| For a cavity ship,since the surface of the submerged body is ventilated by gas,it can achieve a good drag reduction effect.Since the submerged body of the cavity ship is close to the free surface,the process of ventilating the submerged body of the cavity ship involves the interaction of solid,liquid and gas.The main flow phenomenon is the cavitation flow near the free liquid surface.Research on the evolution characteristics of cavitation near the free surface can solve the problems of the stability of cavitation and the interaction of the convection field of the cavity ship encountered in engineering applications,which is of great significance to the design and development of the cavity ship.Therefore,this paper mainly carried out the research on the cavitation dynamics characteristics near free surface,and the research on the cavitation flow field of the submerged body of the cavity ship near the free surface.First,aiming at the near free surface cavitation phenomenon that generally exists around the submerged body of the cavity ship,the hydrofoil is used as the basic model to study the cavitation flow of the NACA66 hydrofoil under the condition of near free surface.By comparing the cavity shape and hydrodynamic performance of the hydrofoil with or without a free surface,it is found that the free surface reduces the length of the natural cavitation,resulting in a decrease in the lift and drag coefficient of the hydrofoil,and an increase in the frequency of cavity shedding.In addition,through the Dynamic Mode Decomposition(DMD)of the velocity field,it is found that the free surface reduces the energy contained in each mode and reduces the scale of the vortex.By comparing the hydrofoil cavitation at different depths,it is found that the free surface has a certain inhibitory effect on the hydrofoil cavitation.In order to study the cavitation flow field characteristics of a near free surface hydrofoil with one end fixed and one end free,the tip leakage vortex(TLV)cavitation flow of NACA0009 hydrofoil was simulated near the free surface.By comparing two working conditions with or without a free surface,the influence of the evolution of the cavitation of the leaking vortex on the tip of the hydrofoil on the free surface and the characteristics of the hydrodynamic load are studied.In the working condition near the free surface,the lift and drag coefficients of the TLV hydrofoil decrease,but the pulsation frequency of the lift coefficient increases.At the same time,by comparing the vortex structure and vortex transport characteristics under the two working conditions with and without a free surface,the flow field evolution mechanism of TLV cavitation near the free surface is analyzed.Through the comparison of the first two modes based on the DMD method,it is found that the main flow characteristics of TLV cavitation(including coherent structure and energy distribution)are closely related to the existence of free liquid surface.Then,based on the research of the cavitation flow of the hydrofoil near the free surface,a ventilated submerged body with a front propeller was designed,and the coupling effect of the front propeller and the ventilated submerged body under no free liquid surface was evaluated.By comparing the vortex structure and cross-sectional vorticity under three different working conditions,it is found that the submerged body will interfere with the development characteristics of the propeller blade tip vortex downstream,and affect the hydrodynamic performance of the structure,including thrust,drag and lift.The hydrodynamic performance of the propeller under three different working conditions was extracted,and it was found that a ventilated submerged body would reduce the efficiency of the propeller and reduce the resistance.At the same time,the ventilated cavitation has a significant wall effect,which keeps the main characteristic areas of the speed field and vorticity field of the propeller away from the surface of the submerged body.The analysis of the blade tip vortex structure shows that the submerged body can suppress the attenuation of the downstream blade tip vortex,so that the propeller blade tip vortex is easier to develop downstream.In the actual case,the part of the underwater submerged body of the cavity ship is close to the free surface during the voyage.Therefore,a comparative study of the ventilated submerged body with or without free surface is carried out,and the mechanism of the influence of the free surface on the submerged body of the cavity ship is analyzed.Compared with the condition of no free surface,the effect of the free surface on the front propeller and the flow field of the ventilated submerged body is studied through the cavity shape,the hydrodynamic performance of the submerged body,the velocity field,and the vortex field.It is found that the function of the free surface promotes the drag reduction effect of the ventilated cavity.The speed change on the surface of the submerged body is suppressed,the flow field reaches a stable state earlier,and the development of the vorticity in the horizontal direction is promoted.Finally,based on the previous research results,a numerical model of a cavitating vessel is designed.Numerical simulations are carried out on the motion response and load characteristics of the cavitating vessel under wave conditions.The pitch,heave motion and acceleration of the cavitating ship under wave conditions are analyzed.The hydrodynamic performance parameters such as lift resistance and surface pressure are analyzed.By comparing the results of numerical simulation between the conventional monohull and the cavitation hull,it is found that the cavitation hull has better seakeeping and stability than the conventional monohull,and can sail more smoothly in the waves.In addition,the floating state of the cavity ship will change after being ventilated,and it has obvious drag reduction and anti-rolling effects,and the surface pressure load of the cavity ship is also reduced. |
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