CFD-Based Investigation of the Drag-Reduction Characteristics in Heterogeneous Autonomous Underwater Vehicle Formations

Abstract To investigate the hydrodynamic interaction and drag-reduction potential of heterogeneous autonomous underwater vehicle (AUV) formations during formation cruising, this study performs a computational fluid dynamics (CFD)-based analysis of a three-AUV heterogeneous formation consisting of one DARPA SUBOFF model and two Zebrafish 150 AUVs. The Reynolds-averaged Navier–Stokes (RANS) equations coupled with the k–ω shear stress transport (SST) turbulence model are employed to conduct numerical simulations and comparative analyses of the resistance characteristics and the pressure-field distributions for four representative configurations: tandem, triangular, parallel, and inverted triangular formations, under various inter-vehicle spacings. The numerical method is first validated against resistance test data of the SUBOFF model. The results show that the CFD prediction errors at the medium and high speeds are within 5%. That satisfies the requirements for relative hydrodynamic comparison among formation configurations in this study. On this basis, the individual drag variation ratio and the overall formation drag variation ratio are introduced to evaluate the drag-reduction performance of various formations. The results indicate that the drag-reduction capability of heterogeneous AUV formations is not determined simply by reduced inter-vehicle spacing, but is primarily governed by wake continuity, wake coverage, and lateral pressure coupling. The tandem formation forms a continuous multi-stage wake-shielding structure, with an overall formation drag variation ratio Cform of approximately from –10 to –13%, making it the most effective drag-reduction configuration among the investigated cases. The triangular formation exhibits coexistence of local wake utilization and lateral interference, and its overall drag-reduction effect is not significant. The parallel formation is mainly affected by lateral pressure coupling and gap-acceleration effects, which can easily lead to an increase in the overall resistance. The inverted triangular formation can provide local drag reduction for the large AUV at specific spacings, but this benefit is offset by the increased resistance of the two small AUVs, with the optimum overall drag variation ratio being –1.563%. The findings provide a reference for drag-reducing cruising-formation design and mission formation control of heterogeneous AUV swarms.