||Numerical simulation of restructuring behavior of non-fractal aggregate in simple shear flow
Lieu, Tu Uyen
Understanding the behavior of colloidal aggregates in fluid flow is one of the fundamental issues for the prediction and control of the dispersion state of colloidal suspension, which is widely encountered in several engineering fields. The structure of the aggregate has a significant role for both the microstructure and macroproperties of the dispersion. An aggregate may vary its structure by three aspects: aggregation with the other to form a larger one, breakup into several smaller pieces, and restructuring i.e. the particles composing aggregate changes their position relatively but still connect to the others. It has been found out that the restructuring not only changes structure of the aggregate itself, but also has a crucial impact on the aggregation and breakup. Therefore, the restructuring of aggregate contributes important role on the overall performance of the dispersion. Recent studies have suggested that in simple shear flow, a widely used model flow system, the aggregate is apt to have high fractal dimension, and the behavior of such aggregates is complicatedly diverse. Consequently, the restructuring behavior of aggregate having high fractal dimension in simple shear flow is important for either understanding the mechanism of restructuring, or developing highly accurate model predicting the properties of dispersion. In this study, the restructuring behavior of non-fractal aggregate in simple shear flow is investigated. Numerical simulation is conducted to examine the effect of aggregate structure and shear flow condition on the restructuring of aggregate, and the underlying mechanism. The aggregate is composed of a number of monosized, spherical, and hard primary particles. The attraction between particles is calculated by the retarded van der Waals potential. For a system of many particles in fluid medium, the hydrodynamic interaction is very important for a dynamical system, yet very complicated. Stokesian dynamics is a simulation technique which is capable of capturing the complex many-body hydrodynamic interaction. Therefore, Stokesian dynamics approach is employed in the study. The dissertation consists of six chapters. Chapter 1 presents the background and the statement of the study. The gap between the recent studies and this work is also given. Chapter 2 gives details of the numerical simulation, including the construction of the mathematical ormulation, the way it is applied to the study, verification of the method, and the simulation conditions of the study. In Chapter 3, the restructuring behavior of a non-fractal aggregate in simple shear flow is explored. The effect of initial coordination number and the intensity of shear flow on the restructuring of aggregate is investigated. The temporal change in internal structure of the non-fractal aggregate, in terms of coordination number, is examined. It shows that after subjected to shear flow, the aggregate rotates around the vorticity axis and deforms along the streamline. The deformation progresses with time: from spherical shape to ellipsoidal one; finally followed by recovering to almost spherical shape. Simultaneously, the coordination number of aggregate alters and reaches a stable value. The restructuring of aggregate originates from the superimposition of rotational and extensional component of simple shear flow. The aggregate restructures so that a stable structure corresponding to the applied shear flow is obtained. At the same fluid shear stress condition, despite of the significant difference in the initial packing properties, the stable structure of aggregate likely exist at quite compact state, and the stable structure reveals slight difference. The dependence of stable structure on shear flow condition shows similar manner for all aggregates. The stable aggregate in weaker flow is more dense than that in the stronger flow. However there is a limit for such compact structure. A kinetic model based on simultaneous formation and disintegration of links between primary particles is proposed. The model results seem suitable to explain the restructuring of non-fractal aggregate within the scope of the study. The kinetic model furthermore exhibits the dynamic characteristic of the aggregate when the aggregate obtains its stable state. Chapter 4 focuses on a systematic approach for predicting the restructuring of non-fractal aggregate and interprets the physical meaning of restructuring. At first, the transition among stable aggregates reveals to have some reversibility. Further analysis on the volume fraction of the aggregate obtained at the weak flow condition indicates that such aggregate expresses kind of inherent characteristic which is suitable to employ in the general relation of restructuring. By considering these inherit structure into a parameter representing the ability of primary particles to form connection with the others, defined as connection capability, a specific relation between the temporal change in connection capability and the tensile strength of aggregate, calculated by Rumpf’s theory, during the restructuring of aggregate is established. Regarding the stable structure of aggregate in simple shear flow, when the inherit structure of aggregate is taken into account, it shows that the prediction of stable structure and the progress toward stable structure are possible to obtain. In chapter 5, the criteria for restructuring of non-fractal aggregate are analyzed by determination of the strength of aggregate and the hydrodynamic stress acting on the aggregate by fluid. As for the condition of the non-fractal aggregate in this study, the effect of penetration of fluid flow on the restructuring of the aggregate is examined. It shows that the penetration of fluid flow on the aggregate is weekly dependent on porosity of the non-fractal aggregate in the study. Based on the contribution of structure of aggregate on the restructuring and the tensile strength of aggregate, criteria for restructuring of non-fractal aggregate are found out. Chapter 6 is the general conclusion of the dissertation.
Hokkaido University（北海道大学）. 博士(工学)