Study on the impact of asymmetric design on the hydrodynamic characterization of the disk-shaped autonomous underwater helicopter surging near the seabed

Hydrodynamic characteristics of autonomous underwater vehicles (AUVs) are significantly changed when they are close to the seabed boundary. In this study, the hydrodynamic performance of a newly developed disk-shaped AUV—the autonomous underwater helicopter (AUH) near the bottom boundary is investigated. The hull-shape modification into an asymmetric hull geometry was proposed to improve the near-bottom hydrodynamic performance and guide future work on AUH profile optimization. It was indicated that multiple zero-velocity regions are formed beneath the vessel when it surges near the bottom boundary. This induced the intricate interactions of boundary layers between the AUH and the seabed. Moreover, abrupt evolution on the profile of the AUH hull could induce strong shear and vortices between the vessel and the boundary, potentially causing phenomena like flow separation and reattachment. The bottom profile of the AUH was designed to reduce the bulge and increase the area of the horizontal region, which considerably reduced the drag and meanwhile increased the stability. Furthermore, two mechanisms depending on the hull profile near the bottom boundary, namely, the blocking effect that leads to both higher drag and lift and the acceleration effect that reduces the drag and lift, were suggested to explain the alteration of the hydrodynamic performance near the bottom boundary.