Copyright © 2018 Authors. This is an open access article distributed under the Creative Commons Attribution License , which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. International Journal of Engineering & Technology, 7 (4.35) (2018) 589-595 International Journal of Engineering & Technology Website: www.sciencepubco.com/index.php/IJET Research paper An Analysis of Two-Dimensional Stratified Gravity Current Flow using Open FOAM W.K Lam 1 *, L. Chan 2 , H. Hasini 3 , A. Ooi 4 1 Department of Mechanical Engineering, UNITEN, Jalan IKRAM –UNITEN 43000, Kajang, Selangor, Malaysia 2 Department of Mechanical Engineering, University of Melbourne, Parkville VIC 3010, Australia *Corresponding author E-mail: lzhchan@unimelb.edu.au Abstract Direct numerical simulations (DNSs) of two-dimensional stratified gravity-current are simulated using OpenFOAM. Three different as- pect ratio, h 0 /l 0 (where h 0 is the height of the dense fluid and l 0 is the length of the dense fluid) are simulated with stratification ranging from 0 (homogenous ambient) to 0.2 with a constant Reynolds number (Re) of 4000. The stratification of the ambient air is determined by the density difference between the bottom and the top walls of the channel ( ρ b - ρ 0 , where ρ b is the density at the bottom of the domain and ρ 0 is the density at the top). The magnitude of the stratification (S=ԑ b /ԑ) can be determined by calculating the reduced density differ- ences of the bottom fluid with the ambient fluid (ԑ b = (ρ b - ρ 0 )/ ρ 0 ) and the dense fluid with the ambient fluid (ԑ = (ρ c -ρ 0 )/ ρ 0 , where ρ c represents the density of the dense fluid). The configuration of the simulation is validated with a test case from Birman, Meiburg & Ungraish and the contour and front velocity (propagation speed) were in good agreement. The gravity current flow in the stratified ambi- ent is analyzed qualitatively and compared with the gravity current in the homogenous ambient. Gravity current in homogenous ambient (S=0) and weak stratification (S=0.2) are supercritical flow where the flow is turbulent and Kelvin-Helmholtz (K-H) billow formed be- hind the gravity current head. The front location of the gravity is reduced as the stratification increase and denotes that the front velocity of the gravity current is reduced by the stratification. Keywords: Aspect ratio; Direct numerical simulation; Multiphase and particle-laden flows; OpenFOAM; Stratification 1. Introduction Gravity current or density current is a type of fluid flow that oc- curs mainly in the horizontal domain due to density difference of two fluids. Gravity current is observed in a natural phenomenon such as sandstorms, powder-snow avalanches, pyroclastic flow and haboob. Simpson [1] has reviewed a wide range of experimental studies on gravity current flow. Gravity current can be categorized into two types which is compositional gravity cur- rent (can be Boussinesq, where the density differences between two fluids are less than 5 % or non-Boussinesq) and particle- driven gravity current (presence of sediments in the fluid). Gravity current flow has been widely studied with a variety of lock con- figurations such as lock shape [2] (elliptical release [3], [4] and circular release [5]), lock depth and initial aspect ratio [6]. Gravity current flow is determined by the Froude number ( Fr) which is a dimensionless parameter defined as the ratio of inertial forces relative to the gravitational forces ( Fr = u f / √h max , where u f is the front velocity and the h max is the maximum height of the current). When the Froude number is less than 1/π (Fr < 1/π), the gravity current resides in the subcritical flow regime. The flow is slow, and the formation of the gravitational waves which propa- gates faster than the front of the gravity current occurs. As the Froude number increases and becomes greater than 1/π ( Fr > 1/π), the gravity current flow reaches the supercritical regime, where the flow becomes turbulent. The critical flow regime occurs in between the subcritical and supercritical regime, when the Froude number is 1/π (Fr = 1/π). The propagation of the gravity current into a stratified ambient is the concern of this study. An experimental and numerical study has been conducted by Maxworthy et al. [7] to study the relation- ship between the Froude number (Fr), density ratio (R) and the effects of the internal waves on the dynamics of the gravity cur- rent flow. He found that the propagation of the wave is faster than the gravity current in the subcritical regime. Ungarish & Huppert [8] conducted a similar study using shallow-water approximation (SW) and the result shows excellent agreement with the experi- mental results presented by Maxworthy et al. [7]. He stated that the stratification will reduced the propagation speed of the gravity current compared to a homogenous ambient where the density is constant in wall normal direction. Ungarish (2005) [9] extended the stratified gravity current study with the ‘Dam-break’ release characteristic. In that study, several extensions of the stratified gravity current has been studied, such as shallow-water (SW) equation extended to non-linear stratification, gravity current re- leased from an elliptical reservoir and axisymmetric configuration. Ungarish (2006) [10] developed the theory for the propagation of a steady gravity current speed into a linearly stratified ambient by generalizing the Benjamin’s (1968) [11] theory on the steady state propagation of the gravity current into a homogeneous ambient. Birman et al. [12] then examined the theory on the dependency of the front velocity on the stratification (S) with varies depth ratio (ɸ=h 0 /L y ) by manipulating the height of the channel (L y ). It was found to work well for the weak stratification (S ≤ 0.5). Ungarish (2008) [13] also analyzed the energy-balances of an axisymmetric gravity current in the homogenous ambient and linearly stratified ambient with a two-dimensional geometry.