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Numerical modelling of the effects of biofouling on ship resistance and propulsion characteristics

The effect of biofouling on the hydrodynamic characteristics of ship resistance and propulsion in calm water is very important from both an economic and environmental point of view. Hydrodynamic performance of a ship is disrupted because of the presence of biofouling organisms, which results in increased fuel consumption, ship speed reduction and increased emission of harmful gases. Presently, there is no comprehensive procedure, which could reliably predict the effect of biofouling on the ship hydrodynamic characteristics. Consequently, International Towing Tank Conference (ITTC) has advised scientists to present new formulae or methods based on the experimental data to determine the effect of biofouling on the ship resistance and propulsion characteristics. Since biofouling depends on many parameters and it is very difficult to predict how long will antifouling coatings prevent fouling of a ship, the proposed research is focused on the effects of predetermined surface conditions on the ship hydrodynamic characteristics. Biofouling can be classified into the soft, hard and composite fouling. In this thesis the effects of biofilm and hard fouling on ship resistance, propeller open water and ship self-propulsion characteristics are investigated. Within the proposed research commercial software package is used. The mathematical model is based on the averaged continuity equation and Reynolds Averaged Navier-Stokes (RANS) equations. Governing equations are discretised utilizing Finite Volume Method (FVM). After the analysis of several turbulence models for the closure of set of equations and their influence on the obtained ship hydrodynamic characteristics has been performed, k − Shear Stress Transport turbulence model is selected as a compromise between accuracy and computational time. Volume of Fluid method is utilized for tracking and locating the free surface. The effects of biofouling are modelled through the implementation of roughness function model within the wall function of Computational Fluid Dynamics (CFD) solver. The research is based on the wall similarity hypothesis which claims that roughness effects are limited to inner layer of turbulent boundary layer. The validity of numerical procedure is examined through verification and validation of the obtained results. The validation of the obtained numerical results for the smooth surface condition is carried out by comparison with the extrapolated towing tank results and other numerical studies available in the literature. The validation of numerical drag characterization study is performed by comparison of the obtained numerical results and experimental ones. Also, the obtained numerical results in terms of the increase in frictional resistance for a flat plate having the same length as a ship are compared with the ones obtained using the Granville similarity law scaling method. Thereafter, the applicability of the proposed CFD approach is demonstrated on the example of three full-scale merchant ships. Also, the newly proposed performance prediction method for fouled surfaces is presented, which can account for fouling effects on the ship performance. The applicability of this method is demonstrated for fouling conditions with lower fouling rates. Thus, a robust and rapid assessment of the effects of biofouling on the ship hydrodynamic characteristics in calm water is enabled.

biofouling ; ship hydrodynamics ; Computational Fluid Dynamics ; Reynolds Averaged Navier-Stokes ; roughness function ; performance prediction method

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