Hydrodynamic characteristics of large-length-to-diameter-ratio unmanned underwater vehicles for seabed energy mineral exploration

Abstract This study numerically investigates the hydrodynamic characteristics of a series of unmanned underwater vehicles (UUVs) with large length-to-diameter (L/D) ratios of 13, 18, 22 and 28 for seabed energy mineral exploration. Four geometric models were investigated by extending the mid-body cylindrical section of the benchmark SUBOFF AFF-1 model. Simulations were conducted by solving the Reynolds-Averaged Navier–Stokes equations with the Shear Stress Transport k-ω turbulence model. Key findings reveal that at zero angle of attack (α), the frictional drag component constitutes over 80% of the total drag, a proportion that increases with the L/D ratio. Lift, drag, and side force increase approximately linearly with increasing α, with the side force being slightly larger than the lift under identical deflection angles. The slope and x-intercept of the line of action of the resultant hydrodynamic force increases with α but decreases with the L/D ratio. Furthermore, the streamwise location of vortex structure inception shifts towards the bow with increasing α, and vortex structures form at smaller α for UUV models with larger L/D ratios. Highlights 1) Frictional drag contributes over 80% of the total drag at zero angle of attack (α), establishing it as the dominant factor for large length-to-diameter (L/D) ratio unmanned underwater vehicles (UUVs).2) Quantitatively elucidates the coupled effect of α and L/D ratio on the slope and intercept of the line of action of the resultant hydrodynamic force.3) Reveals the unique characteristics that vortex structure inception shifts upstream with increasing α, a phenomenon pronounced in large L/D ratio configurations.4) Proposes a novel zonal lift model (bow-generation, mid-body-dominance, stern-loss) for slender body hydrodynamics.5) Provides direct guidance for the hull design and drag-reduction optimization of large L/D ratio UUVs.