Flow around ships and underwater vehicles

The flow generated by a surface or an underwater vehicle has immediate consequences on its drag as well as on the behavior of the ship in waves, and thus on the comfort onboard, and on the stability and maneuvering characteristics. The flow arriving at the propeller plane and that generated by the propeller itself govern the propulsion efficiency but, due to the unsteadiness of the loading, they are also responsible for the transmission of structural vibrations and noise to the interior of the vessel. The noise generated by the propeller may also have an environmental impact, at least on the most busy ship routes. The propeller noise is even more important for submarines, which have to be as silent as possible during their mission. The breaking of the bow wave, beside increasing the ship drag, generates a highly turbulent flow with large air entrainment. The turbulent bubbly flow is a very important source of noise, which affects the signal received by the sonar located in the fore part of the hull. Moreover, the fragmentation process of the entrained air cavities produces tiny bubbles that persist for long time after the passage of the ship and scavenge surfactants while rising toward the free surface. This affects the behavior of the capillary waves and leads to the formation of a long wake, which is easily detectable by radars. When the ship moves in waves or starts maneuvering operations, the flow becomes far more complicated. Viscous effects at the keels generated large vortex structures that contribute to the damping and stabilize the vessel. Motivated by the above considerations, a brief overview of the most recent research advances is provided here. The discussion presented is only qualitative, whereas the interested reader can find a deeper and more quantitative discussion on the cited literature. The basic hydrodynamic aspects that are already well known are omitted. The work is divided into two sections, one dedicated to ships and the second one to underwater vehicles. For the ship, the discussion concerns the breaking of the bow wave, the separated flow at the stern, the flow about propellers, and the interaction with hull and rudders and the flow about ships in maneuver or in unsteady motion. For underwater vehicles, the topics are the flow generated by a submarine in steady motion, the flow about the propeller and the interaction with hull, sail, and rudder, and, finally, some considerations are given for supercavitating vehicles