Numerical investigation of coarse granular flow of the Coandă effect-based collector over logarithmic spiral surface for deepsea mining

As an emerging hydraulic harvester for deep-ocean polymetallic nodules, Coandă effect-based collectors aim to minimize seabed disturbance while maintaining high collector efficiency. This study employs CFD-DEM simulations to analyze a logarithmic spiral collector with non-dimensional parameters (s/R = 1, 2/3, 1/2), focusing on jet dynamics and particle trajectories. Results reveal that the near-wall jet entrains ambient flow, creating a high-pressure zone at the collection pipe inlet. The merged flow ascends asymmetrically toward the tube, inducing a reflux vortex. Jet expansion is quantified via normalized half-width (y_1/2m/h), showing a linear growth rate (∼10 % per s/h increment) along the logarithmic spiral. Turbulence effects are characterized by wall Reynolds number (Re_w) and local Reynolds number (Re_b): Re_b increases monotonically with s, while Re_w first decreases then increases due to delayed boundary layer development, independent of s/R. Particle motion exhibits four distinct lift phases, with higher s/R ratios enhancing sustained horizontal jet velocity components, thereby accelerating particle velocity during lift and revolution stages. These findings systematically clarify interactions between spiral geometry, flow dynamics, and particle transport, offering direct guidance for optimizing Coandă effect-based collectors to reduce seabed environmental impact. The proposed metrics (y_1/2m/h, Re_w, Re_b) establish quantitative benchmarks for evaluating hydraulic performance in deep-sea mining applications.