以數值模擬探討天然沉積之卵礫石顆粒組構

Abstract

顆粒材料其工程行為受組構之影響甚大，以巨觀的角度視之顯然只能以孔 隙率隱性地描述其疏鬆狀況。若以微觀力學著眼，以顆粒間相互作用的關 係為出發點，便能有效率地表示出顆粒的組構性質與外力及變形間之關係 。描述顆粒組構之參數多用接觸數、中心點連線向量分佈、法線向量分佈 、平均中心點連線長度分佈以及長軸向量分佈等來表示，且方向性資料一 般以組構張量來表達最有效率。微觀力學之組合率至目前為止多僅限於大 小相等之圓形顆粒，然而天然的顆粒不純為圓形，多少較接近橢圓形，且 其粒徑大小與級配情形也各不相同。本文即嘗試以二向度之數值模擬研究 在天然之沉積狀況下，單一粒徑與具級配粒徑之顆粒集合體在不同之扁平 率與均勻係數下之幾何組構特性與變化趨勢，期能為未來的研究者提供一 些相關之資訊以供參考。本文研究發現，顆粒形狀與顆粒級配情形均強烈 地控制了顆粒之組構參數。對於同時考慮顆粒粒徑相同及考慮顆粒級配的 情形下，當扁平率小於 0.2時，中心點連線向量分佈與法線向量分佈均十 分類似，均多分佈於45度與135度之方向，故二者可相互取代。然而扁平 率在 0.2 至 0.5 之間時，二者將有明顯的不同，中心點連線向量分佈有 漸往水平方向集中的趨勢，而法線向量分佈則有往垂直方向集中的趨勢。 平均中心點連線長度分佈，在扁平率為 0時接近圓形，隨著扁平率之增大 而逐漸變為橢圓形。長軸向量分佈則多平行河床面，且在扁平率大於0.3 時其分佈以成花生狀之分佈。 The mechanical behavior of granular material largely depends on its fabric. Void ratio alone fail to characterize the fabric of a granular material. from a microscopic view, general papameters defining the fabric of granular material include the following:(i)coordination number,(ii)branch vector distribution,(iii)length distribution of average branch vector, (iv)contact normal distribution,(v)orientation vector orientation. So far, most studies on the fabric of granular material have focused on that of uniform sized spheres that may be quit different from natural deposited graunlar materials. In this thesis, two-dimensional numerical simulations of particle deposition are carried out to study the fabric features of naturally deposited gravel particles.Influences of several factors, including particle size , particle shape, and gradation , on the characteristics of fabric in terms of fabric tensor are examined. From this study , it appears the shape and grain-size distribution of particles strongly control the fabric.For eccentricity below 0.2, the branch vector distribution and the contact normal distribution are almost the same, but when eccentricity is among 0.2 and 0.5, they are quit different. The branch vector distribution and contact normal distribution concentrates along the horizontal and vertical direction respectively. The orientation vector parallels to river bed, and when eccentricity is larger then 0.3,its distribution becomes peanut shaped.