Contents lists available at ScienceDirect Ocean Engineering journal homepage: www.elsevier.com/locate/oceaneng Numerical analysis of ventilated cavity flow over a 2-D wall mounted fence Luka Barbaca ⁎ , Bryce W. Pearce, Paul A. Brandner Australian Maritime College, University of Tasmania, Launceston, Tasmania 7250, Australia ARTICLE INFO Keywords: Ventilated cavity Wall mounted fence CFD ABSTRACT Ventilated cavity flow over a 2-D wall mounted fence is numerically investigated using a viscous approach. An implicit unsteady compressible solver was used with a RANS k ω − SST turbulence model and VOF approach to capture the cavity interface. The simulations were carried out for a fixed fence height based Froude number and constant outlet pressure. Cavity topology, wall pressure distributions and the resulting hydrodynamic forces were determined as a function of ventilation rate, degree of fence immersion in the oncoming wall boundary layer and degree of confinement of the flow domain. It was found that with an increase in ventilation rate, lift increases and drag decreases resulting in a greater hydrodynamic efficiency (lift to drag ratio) of the fence-wall system. With increase in immersion of the fence in the boundary layer, both lift and drag decreased, while the lift to drag ratio increased. Variation in the degree of confinement had a large influence on the flow, with the reduction in lift and hydrodynamic efficiency observed for the more confined conditions. 1. Introduction Ventilated (also termed ‘artificial’) cavities can be utilized for drag reduction in marine applications. Drag can be decreased by forming an air bubble/layer between the solid surface and water to reduce the skin friction (Ceccio, 2010), or by increasing the pressure on the down- stream surface of the cavitating body to reduce the form drag (Franc and Michel, 2004). The main parameter used to characterize these flows is the cavitation number, σ p p ρU =( − )/0.5 c c ∞ ∞ 2 , where p ∞ is the reference free-stream pressure, p c is the pressure inside the cavity, ρ is the liquid density and U ∞ is the reference free-stream velocity. Similar cavities can be formed naturally (vapour filled) when the liquid is subjected to vapour pressure (p v ), with in this case σ σ p p ρU = =( − )/0.5 c v v ∞ ∞ 2 . It has been shown that, for the same σ c value, natural and ventilated cavities exhibit comparable behaviour except for the differences in closure physics (May, 1975; Kunz et al., 1999). To sustain a cavity long enough to be applicable for drag reduction purposes, σ c has to be in the order of 0.1 (Kawakami and Arndt, 2011). As p v is small, to naturally achieve such a low σ c in a practical flow, a high free-stream velocity (i.e. >100 knots) is required. In the ventilated cavity case p c is controlled by the flux of injected air, and such low σ c values can be achieved independent of free-stream conditions. Consequently, devices utilising ventilation can be efficiently used at lower operating velocities and/or higher free-stream pressures (i.e. deeper submersion), leading to a much broader range of potential application. Depending on the location of the cavity closure, a cavity is classified as either a ‘partial’ cavity or a ‘supercavity’. The former is defined where the closure region is located on the surface of the body and if the closure is rather downstream in the wake it is termed a supercavity (Franc and Michel, 2004). The application of both ventilated partial and supercavitation to high-speed underwater bodies has been of particular interest post the second world war for military applications (Reichardt, 1946; Waid, 1957), but also there has been some interest in a commercial context (Brentjes, 1962). Past studies into ventilated flows have mainly focused on the reduction of skin friction in hydrodynamic applications. There has been extensive research into the use of axisymmetric ventilated cavities for drag reduction of underwater projectiles in the second half of last century, with an ongoing interest in the topic. A review of basic physical properties and calculation methods for axisymetric ventilated cavities is given by Semenenko (2002). The other application of substantial interest has been in the use of ventilated partial cavities on the underwater part of ship hulls, referred as ‘air-lubrication’. The injection of air is used to create a stable cavity for ships operating in the range of speeds that are not sufficient to enable detachment of a natural cavity from a geometric discontinuity in the hull. To date several semi- displacement and planning boats using this phenomenon have been built, with a reported resistance decrease in the range of 10–30% (Latorre, 1997; Butuzov et al., 1999; Matveev et al., 2009). Additionally, some work has been done on implementing air-lubrica- tion system on full displacement ships in a commercial context (Mizokami et al., 2010; Surveyor, 2011). A comprehensive overview http://dx.doi.org/10.1016/j.oceaneng.2017.06.018 Received 12 May 2016; Received in revised form 15 February 2017; Accepted 7 June 2017 ⁎ Corresponding author. E-mail address: Luka.Barbaca@utas.edu.au (L. Barbaca). Ocean Engineering 141 (2017) 143–153 0029-8018/ © 2017 Elsevier Ltd. All rights reserved. MARK