紊动水流中细颗粒泥沙絮凝发育特征的试验研究

Experimental research on the flocculation characteristics of cohesive sediment in turbulent flow

本文通过试验,研究了水流紊动剪切对黏性细颗粒泥沙絮凝发育的影响作用。根据我们的实验,得出以下初步认识:(1)在某一剪切条件下,长时间后絮凝发育会渐趋稳定,达到某种平衡状态,这时絮团尺度的统计平均值趋于某一稳定值;(2)在低剪切紊动作用(剪切流速梯度G<30或41s-1),紊动剪切强度的提高促进絮凝发育,絮团尺度的统计平均值增大;相反,在高剪切紊动作用下(G>30或41s-1),紊动剪切强度的提高抑制絮凝发育,絮团尺度的统计平均值减小;在中间某一剪切状况下(G=30或41s-1),絮凝发育达到最佳状态,絮团尺度的统计平均值最大。另外,在试验中也发现:(3)在相同的水流剪切状况下,初始泥沙浓度越大,悬浮体系中形成的稳定的絮团中值粒径统计值也越大;(4)提高电解质浓度或增加电解质阳离子化合价,也会促进絮团发育;(5)形成的絮团处在大小为Kolmogorov尺度的旋涡内,其周围的绕流流态为层流状态,絮团受到的阻力以黏性力为主,破裂方式为表面破坏,同时,絮团最大粒径随紊动强度的增加而降低。

The flocculation characteristics of cohesive fine sediment under different conditions of turbulence shear are investigated.The conclusions are as follow.（1） Under a given shear rate,long time evolution of flocculation approaches a state of equilibrium that the statistical mean of the flocs size is steady.（2） Under low shear rate（G 30 or 41s-1） the mean size increases with shear rate as a result of the domination of aggregation over breakup,while under higher shear rate（G 30 or 41s-1） the mean size decreases with shear rate.At intermediate shear rate（G = 30 or 41s-1）,the mean size and the flocculation rate are maximized.（3） Under the same turbulence shear condition,the mean flocs size increases with the initial sediment concentration.（4） Evolution of flocculation could be improved by increasing the cation of electrolyte or concentration of electrolyte.（5） The formed flocs stay inside the eddies of size in the same order as Kolmogorov scale,and the maximum floc size decreases with turbulence intensity.The flow around these flocs is laminar,which may cause a surface erosion of the aggregates.