| Under the requirements of advocating energy conservation,many companies in shipping industry have paid attentions to fuel consumption,emission reduction and optimization of energy efficiency.For the purpose of improving the ship energy efficiency,this paper focused on the navigation resistance and has analyzed the relationship between this and other factors,which aims to explore the methods of optimizing energy efficiency.In this study,according to the case of 5000 PCTC(Pure Car Truck Carrier)and the relevant theory about computational fluid dynamics,through numerical simulation,navigation resistance,ship propulsion efficiency and energy level have been analyzed under different navigational statuses and external conditions,combing with the towed ship model test.In addition,the susceptibility and mapping relation between external factors and resistance,energy efficiency have been obtained.The surface roughness or morphology of the hull is one of factors which affects the navigation resistance.The simulation result shows that within a certain range of roughness height,with the roughness height increases,the navigational resistance increases and the increased resistance within a certain range can be processed empirically by considering the coefficient of roughness.The roughness constant works as correction factor which can affect the navigational resistance by adjusting the uniformity of the sand.In this paper,three types of trench morphology were selected and the results showed that within the selected range of dimensions,the resistance by applying rectangular trench morphology is less than that of triangle and arc-shaped trenches topography.The rectangular trench has obvious eddy currents on both the flow side and the backflow side.In addition,the velocity at eddy center point is near 0 and the higher the speed is,the more turbulent the trend becomes.The floating state of the ship is also an important factor which affects the sailing resistance and the ship’s energy efficiency.The result showed that the corresponding waterline surface shape and wet surface area will change when a vessel lean over,resulting in changes in hull resistance and energy efficiency.When the trim angle range from-2° to 2°,the resistance is the least when the cruising resistance is about 1°.The resistance reaches the smallest when the sailing resistance is 0.5 while it reaches the highest when the sailing resistance is 1.5°.The analysis based on the optimal heel and the optimal trim provides the theoretical basis for analyzing the effect of this combined contribution to the energy efficiency of the target ship.Based on MATLAB-Simulink,the simulation model of EEOI and the main propulsion system have been established.The hull surface conditions,heel angle,pitch angle,and speed have been taken as the input factors of the model and results have been analyzed from the dynamic mapping relationship.It has suggested that the value of EEOI increases sharply at higher relatively speed compared with that at lower speed,which suggests that the navigation speed has effect on EEOI.With the surface height roughness increases,the value of EEOI for the target ship increases and propulsion efficiency decreases.When the trim angle ranges from-1.5° to-2° and the heeling angle reaches to about 1.5°,EEOI and propulsion efficiency have changed a lot because the wet surface area has changed at this time.In conclusion,combined with simulation cloud picture of the trim/heeling angle,EEOI and propulsion efficiency,the relationship between ship energy efficiency and navigational status under different hull surface conditions and at different speeds has been obtained.It provides analysis basis for adjustment model for ship power efficiency and hull resistance as well as the method system for energy saving and drag reduction of ships based on tribology. |
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