Numerical Research Of Internal Solitary Wave Action On Submerged Body

Internal solitary waves,a common phenomenon in the ocean,are a kind of non-linear large-amplitude fluctuation with long characteristic wavelength along the thermoclines which are horizontal layers of stable density stratiflcation.As they carry enormous energy,large-amplitude internal solitary waves can have a significant influence on the marine engineering structures,especially the fall down behavier of submerged structures located in the thermocline.However,there are few related researches so far.In this paper,a simple time-domain numerical model is established to calculate the wave force and motion response of a horizontal submerged slender circular cylinder under the action of an internal solitary wave based on the assumption that there is no rotation of the cylinder.There are two directions of the internal solitary wave are considered,one is parallel to the axis of the horizontal cylinder and the other is perpendicular to the axisFirstly,the velocity and acceleration properties of water particles of the internal solitary wave which satisfy the typical KDV equation in two-layer fluid are studied.The equivalent wavelengths of internal solitary waves are great larger than the scale length of the submerged structure,so the internal solitary wave force can be calculated by the Morison formula.If the structure pierces the surface of the interfacial solitary wave,the buoyancy variation caused by the density difference between the upper and lower fluids can be calculated by the capture of the intersection position.Through the comparison with the results by CFD,the forces calculated by the present simple approximation method are acceptable.Then,a time-domain numerical model is establishe based on the Netwon's Second Law to calculate the motion response and the four-order Runge-Kutta time step method is used to solve the motion equation.Finally,the motion response of the submerged horizontal cylinder under the action of internal solitary wave with parallel and perpendicular direction are investigated respectively by the developed numerical model.The parameters of typical two-layer density distribution and interfacial depression solitary wave are chosen on the basis of observed data in South China Sea.A simplified horizontal slender cylinder model is selected by referring to the conventional submarine parameters.The motion responses of the submerged cylinder encountering internal solitary wave are calculate with the consideration of the different submerged depth and internal solitary wave direction.The cylinder is located in the upper fluid,lower fluid and the interface fluctuation area respectively.The calculations show that the cylinder has a great downward vertical displacement especially when it is located in the interface fluctuation area.The reason is that when the cylinder intersects the interface,the vertical force includes not noly the internal wave force but also the decrease of buoyance which induced the large downward acceleration.The influence of horizontal travel speed of the submerged cylinder are also investigated.In addition,when the internal solitary wave is coming in the direction perpendicular to the cylinder axis,the submerged cylinder in the upper layer will be subjected to a great horizontal force and has a great horizontal displacement.