Multiscale Simulation of Micro and Nano Gas Flows

微/纳流装置中的气体流动通常是多尺度的，具有连续和高度稀薄的流动区域。在稀薄区域，气体不处于局部热力学准平衡，这意味着传统的Navier-Stokes流体动力学方程是不充分的描述。动力学方法，如直接模拟蒙特卡罗方法，是适当的，但计算昂贵，特别是对低速流动。虽然在混合方法中将精确的动力学方法与计算效率高的连续模型相结合将使实际的多尺度微/纳米流模拟成为可能，但这也存在主要问题，例如，非平衡信息未保留在连续模型中，但动力学方法需要。相反，我们建议在这个项目中创建一个新的多尺度格子玻尔兹曼（LB）技术，在高度稀薄的流动区域中的高阶模型（具有大量的离散速度）和较低阶模型（具有少量的离散速度）之间切换。将开发一种智能切换方法，该方法动态评估局部流场的非平衡水平（随着流场的演变，非平衡水平可能会随时间变化），并在不同阶LB模型之间进行切换，同时考虑所需的精度和计算费用。由于这种多尺度LB方案使用在相同的理论框架中开发的模型，因此将避免动力学-连续混合方法所面临的解决方案耦合问题。我们的技术将使非平衡微/纳米流物理学的开发成为可能：在这个项目中，我们将使用它来优化Knudsen压缩机的设计-一个没有任何运动部件的真空泵。

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.