Research On The Hydrodynamic Characteristic Of A Oscillating Foil Near Free Surface

As a new type propeller,the oscillating wing propeller is of great significance to the application of oscillating-winged ships for its excellent performance in environmental protection and energy saving.The oscillating frequency range needs to be studied for the positive and negative thrust.Meanwhile,the impact on Fluctuations of free surface,uniform flow and other factors are also need to be considered.In this paper,computational fluid dynamics(CFD)software STAR-CCM+ is used,and a proper vibration frequency range of the oscillating wing is determined along with the flow velocity condition,which is instructive for designers to obtain a stabilized propulsion solution.The flow field is solved by the simple Green’s function,and the theoretical basis of the problem of oscillating wing in the unbounded flow data is given.In the aspect of viscous flow,the VOF method based on the theory of viscous flow,the VOF method based on the theory of viscous hydrodynamics is used to deal with the free surface,and the N-S equation is solved in the condition that the hydrofoil makes forced motion.The domain mesh is generated in the STAR-CCM+ platform.For different quantity of trim mesh,the optimal grid scale is calculated and the convergence analysis is carried out with time step taken into account.Considering the actual situation of the oscillating wing under the water surface,proper software parameters and turbulence models are selected to establishan integrate and specific research method.In this paper,the motion of pure lift,pure pitching and coupled motion of heave and pitching is studied by using the above research method.The results are compared with the literatures,and the trend is good and it can be seen that the difference between the potential flow solution and the viscous flow solution at each vibration frequency is approximately equal at each oscillating frequency and the negative value of thrust can be analyzed.In the simulation process,the wave height,velocity distribution and pressure distribution can be observed.This paper simulates pure heave oscillating wing using first order Stokes wave,the maximum lift coefficient,drag coefficient and maximum torque coefficient are analyzed,it is found that the above coefficient decreases with the increase of the Froude number decreases.Therefore,when the oscillating wing moves under the wave,the flow rate is too high or too low to ensure that the oscillating wing has sufficient thrust and the oscillating wing must be moved normally under the appropriate flow rate.The numerical results show that the oscillating frequency is very important to maintain the normal propulsion force in both free surface or regular wave to make sure the propulsion force is positive.It is found that the regular wave can make the propulsion force bigger than before through a long time domain analysis so that the results can give more technical details to maintain a steady thrust in the flow.The numerical results show that the oscillating amplitude of wing motion will be negative at certain frequencies,and in the pure pitch movement and coupled movement of pitch and heave,the thrust is negative.The numerical method can provide a more accurate value for the limit of positive and negative value when the osciallating propeller doing pure heave motion.