20
    上传时间: 2010-12-08 13:33:59 (UTC+8)
    出版者: 國立中央大學
    摘要: 對礫石型土石流、坡地崩塌、落石崩落與工業製造而言,顆粒體的流動及力學特性在實際應用上扮演了重要的角色。為了暸解顆粒流的物理特徵與動力學機制,本研究以實驗方式探討了均勻顆粒物質的巨觀流動行為、混合顆粒之粒徑分離、重力崩落過程與顆粒坡面受水平震動之遞減行為。實驗採用Voronoi細胞為基礎之流動粒子影像法配合理論進行研析。 在旋轉滾筒內的均勻顆粒流中,本研究提出了一個涵蓋了福祿數、相對粒徑與填充率的無因次流量,其可量化流動層中的動態安息角、流動厚度、表面流速以及剪切率。在混合顆粒的旋轉滾筒流動中,實驗分析顯示增加邊壁粗糙度將造成新的軸向流動型態:粗細顆粒完全分離的非對稱帶。而造成軸向分離的粒徑比、幾何尺度與邊壁摩擦等特性也進一步地被討論。在流動軌跡與速度場之分析上,本研究發現固定的邊壁摩擦條件下,軸向對流胞將隨福祿數提高而逐漸擴大。另外結合Péclet數與邊壁粗糙度的尺度定律分析,顯示了藉由改變不同的控制參數可誘發相同的軸向分離現象。 落石崩塌及坡面滑動的崩落歷程與顆粒物理息息相關。對於已知的顆粒粒徑與底床坡度,實驗崩落坡面的退縮歷程變化與本研究所提出之理論曲線相符,其無因次流動長度將隨著無因次時間增加而遞減。暫態的顆粒流動速度量測顯示在接近靜態的底部區域,其剖面為遵循超穩態流變學(SSH)之指數分布,而非Bagnold流變學之範疇。在水平震動的坡面破壞實驗中,本文所提之指數遞減模式可良好地描述表面坡角於非拘限與拘限邊界的遞減歷程,同時反映不同相對加速度、顆粒表面粗糙度與尾檻高度之差異。另外顆粒堆積內部的流體化厚度與最終穩定坡角則顯示了渠寬縮減後的邊壁效應。最後,兩個相異邊壁條件下所引起的不同坡面破壞機制也將進一步被驗證與探討。 Granular flows always play an important role on the flow dynamics and application for both natural phenomena such as stony debris flows, slope avalanches, and rock falls in field and industrial manufactory. In order to understand the physical characteristics and dynamic mechanism in granular flows, this study explores the macroscopic flow characteristic of uniform granular material, size segregation of binary mixture, falling processing induced by gravity, and slope angle decay under horizontal shaking by experimental investigations. Both experimental analyses with digital imaging techniques based on Voronoi velocimetry algorithm and theoretical description are employed in the study. With regard to the granular flow composed of uniform grains in a rotating drum, a new dimensionless flow rate combining the effects of Froude number, relative particle size and volume filling is proposed in this study, which controls the flow characteristics in a rational drum such as dynamic angle of repose, thickness of the flowing layer, relative free surface velocity, and the shear rates in the flowing layer. For a binary mixture of granular material, the experiments demonstrate the enhancement of side-wall friction will cause a new pattern: the asymmetrical banding stripe. The onset of axial segregation in connection with the variations of size ratio of mixtures, drum geometry, and wall friction are also verified. From trajectory reconstructions and velocity field, one can conclude that the lateral convective cell will expand gradually while Froude number increases for a constant wall friction case. By means of the corresponding scaling law combining the terms of Péclet number and wall roughness, the same axial segregation can be obtained by changing the different control factors. On the other hand, the falling processes associated with rock avalanches and the sliding of slopes is closely related to the granular physics. With respect to a given size of particles and bottom slope, the retreating upper granular surface follows the presented theoretical curve, and dimensionless mobile length decreases as the dimensionless time parameter increases. Measurements of transient velocity profiles exhibit an exponential-like tail close to the static region at the quasi-static bottom and obey SSH rheology instead of Bagnold’s rheology. As for the slope angle decay in the confined and unconfined boundary under shaking process, the proposed scaling law can be well described the evolution of surface slope angle and reflects the variation among the relative shaking acceleration, surface roughness of grains, and sill height. In addition, the corresponding thickness of internal fluidized layer and final stable slope angle show the important relationship linking to the existence of side-wall effect. Finally, the different types of slope failure mechanisms of these two experiments are also examined and discussed further.
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