Experimental And Numerical Study On The Gravity Current Around A Cylinder

The development of offshore exploitation has made a significant progress due to the increase in the global economy and technology.The ocean space could be utilized for the construction of offshore airport,immersed tube,and submerged floating tunnel,etc.However,gravity currents occur infrequently and unpredictably in many natural environment and engineering applications.When such current encounter the marine structures,considerable force acting on the structures could pose a challenge to these structures.In the present paper,numerical and experimental studies on the lock-exchange gravity current around a cylinder have been performed.Of particular concern are the experimental and numerical simulations of gravity currents,lift and drag force on a fixed cylinder,motion responses of mooring cylinder and instantaneous flow fields.The hydraulic theory on the gravity current produced by lock exchange has been established using energy-conserving or shallow-water theory.The development of gravity current and front velocity are theoretically obtained.Meanwhile,Large-eddy simulations have been performed to investigate the gravity current around a fixed cylinder.Of particular concerns are the vortex shedding,dynamic loads on the cylinder and flow characteristics of gravity current.Moreover,the effect of cylinder position on the drag and lift forces have been conducted.It is found that the front velocity of gravity current predicted by numerical simulations appears slightly smaller than the theoretical results.And pressure components dominate the impact forces exerted by fluid on the fixed cylinder.This means that Reynolds number in the gravity current is sufficiently high.In the lock-exchange experiment,high speed cameras are used to capture the 3DoF motions of cylinder and the instantaneous flow fields.And the motion responses of cylinder could be quantified using image processing algorithm.Meanwhile,theory on the gravity current is applied to validate our lock-exchange experiment.Moreover,the effects of cylinder position and density ratio on the motion responses of cylinder have been investigated.And the dynamic interaction between the gravity current and mooring cylinder is also presented.It is shown that the experimental front velocity is slightly smaller than theoretical values.And the high speed camera together with the image processing methods could obtain the motion responses of cylinder with accuracy.When the denser salt water in the non-mixing layer encounters the mooring cylinder,such a cylinder could experience a large rotation and positive translation in the x-axis direction.When the flow in the mixing layer impinges on the cylinder,c ylinder could experience a fluctuation rather than a large translation.Also the density ratio of gravity current has a significant effect on the flow-induced motion of cylinder——a large density ratio could induce more intense motion.We present the dynamics of gravity current produced by lock exchange,and the dynamic interactions between the gravity current and cylinder using experimental and numerical methods.Applying the image processing methods,the motion response of a mooring cylinder in the gravity current have been investigated.The knowledge of the gravity current in this paper could be of great value.