Wing body junctions in ship hydrodynamics

Abstract

Starting with the 1st of January 2013, all ships greater than 400 gross tons have to comply with design or operational energy efficiency index, in order to reduce the greenhouse emissions. From the naval architect’s point of view, the emission reduction measures can be hydrodynamic, structural, technological and operational. The hydrodynamic measures, which are the first that can be taken into consideration in order to reduce the EEDI, are materialized through the optimization of the hull. The Naval Architect may interfere on the bulbous bow, hydrodynamic shoulders, bulb stern, transom or appendages, the drag being then modified by reducing the wave, viscous pressure or frictional resistance components. Another way to improve the hydrodynamics of a ship is the use of Energy Saving Devices (ESD). These are appendages, mounted on the ship hull, developed to improve the flow near the propeller, which operate in the non-uniform wake field of the ship. The flow mechanism around ESDs comes down to wing-body juncture flow problems and, due to their application to the ship appendage flow, they have recently received much attention in ship hydrodynamics. Despite its simple geometric configuration, the wing-body junction flow is a very complicate flow due to the so-called horseshoe vortex system determined by the adverse pressure gradient induced by the presence of the obstacle and the three-dimensional boundary layer separations around the junction. The horseshoe vortex flow affects the drag, lift and causes a persistent lack of uniformity in the wake and is also considered as one source of the noises, vibration and unsteady inflow for the propeller.