Study On Hydrodynamic Load Of Cofferdam Structure In Extreme Marine Environment

As a part of ocean engineering,sea-crossing bridge is extremely important to China’s politics,military,especially with the sustained and rapid development of economy in this century.In order to provide a relatively safe and stable construction environment in the sea,large diameter cofferdam as a temporary maintenance structure,has been widely used in the practical bridge project.In the marine environment,cofferdam is often subjected to severe waves and current,and the Reynolds numbers in these conditions are usually in the critical or supercritical zone.But there are no relevant provisions for the calculation of current force for the cases in the bridge specification,and few studies on lift force.Besides,there is a lack of fullscale simulation and research on the wave-current force of cofferdam.Through the simulation calculation on ANSYS fluent and on the basis of the full-scale model,the characters of drag force and lift force under normal and extreme actions of current,wave,wave-current are analyzed,and the influencing factors of the wave-current force along the depth distribution form are studied,the current force,wave force and wave-current force are compared,the main work as follows:(1)The current force and wave force of cofferdam are studied by numerical simulation,and the characteristics of current force and wave force are analyzed.The drag coefficient and lift coefficient of cofferdam in the range of over Reynolds number are calculated by hydrodynamic coefficient calculation formula,and the results show that the drag coefficient is greater than the standard value.The influences of current velocity,aspect ratio and water depth on current force,drag coefficient and lift coefficient are revealed,and the square column is the unfavorable shape of current load,and the key characteristic parameters of current force time history curve are also calculated and summarized.Through Stokes third-order wave theory and Morison equation,the calculation formulas of wave force under nonlinear conditions are obtained,the hydrodynamic coefficient under extreme wave conditions is calculated,and the influences of wave height,aspect ratio and water depth on wave force and wave force coefficient are studied.The result shows that water depth has the greatest influence on wave force coefficient.(2)Numerical simulation is used to study the wave-current force of cofferdam,and the characteristics of wave-current force of cofferdam are analyzed.The positive peak of drag force is mainly caused by the increase of static pressure difference,while the secondary peak is mainly caused by the increase of dynamic pressure difference.The influences of current velocity,wave height,aspect ratio and water depth on the wave-current force are revealed,and the main characteristic parameters of the wave-current force are also calculated and summarized.The distribution of wave-current force along depth direction of cofferdam is studied,and the results show that with the increase of water depth and velocity,the maximum force position moves up,and the lift force is mainly concentrated in the middle of the cofferdam.(3)Through the spectral analysis of the forces of cofferdam under the action of current,wave and wave-current,it is found that the wave-current force under the action of current has more higher-order components than wave force.By dividing the cofferdam evenly along the depth direction and into toroidal surface,and monitoring the pressure and force at different position under different loads,the distribution forms of static pressure,dynamic pressure and total pressure in the cofferdam under the action of current,wave and wave-current are compared,and the influence of current and wave on the pressure at each position of the cofferdam are analyzed.The results show that with the position of the monitoring surface further back,the range of depths in which wave plays a dominant role increases.The distribution forms of the load of cofferdam under the action of current,wave and wave-current along the depth are compared,and the influence of current and wave on the wave-current force of cofferdam at different position is analyzed.The results show that the current has great influence on the force of the upper part of cofferdam.The effects of wave and current at each position of the flow field are analyzed by monitoring the flow velocity at each direction of the characteristic points under these three kinds of loads.