| With the continuous development of economic globalization,ship transportation has become one of the most important material transportation channels.While the ship is moving,its power is mainly used to overcome the frictional resistance caused by the water flow o n the wall.Therefore,carrying out research on ship drag reduction technology and improving energy utilization efficiency is of great significance to the realization of"energy saving and emission reduction"and"green ships".At present,most scholars mainly conduct research on drag reduction technology through bionics.Among them,through the study of the upper layer of the marine life chain,such as sharks,dolphins,and tuna,most scholars have studied the walls of micro-grooves,super-hydrophobic coatings,flexible walls,etc.Single-factor drag reduction technology was studied.The drag reduction method in organisms is mainly coupled drag reduction.For example,a shark swimming at high speed in the ocean has a shield scale structure.In the gap betwe en the shield scales,the shark will continuously secrete mucus,which is essentially a surface structure and Coupling of drag reducers to reduce drag.In the field of coupling drag reduction between trenches and drag reducers,there is less research on th e micron scale（50～100μm）under high Reynolds number,and more in-depth research on drag reduction mechanism is needed to explore the effective range of coupling drag reduction under different factors.Based on the related theories of computational fluid dynamics and viscoelastic fluid dynamics,this paper carried out related researches on two drag reduction methods of surface micro-groove drag reduction and additive drag reduction.Firstly,a numerical model of micro-groove surface drag reduction characte ristics was established,the drag reduction effect of micro-scale grooves under different working conditions was calculated,the flow characteristics of the fluid on the surface of the groove were analyzed,and the drag reduction mechanism on the surface of the micro-groove was explored.The influence of such factors as the dimensionless dimension of the groove and the water flow speed on the drag reduction effect of the groove is discussed.At the same time,a numerical model of viscoelastic fluid was esta blished through UDF,the drag reduction effect of viscoelastic fluid under different working conditions was calculated,and the drag reduction mechanism of the three-dimensional flow of viscoelastic fluid was explored.Based on the above research work,the coupling drag reduction mechanism between the surface of the micro groove and the drag reducer was further studied,and the basic law of coupling drag reduction was summarized.The study concluded:（1）As the Reynolds number increases,the drag reduction rate of the micro-groove structure first increases and then decreases.When Re H=2.8×105 and the dimensionless groove width S+=14.64,the maximum drag reduction rate is 19.11%.When Re H=7.96×105 and the dimensionless groove width S+=34.97,the resistance increasing effect is 6.89%.Because under the appropriate Re number,the micro-groove structure will form fluid"slip"in the longitudinal direction,reducing the frictional resistance between the wall and the fluid;while at high Reynolds number,when a finer vortex structure is formed,the vortex The spin will invade into the trench valley,so there will be an effect of increasing resistance.In addition,the drag reduction effect of the groove structure is also related to the groove tip structure.The tip structure will hinder the movement of the spanwise vortex and improve the drag reduction effect.（2）After the smooth plate and groove structure are set as viscoelastic fluid,the friction coefficient Cf of the smooth plate decreases with the increase of Re number.When Re H=2.8×105,the minimum friction coefficient Cf is 2.44×10-3;and the friction coefficient Cfof the groove structure first decreases and then increases with the increase of Re number.When Re H=7×104,the minimum friction coefficient Cfis 1.34×10-3.It shows that after adding the viscoelastic fluid,the drag reduction effect does not continue to increase with the increase of the Reynolds number,but needs to be within a certain Reynolds number range for the viscoelastic fluid to play a drag reduction effect.（3）When set to viscoelastic fluid,although elastic stress is introduced,the value of elastic stress is smaller,and the original viscous stress is greatly reduced,so the total wall shear stress level drops.From the perspective o f vortex structure,it is mainly believed that the occurrence frequency and intensity of coherent structures near the wall surface in the turbulent drag reduction flow of viscoelastic fluid are greatly reduced,which inhibits the generation of turbulent ba nds,thereby reducing resistance.（4）At the same incoming flow velocity,after the groove structure is set as a viscoelastic fluid,the number of vortex structures close to the groove wall is significantly reduced.The drag reduction effect of the viscoel astic fluid of the groove structure shows a decreasing trend with the increase of S+.When S+is equal to 11.52,the drag reduction agent has the greatest strengthening effect on the groove drag reduction,and the groove drag reduction rate can be strengthened by 29.7%.In this paper,the drag reduction technology of micro groove surface and additives is studied systematically.The research results provide data support for the application of Underwater Bionic drag reduction technology,which has good guiding significance for the application and promotion of Ship Drag Reduction Technology,and lays the foundation for practical engineering application... |
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