Numerical Simulation Of Interactions Between Internal Solitary Wave And Marine Structure

In the actual marine environment,the density is usually distributed in layers.That is to say,the density distribution along the depth of seawater is not uniform.When there is a disturbance in the ocean,under the combined action of reduced gravity and Coriolis force,the water particle that deviates from the original equilibrium position will move to the original position and show the phenomenon of oscillating reciprocating motion under the action of inertia.This kind of marine wave that occurs in the stratified ocean is called ocean internal wave.Internal solitary waves(ISWs)are typical representatives of internal waves because of their frequent activities.The balance between nonlinearity and frequency dispersion is responsible for the propagation of ISWs with a constant waveform and propagation velocity in oceans.However,the existing studies on the interaction between ISWs and marine structures are relatively rare,and there are many problems worth studying.Therefore,in this paper,five aspects including the establishment of the ISW numerical model,the evaluation and analysis of the ISW numerical wave modeling method,the propagation evolution of the IS W,the simulation of the interaction between the ISW and the horizontal cylinder,and the simulation of the interaction between the ISW and the semi-submersible platform are investigated step by step.The numerical model of ISW based on the open-source computational fluid dynamics software OpenFOAM is built in this paper.The establishment of the numerical model is based on the coupling density equation to solve the Reynolds time-averaged Navier-stokes equation.Since the distribution of the initial density field is given during the simulation of internal solitary waves,the density equation only solves the changes in the density distribution in the flow field during the propagation of ISW.The finite volume method is used to discretize the numerical model,and the pressure-velocity coupling in the calculation process is processed by the PISO algorithm.In this paper,two cases including internal seiche wave and Lock-Exchange are used to verify the ability of the ISW numerical model to deal with wave evolution in the non-uniform density stratification environment.In the case of internal seiche wave,the numerical simulation results of flow velocity are in good agreement with the analytical solution.In the case of Lock-Exchange,comparisons of velocity field between ISW numerical model proposed in this paper and ocean internal wave model including MITgcm model and the BOM model indicate that the ISW numerical model established in this paper is accurate in simulating the velocity field during the evolution of Lock-Exchange.By capturing the internal bore,KH vortex and mixed vortex during the evolution of Lock-Exchange,it is shown that the ISW numerical model established in this paper can deal with the wave evolution generated in the density stratified environment.Then,this paper makes a comparative evaluation and analysis of different ISW numerical wave-making methods.The study on the generation of the ISW by gravity collapse indicates that this method is simple and easy to use,and is the same as that in most laboratory studies.However,the ISW generated by this method is susceptible to the collapse zone,the depth ratio of upper and lower layers and the density ratio.And many attempts are needed to generate the target amplitude,and the research efficiency is not high.Considering the ISW theory,the numerical wave boundary of the ISW is established.Compared with the flow field data measured in the laboratory,the ISW-making method established in this paper can efficiently and accurately simulate the generation of ISW.In the following simulation of ISW,the mKdV theory is used in the ISW-making method to generate the ISW.In order to study the characteristics of internal solitary wave propagation and evolution,and to further verify the capability of the numerical model established in this paper to simulate the propagation and evolution of internal solitary wave in complex terrain,the propagation and evolution of the ISW over obstacles including triangle,semicircle and slope with the long platform are simulated.By comparing with the measured wave profiles,the ISW numerical model and the ISW-making boundary can accurately simulate the wave-surface evolution of the ISW in the non-flat terrain.By capturing the phenomena of an internal hydraulic jump,internal bore,wave surface inversion,wave splitting,diffraction and reflection,it is proved that the ISW numerical model can well simulate the flow field evolution of the ISW propagation over the obstacle.Considering that most of the offshore engineering equipment is composed of cylindrical components,the interaction between the ISW and the horizontal cylinder is simulated in this paper.The characteristics of the interaction between the internal solitary wave and the horizontal cylinder are studied by the load on the cylinder,the dynamic pressure distribution around the cylinder and the characteristics of the flow field.In the study of the