# Computational Fluid Dynamics

## Introduction

There are so many methods and models in computational fluid dynamics. In RDC3, we are trying to broaden our ability to solving fluid dynamics related problems in various scales. In this short page, we will introduce some methods and models that we are currently conducting in NECTEC.

## Microscale-Mesoscale fluids

In this scale, the assumption of continuum breakdown (i.e. Knudson number is larger than one, Knudson number, a nondimensional parameter related to the distance between molecules). All equations derived from the continuum assumption can not be used in this scale. There are, however, many methods for solving fluid problems in this scale. Examples for those would be, MD(Molecular Dynamics), DPD (Dissipative Particle Dynamics) and LBM (Lattice Boltzmann Method). Each of them has its own advantages and disadvantages, of course there is no best method to deal with any problems.

We are currently looking at Lattice Boltzmann Method (LBM). LBM is, by name, lattice-based computing. The reasons behind this include vasatility of the method, programming complexity and ability in collaboration. Lattice Boltzmann methods (LBMs) are a class of mesoscopic particle based approaches to simulate fluid flows. They are becoming a serious alternative to traditional methods for computational fluid dynamics. The lattice Boltzmann approach developed from lattice gases. Although it can also be derived directly from the simplified Boltzmann BGK equation. In lattice, particles live on the nodes of a discrete lattice. The particles jump from one lattice node to the next, according to their (discrete) velocity. This is called the propagation phase. Then, the particles collide and get a new velocity. This is the collision phase. Hence the simulation proceeds in an alternation between particle propagations and collisions. The lattice Boltzmann method is a powerful technique for the computational modeling of a wide variety of complex fluid flow problems including single and multiphase flow in complex geometries. The rules governing the collisions are designed such that the time-average motion of the particles is consistent with the Navier-Stokes equation.Traditional methods for solving incompressible flows, such as finite-difference or finite-element, require solution of a Poisson equation for the pressure term, which is induced by the mass-continuity equation and the momentum-conservation equation. In the lattice-Boltzmann approach, this time-consuming step is avoided because the incompressibility requirement has been relaxed and the effects of pressurechanges are controlled by an equation of state rather than a Poisson equation. It can be argued that the conventional methods most closely related to the lattice-Boltzmann method are the pseudocompressible algorithms for solving incompressible fluid flows. Some preliminary results of the Lattice-Boltzmann will be presented in the next update.

## Macroscale fluids, DNS for fluid flow problems

Comments/Questions: Send email to sirod.sirisup@nectec.or.th.