Lattice Boltzmann Studies On The Fluid-solid Interfaces And Fluid-solid Systems

Fluid-solid system is ubiquitous in nature, and is closely related to our life:e.g., the change of climate, flow of river and lakes, and its extensive application in industrial manufacture. Hence, understanding of fluid-solid system plays an important role in developing new technology and revitalization of traditional industry. Among the factors affecting the behaviors of fluid-solid systems, the interface between fluid and substrates is much more complicated and difficult to model. Especially, a number of new interfacial phenomena were observed for newly developed nanomaterials. Therefore the study on the fluid-solid interfaces and fluid-solid systems is not only of scientific significance, but also of industrial values.In this article, using Lattice Boltzmann Method (LBM), we study on the fluid-solid interfaces and fluid-solid systems. Firstly, we develop a new method which has faster computation speed than most of other versions. To prove our method, we first use the method to study some special character of bubbling bed, and simulation results from our method is qualitatively agree with previous studies. Then we simulate fluidized bed which is common in fluid-solid system, and again we obtained simulation results that agree well with experiments quantitatively. Secondly, we use Shen-Chen (SC) version of LBM to simulate the properties of fluids on solid surface. In this aspect, we deal with two questions:(i) how the hierarchical structure of a super-hydrophobic surface affect the contact angle of microdroplets sitting on the surface and transition between Wenzel and Cassie states,(ii) we also check the two assumptions for derivation of the equation of Cassie, and thus check the viability of the equation. Detailed conclusions are given as follows:1:Simulation of macroscale fluid-solid system by using LBM(l)We develop a new method for the calculation of flow. The new method has the advantage of faster computation speed, inherent feature for parallel computing, and shows a good applicability in simulating fluid-solid systems. In this modified LBM the effect of void ratio and sliding velocity between fluid and solid were considered. Solid particles in this method are described as the time-driven hard sphere model, and the couple of fluid and solid is described with the model of EMMS. With the new method, we simulate some characters of single jet bubbling bed. Firstly, we simulate the effect of fluid velocity and particle diameter on the equivalent diameter of generated bubbles. Then, we check the disengaging time in different fluid velocities and particle diameters. Thirdly, we study the shape of bubble in different high and width of bed. At last, inductive effect of cavities is studied. In general, all the results we obtained from the new method agree well with previous experimental and theoretical data, indicating that the method we proposed is correct qualitatively.(2) With the new LBM we simulate the single-hole bubbling bed and obtain equivalent diameter of bubble. Our simulation results again agree well with that from the classical equation. Then we simulate raising tube and statistic void ratio of axial and radial and, which agree well with experimental data. All above mentioned results prove the validity of our new method in quantitative.2. Simulating the fluid behaviors on fluid-solid interfaces(1)We use the SC type of LBM to investigate how the hierarchical structure of a rough solid surface, which in this work is modeled as the microstructure (micropillars) covered with nanostructure (nanopillars), affects the contact angle of microdroplets atop of the solid surface and the wetting transition between the Wenzel and Cassie states. Our result shows that combination of multiscale roughness have important effect on the contact angle of microdroplets and the wetting transition between the Wenzel and Cassie states.(2)We check the two assumptions of the Cassie equation based on SC model. One of the assumption is drop must much larger than the dimension of microstructures. The other is microstructures are required to be uniformly distributed on substrates. Different from previous studies our conclusion is that, only the first assumption is necessary for the validity of the Cassie equation.