Exploring the limits of RANS-VoF modelling for air cavity flows

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.