Floc morphology and size distributions of cohesive sediment in steady-state flow.
Floc morphology and size distributions of cohesive sediment in steady-state flow.
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DOI：
10.1016/s0043-1354(03)00082-4
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发表时间：
2003-06
期刊：
影响因子：
12.8
通讯作者：
M. Stone;B. Krishnappan
中科院分区：
文献类型：
--
作者：
M. Stone;B. Krishnappan
在环形水槽中进行了4种恒定流条件(0.058、0.123、0.212和0.323Pa.)下的淤积试验，研究了粘性泥沙颗粒群的分维。用光学显微镜和图像分析系统测定悬浮物的面积、最长轴线和周长。根据相关变量在双对数图上的回归直线斜率计算出4个分维(D、D_1、D_2、D_k)。反映粒子投射面积(A)和周长(P∝AD/2)的分维D从0.058Pa时的1.25±0.005增加到0.121Pa时的最大值1.36±0.003，然后下降到0.323Pa时的1.34±0.001。D的变化表明，在较高的剪应力水平下，颗粒边界变得更加复杂，较大颗粒的形状更加不规则。在最高剪切应力下，D的减小是由于颗粒碰撞增加导致的絮体破碎。最长轴(L)与颗粒周长(P∝)的分维d1由0.058Pa时的1.00±0.006增加到0.325pA时的最大值1.25±0.003。最长轴与粒子投射面积(A∝Ld2)相关的分维D2从0.058Pa时的1.35±0.014增加到0.323Pa时的最大值1.81±0.005。观察到的D1和D2的增加表明，随着剪切力的增加，颗粒变得更长。由科尔卡克经验定律得到的颗粒群分维值从0.058Pa时的3.68±0.002下降到0.323Pa时的1.33±0.001，表明颗粒大小分布在低剪切时由大小相近的颗粒群向高剪切水平下较大的絮状颗粒转变。结果表明，小颗粒(微团)是水体中较大絮体的形成单元，较大絮体的稳定性是稳态剪应力的函数。
Fractal dimensions of particle populations of cohesive sediment were examined during deposition experiments in an annular flume at four conditions of steady-state flow (0.058, 0.123, 0.212 and 0.323Pa). Light microscopy and an image analysis system were used to determine area, longest axis and perimeter of suspended solids. Four fractal dimensions (D, D1, D2, Dk) were calculated from the slopes of regression lines of the relevant variables on double log plots. The fractal dimension D, which relates the projected area (A) to the perimeter (P) of the particle (P∝AD/2), increased from 1.25±0.005 at a shear stress of 0.058Pa to a maximum of 1.36±0.003 at 0.121Pa then decreased to 1.34±0.001 at 0.323Pa. The change in D indicated that particle boundaries became more convoluted and the shape of larger particles was more irregular at higher levels of shear stress. At the highest shear stress, the observed decrease in D resulted from floc breakage due to increased particle collisions. The fractal dimension D1, which relates the longest axis (l) to the perimeter of the particle (P∝lD1), increased from 1.00±0.006 at a shear stress of 0.058Pa to a maximum of 1.25±0.003 at 0.325Pa. The fractal dimension D2, which relates the longest axis with the projected area of the particle (A∝lD2), increased from 1.35±0.014 at a shear stress of 0.058Pa to a maximum of 1.81±0.005 at 0.323Pa. The observed increases in D1and D2indicate that particles became more elongated with increasing shear stress. Values of the fractal dimension Dk, resulting from the Korcak's empirical law for particle population, decreased from 3.68±0.002 at a shear stress of 0.058Pa to 1.33±0.001 at 0.323Pa and indicate that the particle size distribution changed from a population of similar sized particles at low shear to larger flocculated particles at higher levels of shear. The results show that small particle clusters (micro-flocs) are the formational units of larger flocs in the water column and the stability of larger flocs is a function of the shear stress at steady state.