Scale effects on DARPA Suboff maneuvering characteristics from free-running CFD simulations

This study presents a free-running CFD investigation of scale effects on the steady and dynamic maneuvering characteristics of the DARPA SUBOFF, without relying on empirical corrections. Time-dependent simulations were conducted for four geometrically similar models (λ = 24.0, 12.0, 2.4, and 1.0) using commonly adopted CFD approaches, including the Moving Reference Frame (MRF) approach for propeller action, the Rigid Body Motion (RBM) technique for rudder deflection, and the SST k–ω model with all-y⁺wall treatment for turbulence closure. Turning circle tests (δ = ±20.0 deg) and 20–20 zigzag maneuvers were performed to assess scale sensitivity. Between the smallest-scale model (λ = 24.0) and the full-scale (λ = 1.0), the steady turning diameter (STD') decreased by 28.9%, indicating more compact trajectories at higher Reynolds numbers, while the surge velocity ratio (STS') decreased by 30.5%, reflecting greater forward speed loss during tight turns. In dynamic maneuvers, overshoot angles (OSA) increased significantly with scale, most notably +149.8% for 2nd OSA and +107.3% for 3rd OSA, highlighting stronger inertial effects and slower yaw damping at full scale. These results underscore the necessity of accounting for Reynolds number sensitivity when extrapolating model-scale to full-scale predictions, particularly for fully submerged vehicles where scale effects are more pronounced than in surface vessels.