interaction between the ISW and an isolated cylinder,based on the analysis of the influence of the change of the position of the cylinder on the characteristics of the interaction between the internal solitary wave and the cylinder,the influence of the different amplitude of the internal solitary wave and the thickness of the density layer on the load acting on the cylinder caused by the internal solitary wave is further considered.The horizontal force on the isolated cylinder reaches its maximum value when the cylinder is close to the bottom of the wave tank.The isolated cylinder subjects the maximum vertical force when the cylinder centre is near the interface of stratified fluid.The comparison of loads between the numerical simulation results and experimental data indicate that the ISW numerical model can accurately simulate the interaction between the ISW and isolated cylinder located at different positions.Then,the interaction between the ISW and the cylinder with different extended length located near the interface of stratified fluid is simulated.The results show that when the extension length reaches 1 time of the initial cylinder diameter,the maximum vertical force acting on the extended cylinder increases linearly with the increase of the extension length.When the extension length reaches 2.5 times of the initial cylinder diameter,the maximum horizontal force acting on the extended cylinder reaches a peak.The complexity of the flow field around the extended cylinder increases with increase in the extended length.Vortexes produced near the upper and lower sides of the extended cylinder have a significant effect on the maximum vertical force acting on the extended cylinder.The results of the interaction between the ISW and tandem cylinders with different center-to-center distance located near the interface of the stratified fluid show that the horizontal force of each of tandem cylinders is smaller than that of the isolated cylinder in the same position.When the center-to-center distance of the tandem cylinders reaches 2.5 times of the diameter of an isolated cylinder,the maximum horizontal force on each of tandem cylinders is only 48.9%and 84.4%,respectively,of the maximum horizontal force on the isolated cylinder.At this point,the vortex interaction between the tandem cylinders plays a significant role in reducing the force acting on the tandem cylinders.As the center-to-center distance of the tandem cylinders increases,the maximum vertical force on the tandem cylinders decreases gradually.Comparing the forces between the two cylinders,it can be seen that the larger horizontal force and the smaller vertical force act on the cylinder located at the rear.Based on the self-developed offshore oil 981 deep-water drilling platform in China,the scale is 1:300,and the interaction between the ISW and the semi-submersible platform is simulated and analyzed at the laboratory scale.In the research process,firstly,the load on the semi-submersible caused by the ISW propagation and the distribution of the flow field around it are discussed and analyzed.The results show that when the amplitude of the ISW is similar,the load on the semi-submersible platform caused by the propagation of the ISW decreases with the increase in the fluid depth ratio.The force of the platform caused by the ISW propagation is mainly due to pressure.Compared with the force acting on other components of the semi-submersible platform,the force acting on the support of the semi-submersible platform is negligible.The vertical force acting on the semi-submersible platform is almost entirely derived from the vertical force acting on the pontoon.During the interaction between the ISW and the semi-submersible platform,there will be a region generation with obvious velocity decrease around the platform.A large number of vortices shedding around the semi-submersible platform can also be observed.Then,considering the stochastic characteristics of-the propagation direction of the ISW in the actual marine environment,the interaction between the ISW and the semi-submersible platform arranged according to different angles is simulated,discussed and analyzed.With the increase of the angle ? between the propagation direction of the ISW and the symmetry axis of the semi-submersible platform,the maximum horizontal force and torque induced by the ISW propagation on the semi-submersible platform gradually increase.When the ? reaches 0°,the semi-submersible platform bears the maximum vertical force.The distribution of the flow field around the platform is more complicated when the propagation direction of the ISW is not parallel to the symmetry axis of the semi-submersible platform under the condition of the ISW environment.The distribution of the flow field around the same type of component of the platform is similar.When the platform is arranged according to 0° and 90° angles,the flow field around the platform is symmetrically distributed about the section where the y=0.3 m.The interactions between the positive and negative vortex around the semi-submersible platform arranged according to 30° and 60° angles are more complex than 0° and 90°angles.