Journal of Advanced Research in Fluid Mechanics and Thermal Sciences 45, Issue 1 (2018) 109-115 109 Journal of Advanced Research in Fluid Mechanics and Thermal Sciences Journal homepage: www.akademiabaru.com/arfmts.html ISSN: 2289-7879 Numerical Investigation on the Influence of Gas Area Fraction on Developing Flow in a Pipe Containing Superhydrophobic Transverse Grooves Kok Hwa Yu 1,* , Yew Heng Teoh 1 , Mohd Azmi Ismail 1 , Chih Fang Lee 2 , Farzad Ismail 2 1 School of Mechanical Engineering, Engineering Campus, Universiti Sains Malaysia, 14300 Penang, Malaysia 2 School of Aerospace Engineering, Engineering Campus, Universiti Sains Malaysia, 14300 Penang, Malaysia ARTICLE INFO ABSTRACT Article history: Received 20 March 2018 Received in revised form 14 May 2018 Accepted 15 May 2018 Available online 17 May 2018 This study presents a numerical investigation on the entrance length for developing flow in a pipe having alternating superhydrophobic grooves and ribs arranged transversely to the flow direction. Flows at low Reynolds number (i.e.,  = 1) over superhydrophobic transverse grooves of  = 0.1 are considered. The influences of superhydrophobic surfaces on radial velocity profile development, centerline velocity distribution and hydrodynamic entrance length estimation are examined. Numerical results show that the hydrodynamic entrance length arising from flow over superhydrophobic transverse grooves are longer as compared to that of smooth wall. It is also found that the resulted entrance length is directly influence by the relative surface area occupied by grooves (i.e., gas area fraction). When the gas area fraction is larger, it would yield an increase in the hydrodynamic entrance length. Keywords: Entrance length, water-repellent, surface roughness, laminar flow, wall slip Copyright © 2018 PENERBIT AKADEMIA BARU - All rights reserved 1. Introduction By definition, the length of the hydrodynamic entry region before the flow reaches a fully developed flow condition is often designated as the hydrodynamic entrance length (  ). This length is an essential measurement that determines the distance needed for a flowing liquid before a fully developed flow condition could prevail. In most existing literatures, studies pertaining to the hydrodynamic entrance length have been focused on smooth surface [7, 12] where no-slip condition prevails. For an internal flow passing through smooth wall in a pipe of diameter , the hydrodynamic entrance length can be approximated using correlation presented by Durst et al., [5] given by    = 0.619) . + 0.0567) .  /. . (1) * Corresponding author. E-mail address: yukokhwa@usm.my (Kok Hwa Yu) Penerbit Akademia Baru Open Access