CFD Analysis of Drag and Internal Volume Tradeoffs in a Compact AUV with a Myring Forebody and Flat Stern

Low-slenderness-ratio, flat-ended autonomous underwater vehicles must balance hydrodynamic resistance against internal volume retention, yet classical slender-body criteria do not fully represent their coupled forebody wake response. A generalized Myring forebody was assessed for an AUV with L = 0.8 m, D = 0.2 m, and L/D = 4.0 using 53 steady three-dimensional Reynolds averaged Navier-Stokes simulations with the shear stress transport k-ω model. Gaussian process regression and the non-dominated sorting genetic algorithm II (NSGA-II) were used only for candidate-region screening; production grid direct CFD samples were used to determine nondominance. Strict fold-wise leave-one-out cross-validation gave Q2 = 0.173 globally and RMSE = 0.001399 and Q2 = 0.683 in the predefined 18-sample decision region, indicating local screening utility rather than global surrogate validation. The direct CFD audit identified 13 globally and seven locally nondominated samples; both previously selected test configurations were dominated after CFD back-substitution. Their three grid drag sequences were monotonic but non-asymptotic. Pressure drag comprised 75.88–79.20% of total drag. However, the reduction in the low-drag test configuration relative to the baseline arose mainly from a lower viscous contribution; axial pressure fields therefore indicate redistribution rather than exclusive drag-reduction causation. Paired CFD samples showed that the sign of the drag responded to N reversal between the two sampled Lnose values, whereas analytical volume increased with N in both pairs. The results reveal a discrete, configuration-dependent drag volume trade-off and local N-Lnose coupling. The rectangular regions are sampling envelopes rather than validated optimum windows, and the conclusions are restricted to steady, deeply submerged, smooth-wall bare-hull conditions.