This paper reports on the comparison between computational simulations and experimental measurements of a surface vessel in steady turning conditions. The primary purpose of these efforts is to support the development of physics-based high fidelity maneuvering simulation tools by providing accurate and reliable hydrodynamic data with relevance to maneuvering performances. Reynolds Averaged Unsteady Navier Stokes Solver (URANS): CFDSHIPIOWA was used to perform simulations for validation purposes and for better understanding of the fundamental flow physics of a hull under maneuvering conditions. The Propeller effects were simulated using the actuator disk model included in CFDShip-Iowa. The actuator disk model prescribes a circumferential averaged body force with axial and tangential components. No propeller generated side forces are accounted for in the model. This paper examines the effects of actuator disk model on the overall fidelity of a RANS based ship maneuvering simulations. Both experiments and simulations provide physical insights into the complex flow interactions between the hull and various appendages, the rudders and the propellers. The experimental effort consists of flow field measurements using Stereo Particle-Image Velocimetry (SPIV) in the stern region of the model and force and moment measurements on the whole ship and on ship components such as the bilge keels, the rudders, and the propellers. Comparisons between simulations and experimental measurements were made for velocity distributions at different transverse planes along the ship axis and different forces components for hull, appendages and rudders. The actuator disk model does not predict any propeller generated side forces in the code and they need to be taken into account when comparing hull and appendages generated side forces in the simulations. The simulations were compared with experimental results and they both demonstrate the cross flow effect on the transverse forces and the propeller slip streams generated by the propellers during steady turning conditions. The hull forces (include hull, bilge keels, skeg, shafting and strut) predictions were better for large turning circle case as compared with smaller turning circle. Despite flow field simulations appear to capture gross flow features qualitatively; detailed examinations of flow distributions reveal discrepancies in predictions of propeller wake locations and secondary flow structures. The qualitative comparisons for the rudders forces also reveal large discrepancies and it was shown that the primary cause of discrepancies is due to poor predictions of velocity inflow at the rudder plane.

The hydrodynamic design of the bilge keels is important for the ship’s resistance and roll performance. It also affects the ship wake field at the propeller plane and can greatly influence the propulsor performances in terms of noise, efficiency and cavitation. The objective of this work is to investigate the effect of bilge keels on the hull flow during steady turns for a displacement ship with a skeg and a bow dome. An Unsteady Reynolds Averaged Navier Stokes Solver (URANS) CFDShip-Iowa, Version 4, developed at the University of Iowa is used to simulate the flow around the Naval Surface Warfare Center-Carderock Division (NSWCCD) ship model# 5617 with bilge keels at different steady turning conditions. The effect of separated flows caused by the bilge keels and the skeg during steady turns on the flow distributions at the propeller plane will greatly influence the flow at the propeller planes. It was observed that during a high speed port turn at full rudder angle, the onset flow to the port side bilge keel was mainly influenced by the flow around the bow dome and the onset flow to the starboard side bilge keel was subject to the free stream hull flow. The drift angle varies along the bilge keel span during steady turning and complex vortical flow structures were developed on the leeward side of the bilge keels due to flow separations caused by the flow over the tip of the bilge keel from the windward side to the leeward side. The vortical flow generated by the starboard bilge keel also merged with the separated flow caused by the skeg and form a streamwise vortical structure that was convected downstream into the propeller plane. The wake field at both port and starboard propeller planes were analyzed from the simulation results. It can be concluded from the analysis that the starboard side propeller plane was subject to a uniform cross flow and the port side propeller plane was subject to a cross flow that consisted of both cross flow component and a mean swirl that was caused by the streamwise vortical flow generated by the flow separation upstream. The cross flow component at the propeller planes can effectively produce side force affecting the lateral motion of the ship. It can be concluded from the simulations that the bilge keels have great influence on the wake distributions at the propeller planes and can affect the propeller performance during maneuvering in terms of hydrodynamic and structural loadings. Great care should be taken to ensure that the bilge keels be designed properly in the future not just for both seakeeping and propulsion, but also for maneuvering.

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