The hydrodynamic performance of an underwater vehicle is critically influenced by flow discontinuities such as flood holes. This study numerically investigates the effects of tandem square flood holes on the forces, moments, and flow physics of a micro underwater vehicle model. Large-eddy simulations are performed for one to three holes over a jet velocity ratio range of 0–2.5. The results reveal distinct trends: the lateral force, vertical force, and residual drag coefficients increase monotonically, whereas the total drag, frictional drag, and pitching moment coefficients first decrease and then increase, exhibiting a clear optimum. This non-monotonic behavior is attributed to the competition between local friction reduction due to jet-induced flow sheltering and the subsequent drag rise from enhanced mixing. The underlying vortex dynamics show that the jets form a dominant pair of counter-rotating vortices, which breaks the wake symmetry and propagates downstream. At high jet velocities, this vortex system asymmetrically modulates the rudder-tip vortex, directly generating significant lateral forces and pitching moments. By controlling the total flow rate, we demonstrate that the long-range impact on the rudder is governed by the total momentum flux rather than the number of holes. These findings provide physical insight into the fluidic sheltering–mixing competition of tandem flood holes.
Lin et al. (2026) studied this question.