ARTICLE IN PRESS JID: IJMF [m5G;April 28, 2018;3:57] International Journal of Multiphase Flow 000 (2018) 1–16 Contents lists available at ScienceDirect International Journal of Multiphase Flow journal homepage: www.elsevier.com/locate/ijmulfow An experimental study of cavity flow over a 2-D wall-mounted fence in a variable boundary layer Luka Barbaca ∗ , Bryce W. Pearce , Paul A. Brandner Australian Maritime College, University of Tasmania, Launceston, Tasmania, 7250, Australia a r t i c l e i n f o Article history: Received 22 September 2017 Revised 21 February 2018 Accepted 13 April 2018 Available online xxx Keywords: Cavitation Ventilation Wall-mounted fence Experiment a b s t r a c t Ventilated and natural cavity flow over a 2-D wall-mounted fence immersed in a boundary layer is exper- imentally investigated in a cavitation tunnel. Cavity topology, upstream wall pressure distribution and the resulting hydrodynamic forces were determined as a function of ventilation rate, fence immersion in the oncoming boundary layer and free-stream conditions. Cavities exhibit a typical re-entrant jet behaviour, which is the primary mechanism of air/vapour entrainment into the main flow. Some entrainment is also observed via the turbulent break-up at the cavity surface, the intensity of which increases with deeper immersion of the fence within the wall boundary layer. A similar cavity topology, apart from some dif- ference in the wake, is observed for ventilated and natural cavities at the same flow conditions. This similarity is also present in the relations between all other parameters investigated. It was found that with a decrease in cavitation number lift (i.e. force normal to the wall) increases and drag (i.e. force nor- mal to the fence) decreases, resulting in an increased hydrodynamic efficiency of the wall/fence system. With an increase in fence immersion in the boundary layer, lift and drag both increase at the same rate, resulting in a constant lift-to-drag ratio. © 2018 Elsevier Ltd. All rights reserved. 1. Introduction Efficient sea transport and drag reduction of marine vehicles are interrelated topics of interest to the maritime community. Various methods based on the use of gaseous layers/bubbles for reduction of the skin friction of the wetted part of a vessel have been ex- tensively investigated since mid-last century. With reference to the extent of gaseous layer/bubble, these methods can be categorized into four groups: bubble drag reduction (BDR); gas layer/film drag reduction (GLDR); partial cavity drag reduction (PCDR) and super- cavity drag reduction (SCDR) (Ceccio, 2010; Mäkiharju et al., 2013; Murai, 2014). Of particular interest for this study are the latter two, which involve creation and maintenance of large gaseous pockets covering a significant portion (i.e. partial cavity) or the whole body (i.e. supercavity). Additionally, partial cavity drag reduction tech- niques can be divided into those using the body designs with and without cavity lockers (also labelled as ‘arrestors’ or ‘sloped beach’) used to control the flow at the cavity closure (Kopriva et al., 2008; Mäkiharju et al., 2013). The origin of the gaseous cavity can be twofold. A cavity can form naturally, i.e. due to the phase change of the water to vapour, or artificially by injecting an incondensable gas (typically air) into ∗ Corresponding author. E-mail address: luka.barbaca@utas.edu.au (L. Barbaca). the wake of a cavitator. The latter process is commonly referred to as ‘ventilation’ and the resulting cavity termed a ventilated cav- ity. The main parameter used to characterize these cavitating flows, both natural and ventilated, is the cavitation number, σ c = ( p ∞ − p c )/0.5ρU 2 ∞ , where p ∞ is the reference free-stream pressure, p c is the pressure inside the cavity, ρ is the liquid phase density and U ∞ is the reference free-stream velocity. Past studies have shown that both natural and ventilated cavities present at the same flow conditions have a largely similar behaviour except for differences in the closure physics (May, 1975; Kunz et al., 1999). To form a cavity applicable for drag reduction in high-speed applications the σ c value usually has to be of the order of 0.1. For a naturally cav- itating flow p c = p v (where p v is vapour pressure) and achieving such a low σ c generally requires impractical operational speeds in excess of 90 knots (Kawakami and Arndt, 2011). In contrast, for the ventilated case p c is also controlled by the flux of injected air and sufficiently large cavities can be formed at lower, more practical, speeds making the technique applicable to a broad range of ap- plications. Ventilation has also been investigated for drag reduc- tion of low-speed ships (Butuzov et al., 1999) and lifting surfaces (Kopriva et al., 2008), where practicable cavities were achieved for somewhat different cavitation number values (i.e. negative σ c for slow ships and σ c ≈ 1 for lifting surfaces). The use of ventilated supercavities (SCDR) has been extensively studied in the context of axisymmetric underwater projectiles con- https://doi.org/10.1016/j.ijmultiphaseflow.2018.04.011 0301-9322/© 2018 Elsevier Ltd. All rights reserved. Please cite this article as: L. Barbaca et al., An experimental study of cavity flow over a 2-D wall-mounted fence in a variable boundary layer, International Journal of Multiphase Flow (2018), https://doi.org/10.1016/j.ijmultiphaseflow.2018.04.011