Multiscale Simulation of Micro and Nano Gas Flows

Gas flows in micro/nano-flow devices are often multiscale, with both continuum and highly rarefied flow regions. In rarefied regions, the gas is not in local thermodynamic quasi-equilibrium, meaning the conventional Navier-Stokes fluid dynamic equations are an inadequate description. Kinetic methods, such as the direct simulation Monte Carlo method, are appropriate but computationally expensive, especially for low-speed flows. While combining accurate kinetic methods with computationally efficient continuum models in a hybrid approach would enable practical multiscale micro/nano flows simulation, this also has major problems, e.g. non-equilibrium information is not retained in the continuum model but is required by the kinetic methods.

Instead, we propose in this project to create a new multiscale lattice Boltzmann (LB) technique that switches between higher-order models (with a larger number of discrete velocities) in highly rarefied flow regions, and lower-order ones (with a smaller number of discrete velocities) in less rarefied regions. An intelligent switching method will be developed that dynamically assesses the level of non-equilibrium in the local flowfield (which may vary in time as the flowfield evolves), and switches between different-order LB models in response, taking into account the accuracy required and the computational expense. As this multiscale LB scheme uses models developed in the same theoretical framework, the solution coupling problems faced by kinetic-continuum hybrid approaches will be avoided. Our technique will enable the exploitation of non-equilibrium micro/nano-flow physics for new technologies: in this project, we will use it to optimise the design of a Knudsen compressor - a vacuum pump without any moving parts.

This research escalates new understanding of fundamental flow physics to the release of validated software to the international engineering simulation and design community. The multidisciplinary applications of non-equilibrium flows mean this research will be relevant to the pharmaceutical, manufacturing, mechanical, chemical, environmental and electronics industries. Designers, developers and manufacturers will benefit from an enhanced modelling capability for system design purposes, e.g. designing next-generation space vehicles, gas chromatographs, and micro-sensors. Government and non-government agencies may also benefit from this work: for example, the Beagle 2 spacecraft crash may have been avoided if a non-equilibrium aerodynamic instability in its flight through the Martian atmosphere had been taken into account. In the longer term, our work can be incorporated into simulation tools in cognate industries concerned with non-equilibrium transport physics, including modern materials processing, chemical and environmental engineering (e.g. fluidised chemical reactors, pollutant monitoring), and nano-devices (e.g. quantum point contact nano-devices).

This project will contribute to the training and education of a UK workforce with the necessary multidisciplinary knowledge and skills to develop future non-equilibrium flow technologies. The research findings will be incorporated into our postgraduate and undergraduate courses at the University of Strathclyde. To engage beneficiaries and enhance knowledge exchange in the wider community, a seminar series will be organised throughout the project. We will also organise a 2-day international workshop on non-equilibrium transport physics in the final year of the project. We will engage with industrial researchers via our existing industrial collaborative projects on non-equilibrium flow technologies (see letter of support), and exploit opportunities for Knowledge Transfer Partnerships with them to take the work forward after the end of the project.

The results of our work will be published in international peer-reviewed journals that are accessed by most researchers in the field, and will be presented at the most important international conferences related to non-equilibrium flow physics. An interactive project website will be constructed to report our latest results and methodologies. Engagement with the public will also be facilitated through SciTopics, a free expert-generated knowledge-sharing service produced by Elsevier. As the field of application of this project is at the frontiers of engineering (i.e. nanotechnologies), and is the subject of much current debate and discussion, we will arrange other out-reach activities to engage the general public and school children via e.g. the University's School programmes.