DISCRETE ELEMENT METHOD FOR MOLECULAR SCALE VISUALIZATION OF MICRO-FLOWS

摘要

According to the Knudsen number four different types of flow regimes can be identified: continuum, slip, transition, and free-molecular flow. The continuum flow regime is well described by the Navier-Stokes equations. The slip flow can also be described by the Navier-Stokes equations, provided that some special boundary conditions are prescribed. In the transition and the free-molecular flow regimes, the flow is described by the Boltzmann equation, which is a molecular-based model. By using this model, it is possible to solve the high Knudsen number flow problems through molecular-based direct simulation techniques. However, independent of micro-flow research the particulate-solids research community has developed the so-called Discrete Element Method. In recent years, QMUL and MIT research groups (Munjiza, Williams) have revolutionarized these methods by inventing a set of linear packing-density-independent search algorithms, which have enabled systems comprising billions of particles to be considered on a desktop machine. Recently the QMUL group has applied the method to micro-flows. The most important aspect of this new method is accurate integration of motion of individual molecules including interaction between molecules. As temporal and spatial constraints make the visualization of micro-flows in experimental research difficult, the new method is an ideal tool for visualization of micro-flows. The power of these new visualization tools is best demonstrated through the so-called "virtual movies" obtained from simulations. Through these movies the observer is given an opportunity to see the motion of individual atoms of a fluid and their interaction with each other and with the boundary.