دانلود رایگان مقاله ریز قطره دینامیکی در جریان های چرخشی

Abstract

This paper deals with investigations of droplet dynamics in rotating flows. In many previous studies droplet dynamics was analyzed in simple unidirectional flows. To fill this gap, the focus of this study is an overview on investigations of droplet dynamics in a complex rotating flow. A Lattice Boltzmann Method with high potential in simulation of two-phase unsteady flows is applied to simulate the physics of the problem in a lid-driven cavity. In spite of its simple geometry, there is a complex rotating flow field containing different vortices and shear regions. The Reynolds number based on the cavity length scale and the upper wall velocity, ReL, is considered to be 1000. We discuss here effects of different parameters such as: density ratios (1, 5, 10, 100, and 1000), droplet sizes (D/L = 0.097, 0.114, 0.131 and 0.2), and droplet initial positions (1/8, 2/8, and 3/8 of the cavity length, L, out of center). The results are discussed in terms of global flow physics and its interaction with the droplet, drop deformation during its motion along with the main flow, and droplet trajectories. It is shown that there are strong interactions between the droplet and the main carrying flow. During motion, the droplets pass through different flow regions containing acceleration/deceleration zones. Consequently, the droplets experience different shear forces resulting in stretching, shrinking, rotating and dilatation which all contribute to the dynamics of the droplet.

5. Summary

This paper discusses the droplet dynamics and its complexity in a rotating flow, including the physics of the rotating main flow, droplet deformation and droplet trajectories. It is shown that at a density ratio of 1, there is no relative velocity between the droplet and the main carrying flow and the droplet moves with the same velocity as the main flow does. The pathline on which the droplet is moving is the main flow pathline starting from the initial position of the droplet. However, with increasing time of the rotating flow, the droplet is affected by the vorticity field and finally when it reaches the steady state the droplet lies between constant streamlines of the main flow. Hence, its shape remains unchanged while rotating at a constant radius around the center of the cavity depending on its initial position. By increasing the density ratio, the global flow physics becomes affected by the droplet. Since the densities of two fluids are different, there is a relative velocity between them. Here, due to centrifugal forces, the droplet is thrown toward the wall by the main flow. At higher density ratio of the droplet, the initial inertia is greater and has tendency to save the droplet's initial shape, i.e. it helps to resist against deformation or movement. Comparison of the droplets with different density ratios demonstrates that droplets with higher density ratios have deformed less even in accelerating zones. In addition, the centrifugal force on the droplet is increasing with time which means that the final position of the droplet cannot be identified. At very large density ratios, e.g. ρh/ρl= 100, the droplet behaves like a very heavy solid sphere causing a chaotic unsteady flow pattern at any time.