Effect of Length-to-Diameter Ratio on the Hydrodynamic Resistance of an AUV at Constant Displacement: A Validated CFD Study

Abstract

This paper examines the influence of the hull length-to-diameter ratio (L/D) on the hydrodynamic resistance of a generic axisymmetric AUV, represented by the DARPA SUBOFF bare hull. Three-dimensional steady Reynolds–averaged Navier–Stokes (RANS) simulations were conducted in STAR-CCM+ using the shear-stress transport (SST) k–ω turbulence model. The numerical procedure was validated against the SUBOFF towing-tank data of Liu and Huang: after a grid-independence study, the predicted resistance differs from the measurements by up to 3.1% across six forward speeds (1.27% at the reference condition). A family of five hulls spanning L/D = 6.44–11.76 was then generated by affine scaling at constant displacement, and each hull was simulated at six forward speeds. At every speed the resistance, expressed through a volumetric drag coefficient, has a minimum at L/D = 8.57, with off-optimum penalties below about 2%; the coefficient decreases with increasing Reynolds number. A slight change in the relative ranking of the off-optimum hulls was observed between low and high speeds, suggesting a shift in the balance between frictional and pressure-related resistance mechanisms. The findings provide quantitative guidance for selecting the slenderness of axisymmetric AUV hulls.