In this work, the slug flow regime in an air-water horizontal pipe flow has been simulated using the CFD technique. The variables identified to characterise the slug regime are the slug length and slug initiation. Additionally, the pressure drop and the pressure distribution within the simulated pipe segment have been predicted. The volume of fluid method was employed assuming unsteady, immiscible air-water flow, constant fluid properties and coaxial flow. The model was developed in the STAR-CCM+ environment, and the grid was designed in the three dimensional domain using directed mesh. A grid independency study was carried out through the monitoring of the water velocity at the outlet section. 104,000 hexahedral cells for the entire geometry were decided on as the best combination of computing time and accuracy. The simulated pipe segment was 8 m long and had a 0.074 m internal diameter. Three cases of air-water volume fractions have been investigated, where the water flow rate was pre-set at 0.0028 m3/s, and the air flow rate was varied at three dissimilar values of 0.0105, 0.0120 and 0.015 m3/s. These flow rates were converted to superficial velocities and used as boundary conditions at the inlet of the pipe. The simulation was validated by bench marking with a Baker chart, and it had success- fully predicted the slug parameters. The computational fluid dynamics simulation results revealed that the slug length and pressure were increasing as the air superficial velocity increased. The slug initiation position was observed to end up being shifted to a closer position to the inlet. It was believed that the strength of the slug was high at the initiation stage and reduced as the slug progressed to the end of the pipe. The pressure gradient of the flow was realised to increase as the gas flow rate was increasing, which in turn was a result of the higher mean velocity.
hexahedral mesh, slug flow, slug flow characteristics, superficial velocity, two-phase flow
 Ghorai, S. & Nigam, K.D.P., CFD modeling of flow profiles and interfacial phenomena in two-phase flow in pipes. Chemical Engineering and Processing: Process Intensification, 45(1), pp. 55–65, 2006. http://dx.doi.org/10.1016/j.cep.2005.05.006
 Brennen, C.E., Fundamentals of Multiphase Flow, Cambridge University Press: New York, 2005. http://dx.doi.org/10.1017/CBO9780511807169
 Hill, T.J., Fairhurst, C.P., Nelson, C.J., Becerra, H. & Bailey, R.S., Multiphase Production through Hilly Terrain Pipelines in Cusiana Oilfield Colombia. SPE Annual Technical Conference and Exhibition. Society of Petroleum Engineers, 1996. http://dx.doi.org/10.2118/36606-MS
 Orell, A., Experimental validation of a simple model for gas–liquid slug flow in horizontal pipes. Chemical Engineering Science, 60(5), pp. 1371–1381, 2005. http://dx.doi.org/10.1016/j.ces.2004.09.082
 Baker, O., Simultaneous flow of oil and gas. Oil and Gas Journal. 53, pp. 185–195, 1954.
 Rashimi, W., Choong, T.S.Y., Chuah, T.G., Aslina, S. & Khalid, M., Effect of interphase forces on two-phase liquid- liquid flow in horizontal pipe. Journal - The Institution of Engineers, Malaysia, 71(2), pp 35–40, 2010.  Al-Hashimy, Z.I., Al-Kayiem, H.H., Kadhim, Z.K. & Mohmmed, A.O., Numerical simulation and pressure drop prediction of slug flow in oil/gas pipelines. WIT Transactions on Engineering Sciences, 89, 2015, ISSN 1743-3533.
 Wongwises, S., Khanhaew, W. & Vetchsupakhun, W., Prediction of liquid holdup in horizontal stratified two-phase flow. Thammasat International Journal of Science and Technology, 3(2), pp. 48–56, 1998.
 Ranade, VV., Computational Flow Modeling for Chemical Reactor Engineering, Vol. 5. Academic press, 2001.
 Iacovides, H., Kelemenis, G. & Raisee, M., Flow and heat transfer in straight cooling passages with inclined ribs on opposite walls: an experimental and computational study. Experimental Thermal and Fluid Science, 27(3), pp. 283–294, 2003.
 Brackbill, J.U., Kothe, D.B. & Zemach, C., A continuum method for modeling surface tension. Journal of Computational Physics, 100(2), pp. 335–354, 1992. http://dx.doi.org/10.1016/0021-9991(92)90240-Y
 Menter F.R., Two-equation eddy-viscosity turbulence models for engineering applications. AIAA Journal, 32(8), pp. 1598–1605, 1994. http://dx.doi.org/10.2514/3.12149
 Yang, Yi., Ming G. & Xinyang J., New inflow boundary conditions for modeling the neutral equilibrium atmospheric boundary layer in SST k-w model. Proceedings of the Seventh Asia Pacific Conference on Wind Engineering, Taipei, Taiwan, November 8–12, 2009.
 Vallée, C., Höhne, T., Prasser, HM. & Sühnel T., Experimental investigation and CFD simulation of horizontal stratified two-phase flow phenomena. Nuclear Engineering and Design, 238(3), pp. 637–646, 2008. http://dx.doi.org/10.1016/j.nucengdes.2007.02.051
 Wilcox, D.C., Turbulence Modeling for CFD, DCW Industries Inc: La Canada, California, 2006.
 Science Flow, Flow-3D, available at http://www.flow3d.com/cfd-101/cfd-101-implicitexplicit-schemes.html, 2013 (accessed 20 October 2013).
 Aagaard, O., Hydroelastic analysis of flexible wedges. PhD thesis, Norwegian University of Science and Technology, 2013.
 Chica, L., FSI study of internal multiphase flow in subsea piping components. PhD diss, University of Houston, 2014.