Numerical Study On Drag Reduction Characteristic Of Two-phase Flow Of Underwater Moving Body Based On Entropy Generation Analysis

Compared with the conventional moving body in single-phase fluid such as air or water,the body around bubbles attached on the surface often has many advantages,such as low resistance,low energy consumption,long voyage,etc,which leads it have a good development prospect in the development of underwater weapon system.However,the unstable phenomena of bubble formation,development and collapse in the near-wall two-phase flow of this kind of moving body will have a series of effects on its performance.In this context,the numerical method is applied to study the typical two-phase flow characteristics such as cavitation and boiling of near-wall flow of moving body,and the flow entropy generation in thermodynamics is associated with the total drag of the moving body.From the point of view that drag works as an irreversible process of energy-consumed,the entropy generation is used to evaluate the drag of the moving body in the two-phase flow and to explore a new idea for predicting the drag of the two-phase flow.Firstly,the overseas and domestic research status and method about cavitation drag reduction,boiling drag reduction and entropy generation are introduced in detail.At the same time,the paper introduces cavitation model,evaporation-condensation model and entropy generation model according to need for drag reduction of two-phase flow based on entropy generation.On this basis,several typical moving bodies are modeled and meshed.The computational model is validated by grid independence analysis,and the computational results are compared with the literature results to verify the computational model.For the cavitation drag reduction problem,in addition to comparing with the experimental and computational results in the literature,the existing experimental conditions are used to inject the moving body from air into water.In this process,the cavity configuration is compared with the results of numerical simulation to further ensure the correctness of the cavitation model.Then,the cavitation flow characteristics of moving body are calculated.The range of cavitation number is σ≥0.04,which includes supercavitation zone(0.04≤σ≤0.22),partial cavitation zone(0.22≤σ≤1.37)and non-cavitation zone(σ≥1.37).In the supercavitation,the curves of total drag,relative length and relative diameter of cavity versus the cavitation number are plotted,and the calculated results are compared with the experimental values of Savchenko.This again verifies the accuracy of the cavitation model.The calculation results show that the total drag coefficient is almost linear with cavitation number in the supercavitation region,and the relative length and diameter of cavity increase with the decrease of the cavitation number;the generation of cavity has a great influence on the friction resistance,and the friction resistance increases with the decrease of the number cavitation number in the non-cavitation region;In the partial cavitation,as the low values of the cavitation number are approached,friction resistance increases first,then decreases.In the supercavitation,friction resistance increases with the cavitation number decreasing;The flow entropy generation mainly concentrates on the liquid-vapor interface: about half of the entropy generation concentrates on the head of the cavity,then the tail of the cavity;Under the help of the contours,we find that the existence of vortices has a greater impact on the entropy generation: the same as the flow entropy generation concentrate,vortices distributes at the head and tail of the cavity.Finally,the calculation of boiling drag reduction for spheres and cylinders was carried out.Water temperature 98 C,moving body temperature 98 C,125 C,150 C and 175 C are selected as the temperature boundary conditions.Single-phase flow occurs at 98 C,and two-phase flow occurs at 125 C,150 C and 175 C.The results show that the total drag,frictional resistance and pressure drag of the moving body decrease with the increase of temperature,and the ratio of differential pressure drag to total drag is over 90%.The frictional resistance is greatly affected by temperature.Combining with Newton’s frictional law,the variation of frictional resistance can be well explained from the viscosity coefficient and velocity gradient.There is no obvious relationship between the entropy generation of the temperature difference and the total drag of the moving body,but the relationship between the total drag and the flow field entropy generation is linear,which is still valid at different temperatures and angles of attack.