Molecular Dynamics Simulations On Fluid Boundary Layer

Nowadays, fluid mechanics has been applied in various fields. It plays an important role in water conservancy project, vehicle exterior design, air dynamics, environmental engineering, and so on.The computational fluid dynamics (CFD) is a research in computer, depending mainly on CFD software. The working procedure of CFD needs to establish the control equations and to set boundary conditions. The no-slip boundary condition is currently adopted extensively. But in recent years, a large number of experiments have confirmed exist of slip on microscopic scales.Recently, the phenomenon of microscopic slip of fluids has arisen much attention. In this thesis, we firstly introduce the theory of fluid mechanics and the history of development. Then, we introduce the progress of the theory of fluid boundary layer.Thirdly, using Langevin molecular dynamics, we investigate systematically the slip dynamics of fluid monolayer on the surface of disordered boundary. With an increase in the strength of pinning in the boundary, we find for the first time a crossover from elastic crystal to smectic flows as well as a crossover from smectic to plastic flows above the depinning. For the weak pinning, the fluids move in an elastic crystal near the depinning, and fluids flow in a set of parallel channels along the external driving force; the averaged velocity of fluids increases with the external driving force linearly, a power-law scaling relationship between the averaged velocity and the driving force could be obtained and the apparent scaling exponent is found to be less than 1. On increasing the strength of pinning, a smectic flow takes place. In such a case, although the fluids move in parallel channels along the drive, the channels perpendicular to the drive are distorted; the averaged velocity of fluids increases with the external driving force still linearly and the power-law scaling relationship could be also obtained, but the apparent scaling exponent is found to be larger than 1. When the strength of pinning becomes strong enough, plastic flows occur. In this case, fluids move in the channels between different pinning centers; the channels are not stable and changeable with time, even some channels are perpendicular to the external driving force; the averaged velocity increases with the drive nonlinearly and the power-law scaling relationship could not be derived. In addition, we find that the critical force Fc increases with the strength of the pinning, and sudden increase is found in Fc within the crossover from the smectic to plastic flows. More than that, we find that the averaged strength of the Bragg peaks decreases with increasing the strength of the pinning and sudden decreases are observed within the crossover from elastic crystal to smectic flows and within the crossover from smectic to plastic flows.Finally, using Langevin molecular dynamics, we investigate systematically the slip dynamics of fluid monolayer on the surface of periodic pinning boundary. We find a plastic flow above the depining for a weak pinning, and with an increase in the strength of the pinning, a crossover from elastic to smectic flows as well as a crossover from smectic to plastic crystal flows is found. This is quite same as that of disordered boundary.The obtained results are helpful for studying the slip of fluids on the surface of boundary as well as research of turbulence.