Computational uncertainty in hybrid atomistic-continuum frameworks

This paper was presented at the 3rd Micro and Nano Flows Conference (MNF2011), which was held at the Makedonia Palace Hotel, Thessaloniki in Greece. The conference was organised by Brunel University and supported by the Italian Union of Thermofluiddynamics, Aristotle University of Thessaloniki, University of Thessaly, IPEM, the Process Intensification Network, the Institution of Mechanical Engineers, the Heat Transfer Society, HEXAG - the Heat Exchange Action Group, and the Energy Institute.Over the past decade micro and nanofluidics emerged as vital tools in the ongoing drive towards the development of nano-scale analysis and manufacturing systems. Accurate numerical modelling of the phenomena involved at these scales is ssential in order to speed up the industrial design process for nanotechnology. However a parameter often ignored in hybrid simulations is the uncertainty level introduced in the numerical modelling of phenomena taking place at micro and nanoscales. The main interest of the present study is the propagation of the inherent atomistic fluctuations to the continuum solver in the case of multiscale modelling and hybrid solvers

Microchannel fluid flow and heat transfer by lattice boltzmann method

This paper was presented at the 4th Micro and Nano Flows Conference (MNF2014), which was held at University College, London, UK. The conference was organised by Brunel University and supported by the Italian Union of Thermofluiddynamics, IPEM, the Process Intensification Network, the Institution of Mechanical Engineers, the Heat Transfer Society, HEXAG - the Heat Exchange Action Group, and the Energy Institute, ASME Press, LCN London Centre for Nanotechnology, UCL University College London, UCL Engineering, the International NanoScience Community, www.nanopaprika.eu.Micro flow has become a popular field of interest due to the advent of micro electromechanical systems (MEMS). In this work, the lattice Boltzmann method, a particle-based approach, is applied to simulate the two-dimensional micro channel fluid flow. We simulated fluid flow and heat transfer inside microchannel, the prototype application of this study is micro-heat exchangers. The main incentive to look at fluidic behaviour at micron scale is that micro devices tend to behave much differently from the objects we are used to handling in daily life. The choice of using LBM for micro flow simulation is a good one owing to the fact that it is based on the Boltzmann equation which is valid for the whole range of the Knudsen number. Slip velocity and temperature jump boundary conditions are used for the microchannel simulations with Knudsen number values covering the slip flow. The lattice Bhatnagar-Gross-Krook single relaxation time approximation was used. The results found are compared with the Navier-Stokes analytical and numerical results available in the literature and good matches are observed

Molecular dynamics simulations of confined liquids in nanochannels with rough walls

During the past few decades Micro-Electromechanical systems (MEMS) have been increasingly used in various engineering domains ranging from electronics to biological sciences as nowadays they can be massively produced in numerous shapes and with various compositions. Additionally, the development of the manufacturing techniques has allowed MEMS to be easily integrated into devices and expand their applications as sensors and actuators. The future of MEMS seems to be more than promising; however the small scales involved in this type of devices give rise to phenomena that cannot be treated by continuum simulations such as Computational Fluid Dynamics (CFD) or Computational Structural Dynamics (CSD). On the contrary, Molecular Dynamics (MD) Simulations are considered to be an effective approach in investigating the flow behaviour and the rheological properties of liquids in the nanoscale. The aim of this PhD project is to establish and implement Molecular Dynamics Models for the investigation of nano-scale liquid flows and the fluid properties in nanochannels with rough walls. This thesis uses MD to investigate the effect of nano-scale roughness on the slip length, the fluid viscosity and the Kapitza resistance. Rough nanochannel walls have been modelled with the help of the multivariate Weierstrass - Mandelbrot (W-M) function which has been used in the past to describe fractally rough surfaces being common in nature. A number of different approaches have been used to extract the aforementioned thermodynamic and flow properties including Equilibrium Molecular Dynamics (EMD) and Non-Equilibrium Molecular Dynamics (NEMD) Simulations. The outcomes of this research suggest that surface roughness can greatly affect the flow behaviour of highly confined liquids as well as their thermodynamic behaviour. Therefore they could potentially be used as a first step for the selection of the surface treatment and finishing techniques of MEMS devices according to the desired fluid behaviour

Advances and challenges in computational research of micro and nano flows

This paper presents an overview of past and current research in computational modelling of micro- and nanofluidic systems with particular focus on recent advances in multiscale modelling. Different mesoscale and hybrid molecular-continuum methods are presented. The contributions of these methods to a broad range of applications, as well as the physical and computational modelling challenges associated with the development of these methods, are also discussed

DSMC-LBM mapping scheme for rarefied and non-rarefied gas flows

We present the formulation of a kinetic mapping scheme between the Direct Simulation Monte Carlo (DSMC) and the Lattice Boltzmann Method (LBM) which is at the basis of the hybrid model used to couple the two methods in view of efficiently and accurately simulate isothermal flows characterized by variable rarefaction effects. Owing to the kinetic nature of the LBM, the procedure we propose ensures to accurately couple DSMC and LBM at a larger Kn number than usually done in traditional hybrid DSMC-Navier-Stokes equation models. We show the main steps of the mapping algorithm and illustrate details of the implementation. Good agreement is found between the moments of the single particle distribution function as obtained from the mapping scheme and from independent LBM or DSMC simulations at the grid nodes where the coupling is imposed. We also show results on the application of the hybrid scheme based on a simpler mapping scheme for plane Poiseuille flow at finite Kn number. Potential gains in the computational efficiency assured by the application of the coupling scheme are estimated for the same flow.Comment: Submitted to Journal of Computational Scienc

A time-parallel framework for coupling finite element and lattice Boltzmann methods

International audienceIn this work we propose a new numerical procedure for the simulation of time-dependent problems based on the coupling between the finite element method and the lattice Boltzmann method. The procedure is based on the Parareal paradigm and allows to couple efficiently the two numerical methods, each one working with its own grid size and time-step size. The motivations behind this approach are manifold. Among others, we have that one technique may be more efficient, or physically more appropriate or less memory consuming than the other depending on the target of the simulation and/or on the sub-region of the computational domain. Furthermore, the coupling with finite element method may circumvent some difficulties inherent to lattice Boltzmann discretization, for some domains with complex boundaries, or for some boundary conditions. The theoretical and numerical framework is presented for parabolic equations, in order to describe and validate numerically the methodology in a simple situation

Meshfree method for the stochastic Landau-Lifshitz Navier-Stokes equations

The current study aimed to develop a meshfree Lagrangian particle method for the Landau-Lifshitz Navier-Stokes (LLNS) equations. The LLNS equations incorporate thermal fluctuation into macroscopic hydrodynamics by addition of white noise fluxes whose magnitudes are set by a fluctuation-dissipation theorem. Moreover, the study focuses on capturing correct variance and correlation computed at equilibrium flows, which are compared with available theoretical values and found very good agreement