Study On Hydrodynamic Mathematical Model And Its Characteristics Between Sediment And Overlying Water In The Offshore And Estuarine Areas

The sediment-water interface layer is the significant place for physical,chemical,and ecological interactions between sedimentary layer and overlying water.The sediment-water interface is an important interface for transport and exchange of substances.The offshore and estuarine areas are crucial transitional areas between land and sea.Freshwater flows through the estuarine area and offshore area,and reaches the remote continental shelf.There are commonality and specificity between offshore area and estuarine area.In the offshore area,the salinity variation is small,and tidal effect is significant.The pressure gradient and shear force are the main dynamic between sedimentary layer and overlying water.The estuarine area is not only affected by tide,but also influenced by runoff and salinity gradient variation.The exchange of substances between offshore area and estuarine area at the sediment-water interface is affected by the hydrodynamic environment between the sedimentary layer and overlying water.At present,field observations and physical experiments have limitations in terms of observation technologies and operating conditions.There is still few researches in studying hydrodynamic characteristics of sediment-water interface layers under the influences of tidal current and salinity gradient variation.Therefore,investigating the hydrodynamic mechanism is of great practical significance to research of marine environment dynamic theory and forecast analysis between the sedimentary layer and overlying water in the offshore and estuarine regions.In this study,the technical thinking were that from offshore waters with relative simple dynamics to estuarine waters with complicated dynamics,and from overlying water to sediment-water interface layer.The mathematic model on the flow process in the sedimentwater interface layer under the effects of tidal current and salinity gradient variation was studied.And the hydrodynamic characteristics of sediment-water interface layer were discussed.The main research work and results were as follows.(1)Based on the unsteady Reynolds averaged Navier-Stokes equation,the hybrid meshes were used to generate local fine model for irregular structures and irregular submarine topographic change.A small-scale fine model of tidal current motion in offshore waters was established.The model was validated with measured data,and was applied to simulate flow movement around the region of submarine excavation engineering.The calculation indicated showed that the small scale model of the tidal currents motion can reasonably describe the flow characteristics of the tidal currents,and obtain the local small scale circulating currents and the turbulence characteristics in the submarine excavation engineering.(2)Based on the unsteady Reynolds averaged Navier-Stokes equation with porosity and Reynolds stress transport equation,the fluid-domain stratification method was used,the sedimentary layer,overlying water and air layer weere all treated as fluid domain,and the mathematical model on the flow dynamics around the sediment-water interface under tidal current in the offshore waters was established.The model was validated by measured data,and used to compared the velocity distribution without considering the effect of sedimentary layer.The simulated results indicated that the model palyed a strong performance in simulating the hydrodynamics of coupled flow around sediment-water interface under tidal current,and was good to simulate the velocity vertical distribution near seabed.Compared with models that used Darcy's Formula,the model that used Darcy-Forchheimer Formula could be better to express the turbulent characteristics in the sediment-water interface layer.The turbulence characteristics of sediment-water interface layer such as turbulent kinetic energy,turbulent dissipation rate,turbulent kinetic energy productivity have periodic change with the tidal cycle.The momentum exchange in the sediment water interface is dominated by the turbulent diffusion,thus it can be seen that the tidal movement has a direct effect on the hydrodynamic characteristics of the sediment-water interface layer.(3)Based on the unsteady Reynolds averaged Navier-Stokes equation with porosity and salinity transport equation,a small-scale numerical model on saltwater intrusion movement in the estuarine area was established.The model were validated with two situations.One was with no tide.And the other one was with tide.The simulated results indicated that the flow characteristics and morphological characteristics were accordance with experimental data and previous research results.The model could be reasonably used to simulate the saltwater intrusion movement under the tide and salinity gradient.And the hydrodynamic characteristics of saltwater intrusion head were studied.It was obtained that the variation of shear force could cause the strong vertical mixing in the head of saltwater intrusion.(4)The numerical model on saltwater intrusion movement in the estuarine area was applied to simulate the flow movement in the sediment-water interface layer with constant salinity at the upstream boundary condition and downstream boundary condition respectively.The results indicated that the velocity vertical structures were more complicated in the estuarine area when comparing with that in the offshore area.During the stage of flooding,there was salinity discontinuous structures existing in the surfacial sedimentary layer,and the position of discontinuous structures would be deeper into the sedimentary layer.But it did not exist during the stage of ebbing.