珠江口黏性泥沙絮凝的湍流动力机制

The turbulent dynamic mechanism of flocculation of cohesive sediment in the Zhujiang River Estuary

本文利用2010年枯季在珠江口进行的大、中、小潮LISST剖面及底边界层观测资料,分析了磨刀门河口枯季稳定存在的絮团三峰结构,即构建絮团的基本粒子的平均粒径约为8.3~9.0μm,小絮团为36~100μm,大絮团大于180μm。小潮期,盐跃层捕集的悬浮泥沙以强絮凝过程为主,大絮团含量占优;中、大潮期,平均粒径普遍增大,絮凝占优。潮内的动力变化对絮团多峰结构及形态参数的影响不明显,絮凝与解凝处于动态平衡。结合坐底三角架的湍流资料和简化的群体平衡模型(Population Balance Equation,PBE),进一步揭示了絮团变化的湍流动力机制。高流速下的强紊动剪切力,直接导致大絮团被破坏形成小絮凝体,絮凝体平均粒径减小,反之絮凝强于解凝作用。同时,基于高斯矩积分方法求解PBE,得到的粒径分布基本与观测值吻合,说明在有较好的现场湍流与粒径观测资料的条件下,PBE包含的湍流动力机制可以用来研究黏性泥沙的絮凝过程。

This thesis aimed to analyze the stable multimodal(3-peaked)particle size distributions(PSDs)of flocs in the Zhujiang River Estuary with the field observation data getting by LISST and the bottom boundary layer observation system during the dry season in 2010.The results show that the mean diameter of the basic building blocks of flocs,so-called primary particle,is about 8.3–9.0μm;the mean diameter of microflocs in a range of 36–100μm,and macroflocs have a size range of 180μm to thousands of micrometers.In the neap tidal periods,the suspension sediment of halocline is dominated by the macroflocs with strong flocculation process;the mean diameter of flocs is increases and is controlled by flocs during the moderate and spring tide.The dynamic change of the tide has little impact on the multimodal PSDs and morphological parameters,with aggregation and breakage of the flocculation in the dynamic equilibrium.Study results further demonstrate the turbulent dynamic mechanism of flocculation by combining the turbulence data collected by the bottom tripod and the simplified Population Balance Equation(PBE).It is that,the high shear of the peak flow would enhance breakage of macroflocs to microflocs and decrease the mean diameter of flocs,on the contrary,aggregation is much stronger than breakage.It also shows that PSDs are in according with observation by solving PBE based on gaussian moment integral method.It turns out that PBE which containes the turbulent dynamic mechanism can be used to study the flocculation of cohesive sediment with turbulence and PSDs data.