engleski

Comparing Ship Self-Propulsion Modelling Using The Actuator Disc And Fully Discretized Propeller Model

Accurate prediction of the self-propulsion point of a ship represents a challenging task in the field of naval architecture. The significance of this task is high considering the new regulations regarding lower greenhouse emissions. Computational Fluid Dynamics (CFD) based on the Finite Volume (FV) method is often used to describe the ship-propeller interaction and to predict self-propulsion characteristics due to higher costs and complexity of the experimental methods, especially in a preliminary design stage. In this study, detailed analysis of the global and local flow characteristics of a self-propelled ship at Fn = 0.26 is presented. The ship used in the present study is a model-scale KRISO Container Ship (KCS). A fully discretized propeller and actuator disc (AD) models for propeller modelling are used and compared in this study. Two different implementations of the AD models were used: a model which mimics the thrust effect only via the pressure jump, and model where both thrust and torque effects are taken into account, by applying pressure and tangential velocity jump boundary conditions. The actuator disc approach is usually sufficient for obtaining accurate self-propulsion integral values such as thrust and torque [1]. However, if detailed local flow features are important, the ship propeller needs to be modeled as fully discretized and rotating. Therefore, to reduce overall computational time while achieving accurate representation of global and local characteristics at the same time, a procedure combining the AD and discretized propeller methods with the use of dynamic overset grids [2, 3] and PI controller is proposed. The aforementioned models and PI controllers are implemented within the Naval Hydro Pack library, which is based on foam- extend, a community-driven fork of the OpenFOAM software. Overset mesh approach uses multiple separately meshed grids, usually one background and one or more body-fitted grids, to discretize the computational domain. Interpolation of the field data between the grids is performed in the fringe layers. An incompressible, two-phase, viscous numerical model is applied in this study. Turbulence is taken into account with the k − omega SST model [4]. Level Set (LS) [5, 6, 7, 8] approach is applied in order to capture the interface between two simultaneously solved phases, while the Ghost Fluid Method (GFM) [9] is employed to ensure sharp distribution of pressure and density at the free surface. Coupling of the pressure field and rigid body motion is resolved with enhanced approach introduced by Gatin et al. [10], while the coupling of flow and rigid body equations is resolved with the PIMPLE algorithm. The application of LS and GFM proved to be reliable for capturing the sharp interface between the two phases, while the interpolation of the field data between the overset grids did not affect local and global self-propulsion characteristics significantly. Overall, good agreement with the experimental data for both local and global characteristics is achieved (Table 1 and Fig. 1).

Self-propulsion ; Discretized propeller ; Actuator disc ; Overset Grid ; Level Set

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