Background: Air lubrication techniques have the potential to significantly reduce frictional drag, benefiting sustainable employability of ships. However, these techniques are not yet widely applied in the shipping industry, since a complete understanding of the relevant two-phase flow physics is still lacking. Objective: This article aims to explore the limitations and capabilities of RANS-VoF modelling to numerically model air cavity flows. Methods: Simulations were performed including numerical uncertainty verification and compared to experimental data for an external cavity. To study the effect of reduced eddy-viscosity at the cavity interface, two types of eddy-viscosity correction functions were applied next to a base case, i.e., a power and a Gaussian function. Results: The cavity length and thickness as well as the velocity profiles in the boundary layer just upstream, in the middle and downstream of the external cavity compare well to experimental data. However, in contrast to what was found experimentally, a too strong coupling was found between the computed cavity profile and the air pressure at the nozzle and too much air leaks out of the cavity. For the same nozzle air pressure as in the experiments, similar cavity dimensions were found, but the air flow rate is overestimated by a factor of five. Conclusions: The used methodology is capable of predicting the cavity profile and velocity profiles at different stream-wise locations in the boundary layer around the cavity with respect to experimental findings. However, a mismatch was found in the determination of the required air flow rate for the cavity, which is hypotesized to be mainly caused by the incorrect turbulence modelling around the interface and the advection of a smeared air-water interface in the reattachment zone. This is a direct consequence of the used VoF method. The exact mechanism for air discharge at the cavity closure is still not clear.

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This article aims to link the physical modelling of air cavities to their numerical modelling with a RaNS solver. The largest challenge in predicting the cavity characteristics numerically lies in correctly modelling their closure region. According to Reboud et al. [1], the commonly over-predicted eddy-viscosity at the water-gas interface reduces the development of the re-entrant jet and can thus prevent the occurrence of shedding. Computations were carried out using two different eddy-viscosity correction functions: a power function as described by Reboud et al. [1] and Coutier-Delgosha et al. [2] and a Gaussian function proposed by Rotte et al. [3]. The Gaussian correction function does not contain large gradients when the local density approaches pure gas or liquid. It also has no bias towards one of the fluids and is expected to improve the stability of the simulation and the physical modelling of the problem.@en

Air lubrication techniques are very promising in reducing ship drag. It has been demonstrated that air cavity applications can realise propulsive power reduction percentages of 10-20% due to the reduction of the frictional resistance [1, 2]. However, a complete understanding of the two-phase flow physics involved with air cavity flows is still missing. Multiphase CFD methods can help to get a better understanding of these physics. The largest challenge in predicting the air cavity characteristics lies in the correct modelling of their closure (reattachment) region [3, 4]. In this region the separated air-water flow transforms into a more dispersed flow. The transformation is partly caused by instabilities in the two-phase flow. This article aims to link the physical modelling of the relevant phenomena to their numerical modelling. The link to the numerical modelling is addressed with an emphasis on different RaNS and hybrid RaNS-LES turbulence models. The article is based on the available literature in the public domain and knowledge gained in research projects carried out at Delft University of Technology and Maritime Research Institute Netherlands (MARIN).@en