Study Of The Strongly Nonlinear Interaction Between Wave-current And Horizontal Cylinder

Wave current interaction is a common phenomenon in real sea.The load on the offshore structures mainly results from wave and current.The ocean structures,such as the oil platforms,consist of various horizontal and vertical cylinders.In recent years,the researches mainly focus on the load on vertical cylinders in the wave and current environment.However,very limited amount of researches have been conducted on the horizontal cylinders.In engineering,the Morison's equation is usually used to calculate the wave-current load on the horizontal cylinder,which is suitable for the weak interaction between wave and cylinder.However,when the cylinder is located near the free surface,the interaction between wave-current and cylinder may become strongly nonlinear.Thus the assumption of Morison's equation is no longer satisfied and it is difficult to accurately calculate the load on the cylinder.In this paper,experiments and numerical simulations are carried out concerning the strongly nonlinear interation between wave-current and horizontal cylinder.The main contents are as follows.Firstly,the strong interaction between regular wave and horizontal cylinder is studied experimentally and numerically.Both the experiments of wave-cylinder interaction and wave-current-cylinder interaction were carried out in a circulating water channel.The wave force and wave-current force acting on the cylinder with various submerged depths were measured in the experiments.In addition,using the commerical software Fluent,the strongly nonlinear phenomena in the interaction between regular wave and horizontal cylinder,including the wave reflection,wave blockage,large deformation of the free surface and the vortex field around the cylinder,are studied.The influence of the above nonlinear phenomena on the nonlinear wave force is also discussed.Finally,the variation of the wave-current force and its peak value with the submerged depth are summarized based on the experimental data.The influence of the wave reflection and blockage on the peak value of the force is discussed in detail.Freak wave is an extreme wave with great energy and destructiveness.When the horizontal cylinder encounters the freak wave,an even stronger nonlinear interaction phenomenon will appear.In the experiment and numerical simulation,related researches are conducted using focused wave instead of the freak wave.In this paper,the study of the strongly nonlinear interaction between focused wave and horizontal cylinder is carried out.Firstly,the instantaneous flow field under the interaction between focused wave with large wave steepness and cylinder is measured by PIV.The formation and development of the vortex around the cylinder are summarized.The similarities and differences between the vorticity fields due to regular waves and focused wave are discussed.Then,the influence of the following factors on the force acting on the cylinder,such as the submerged depth and the current velocity,are studied.Finally,the time-frequency energy characteristics of the propagation and evolution of the focused wave and the interaction between focused wave and horizontal cylinder are studied by wavelet transform.Accurate simulation of the waves is the important basis of both the study of the wave-cylinder interaction and other hydrodynamic problems in ocean engineering.In order to better meet the requirement of the accurate simulation of wave in the hydrodynamic research,a high-accuracy large eddy simulation program is developed in this paper.In this model the filtered incompressible Navier-Stokes equations are solved numerically on non-staggered grids based on finite volume method.The compact fourth-order accuracy scheme is used for spatial discretization and the fourth-order Runge-Kutta integration is adopted for time advancement.The momentum interpolation method is used to deal with the coupling between pressure and velocity.The turbulence is solved by Smagorinsky subgrid stress model.The PLIC-VOF method is used to capture the free surface of the wave.In addition,various waves are generated by giving the boundary condition of the velocity and VOF value.Finally,the Imersed Boundary Method is used to simulate the solid structure.Finally,the numerical simulations of square cavity flow,moving surface tracking and the flow around cylinder are carried out to verify the effectiveness of the large eddy simulation model developed in this paper.Then,this numerical model is first applied to the simulation of waves,such as linear wave,cnoidal wave,solitary wave and focused wave,and then applied to more complicated simulations such as the wave-cylinder interaction,wave-current-cylinder interaction,flow field around the cross section of ship and a moving cylinder,in order to verify the reliability and applicability of the present model.In addition,based on the uncertainty analysis and comparison of flow field,the present model is validated and proven to have superiority over the commercial CFD software Fluent in the simulations of solitary wave and flow around cylinder.