Research On The Discrete Vortex Method For Simulation Of Free-surface Forced Roll Cylinders

Vessels navigating in the ocean are prone to roll motion.Significant amplitude roll motion of a ship even poses a threat to sailing safety.The flow field induced by ship roll motion is accompanied by strong flow separation and vortex shedding,and affected by the amplitude of roll motion,wave frequency and hull appendages.So the hydrodynamic characteristics of ships roll motion are extremely complicated,and the roll motion is therefore difficult to be predicted accurately.In this paper,simulation of free-surface forced-roll cylinders is investigated,and the main technical difficulties are divided into three categories.The first is convection and diffusion of unsteady flow vortex.The second is the simulation of free-surface perturbation wave.And the third is the simulation of separated flow caused by forced roll motion.The discrete vortex method is applied in order to solve these problems,and we have done some work as follows.The basic theories and research methods of viscous flow are summarized,which fall into two general categories:the ones that need to discretize the flow(field discretization methods)and the ones that only need to specify the geometry of the domain(grid-free methods).It is concluded that the discrete vortex method has the advantage for the large Reynolds number flow separation problems,and hence it can be applied to study the flow field of roll cylinders.The fundamental principle of solving the two-dimensional viscous flow field by discrete vortex method(RVM)is then introduced.The Poisson equation is established by the stream function.The flow field is decomposed into two parts:a rotational part and an irrotational part.The rotational part is solved by discrete vortex method,and the irrotational part is solved by Complex Variable Boundary Element Method(CVBEM).Then comes the convection and diffusion of vortex in the unsteady flow field.A uniform stream past a circular cylinder is simulated.By computing the Strouhal number,the drag force,the pressure distribution around the cylinder,the formation of the vortex and the characteristics of vortex shedding,the Reynolds number range for RVM is obtained.The oscillating flow around a fixed cylinder in the unbounded domain is simulated to validate the hydrodynamic calculation results and the motion characteristics of the vortices in the unsteady flow are analyzed.For the free-surface wave perturbation problem,the reliability of the simulation of the irrotational part is verified by three numerical wave makers in the potential flow.The free surface wave perturbation is studied based on the simulation of a floating cylinder of forced oscillating motion in inviscid flows.Based on the above methods and theories for unsteady vortex motion and free-surface wave perturbation,a surface-piercing plate oscillating in the presence of surface waves with consideration of fluid viscosity is simulated and the results are in good agreement with the literature.The results of the simulation of a free-surface forced-roll cylinder show that the steady flow field is successfully obtained and the same conclusion is reached in the literature.Note that inviscid computations consistently over-predict the amplitude of waves and hydrodynamic moments.