Numerical Analysis of Water-Injection Drag Reduction on a Flat Plate

Water injection is a promising alternative to traditional air lubrication for reducing ship hull drag and improving energy efficiency. Addressing the limited research on the efficacy of water lubrication on ships, this novel study is the first to numerically evaluate its performance on a flat-plate model, systematically investigating key operational and geometrical parameters. The rectangular flat plate model of finite thickness represents a 1:56 scale of the Japan Bulk Carrier hull. The study conducts Reynolds-Averaged Navier–Stokes (RANS) simulations using the commercial CFD package STAR-CCM+ and systematically investigates the effects of injection angle, velocity ratio, flow rate, Reynolds number, and plate orientation. The results indicate that an injection angle of 60–90° is optimal, with an ideal velocity ratio (UInj/Ub) of approximately 1.5, resulting in a drag reduction of up to 38.8%. The flow-rate ratio (QInj/Qw) also serves as a pertinent scaling parameter, with an optimum at 1.1. The study found that the primary drag reduction mechanism is the decrease in skin friction, which, unlike pressure-driven effects, is robust across different plate orientations. These findings underscore the potential of water injection as a scalable and effective strategy for maritime decarbonisation, exhibiting performance that is robust and stable across a wide range of Reynolds numbers and plate orientations.