Achieving a High Accuracy Numerical Simulations of the Flow Around a Full Scale Ship

Abstract

The hydrodynamic performance of ships may be improved by the retrofit of Energy Saving Devices (ESDs). These devices are typically seen in the aft part of the ship hull and act by lowering the ship resistance, conditioning the fluid in front of the propeller and/or recovering energy from the rotational swirl of the fluid leaving the propeller. In the case of a retrofit of an existing ship no straight forward solution exists. In order to find a beneficial design that will improve hydrodynamic performance, a successful and accurate initial assessment of the flow around a hull is of the most importance. Once the flow around the hull is scrutinized in detail, and required flow changes are determined, a ship designer can progress with designing an Energy Saving Device specifically tailored to have a desired effect. This paper presents a high quality numerical evaluation of the flow around a ship hull in the full scale using a sophisticated DES model that was successfully validated against the sea trials. The findings from the numerical analysis will identify the potential improvements in the hydrodynamic performance of the ship that could be achieved by ESD. INTRODUCTION In the shipping industry greenhouse gas emission is already subject to the International Maritime Organization (IMO) regulation. The existing criteria and the one coming into force are stringent. In order to enhance ship efficiency, all possible improvements are required. Among them, fuel cells, air lubrication systems, the use of natural resources such as wind, solar energy and magnus effect cylinders can be found on the market. These devices are promising; however, one of the most inexpensive and convenient is to improve hydrodynamic efficiency of the Hullform itself. 1 Contact author: [email protected] This can be achieved with the hydrodynamic Energy Saving Devices (ESDs) (1, 2). These devices can be retrofitted to the propeller, hull or rudder with the primary goal of improving hydrodynamic efficiency and reducing the fuel consumption of the ship. They are typically seen in the aft part of the ship hull and act by lowering the ship resistance, conditioning the fluid in front of the propeller and/or recovering energy from the rotational swirl of the fluid leaving the propeller. In order to find a beneficial design that will improve hydrodynamic performance, it is essential to conduct detailed and accurate initial assessment of the flow around a hull. This can be achieved by performance assessment both, experimentally and/or numerically. When using CFD numerical simulations, achieving a high quality simulations is paramount. The commercial CFD codes provide various turbulence modeling approaches, however, not all of them are suitable for these scenarios. Duvigneau et al. (3) compared between the RANS k-ω and Reynold’s Stress (RSM) turbulence models at ship model scale. For the case of a tanker with a full hullform that generates a strong bilge vorteces, the applied turbulence model had a large influence on how the flow field at the stern resolved – mainly depending on the model’s ability to capture anisotropic vortical flow. They concluded that the RSM was found to be superior than the k-ω model. Starke et al. (4) conducted a CFD at both, a model and a full scale. He remarked the importance of capturing the anisotropic vorticity in the wake, and that the difference between turbulence models was of a lesser magnitude at full-scale. Yang et al. (5) evaluated the performance difference of a RSM and a Realizable k-ε model for nominal wake at model scale. They found that the RSM provided better agreement with wake measurement data at model scale.