A molecular dynamics study of flow regimes with effects of wall properties in 2-D nano-couette flows.

Abstract

The transitional Reynolds number and range for macrochannel flows are around 1200 and 1000 respectively. Several studies have shown that for microchannel flows, the transitional Reynolds number drops to around 500 and the transitional range around 300. This thesis provides some data on the laminar to turbulent transition for nano-channel flows through molecular dynamic simulations. The Lennard-Jones potential is used for fluid-fluid and wall-fluid interactions, and a non-linear spring potential is used for wall-wall interactions. The mixing is characterised by averaging the maximum transverse movement for all fluid molecules. Six nano-separations were simulated and the flow and mixing behaviours examined. The results show that the transitional Reynolds number and the range increase with increasing diameter. The effects of wall properties on flow regimes were also investigated using molecular dynamic simulations. The results show that the transitional Reynolds number and range increased with increasing wall density. For increasing wall interaction strength, the effect on the transitional Reynolds number was inconclusive. However, the transitional range increased. With an increase in wall wettability, which corresponds to an increase in hydrophilicity, there was an increase in both the transition Reynolds number and range. The wall roughness was modeled as sinusoidal. For an increase in the amplitude of the wall roughness, there was a decrease in the transitional Reynolds number. The effect on the transitional range was inconclusive. For an increase in the period of the wall roughness, both the transitional Reynolds number and range increased. Errors involved in MD simulations arise from several sources. They include the size of the time-step, thermostat model, wall model, and viscosity calculation method. No experimental results at the nano-scale are available for direct comparison however they provide a basis for future work. As computation power improves, MD simulations at higher Reynolds numbers may be compared with experimental results of flows through microchannels. Also, as laboratory technology and measurement accuracy improves, experimental results of flows through nano-channels may be conducted and used for comparison.