As a critical component of unmanned naval warfare, Unmanned Underwater Vehicles
(UUVs) have garnered significant attention from major military powers. When
navigating through pycnoclines—a widespread vertical density stratification in
marine environments—UUVs generate volume effect internal waves that influence
hydrodynamic resistance. Therefore, investigating the hydrodynamic
characteristics of UUVs in pycnoclines is essential. Despite substantial
research progress, most studies focus on internal wave patterns and their
impacts on submerged vehicles, with limited exploration of UUV resistance and
surface pressure distribution. This work establishes a numerical method
according to the Reynolds-Averaged Navier-Stokes (RANS) equations, employing the
Realizable k-ε turbulence model and the Volume
of Fluid (VOF) method to capture fluid density interfaces, thereby analyzing the
hydrodynamic characteristics of UUVs in pycnoclines. Furthermore, a numerical
method was constructed, and the convergence regarding the grid and time-steps
were verified. Additionally, numerical experiments under varying navigation
speeds and depths are conducted to investigate the total resistance, frictional
resistance, wave-making resistance coefficients, and spatial variation of
surface pressure. Based on the results, the total resistance of a UUV is
positively correlated with its navigation speed. When navigating in the upper
seawater layers, the total resistance also exhibits a positive correlation with
navigation depth. However, when operating in the lower seawater layers, the
total resistance initially increases and then decreases with increasing depth,
reaching its peak level at a navigation depth of 13 m. Both increasing
navigation speed and approaching the density interface can enhance the
sensitivity of total resistance to navigation depth. The alteration in total
resistance stems primarily from changes in wave-making resistance while showing
a weaker correlation with frictional resistance. The UUV’s speed positively
correlates with pressure at locations with abrupt curvature changes on its
surface, but it has a negligible influence on pressure distribution in smooth
surface regions. Besides, navigation depth positively correlates with surface
pressure magnitude yet exerts a limited impact on pressure distribution
patterns. The findings contribute to a more complete picture of the hydrodynamic
properties of UUVs navigating through pycnoclines, offering valuable references
for optimizing UUV design and operational strategies.