The Influence Of Sand Wave Morphology On Hydrodynamic Characteristics Of The Overlying Water In The Offshore And Estuary Area

The offshore and estuary area is an important transition area for ocean and land,and it is also one of the most complex water-field in the hydrodynamic environment.One manifestation of the hydrodynamic complexity of the area,is that river runoff,tidal currents and waves affect the transport and deposition of sediment,which in turn causes changes in the morphology of the estuarine shore to form sand wave topography.Sand wave topography,in turn,causes great disturbance to the near-bottom flow structure,especially from the ocean side,and the entry of waves into the area causes dynamic changes in the velocity and pressure within the water,exacerbating the turbulence of the water body overlying the sand wave topography.At present,due to the lack of technical and operational conditions,field observations and physical experiments are restricted,the research on the hydrodynamic characteristics of sand wave turbulence is still very limited.Therefore,it is of great practical significance to study the dynamics of marine environmental theory,and to study the characteristics of sand wave turbulence by using numerical simulation techniques to investigate the influence of dune terrain on the hydrodynamic characteristics of overlying water bodies in offshore and estuary areas.Based on the Reynolds stress turbulence model and the VOF method to track the free surface,a two-dimensional generalized fixed dune terrain numerical flume is established in this paper.The boundary wave-making method and the setting of a damping zone are used to simulate current flow,wave propagation,and current-wave flow in a numerical tank.The structure and characteristics of the sand wave turbulence under different hydrodynamic conditions are analyzed.A variety of different working conditions are simulated by changing the boundary conditions,and the effects of changes in hydrodynamic factors such as velocity,shape of the dune,wave height and period on sand wave turbulence are discussed.The main research results are as follows:(1)Under the effect of the current,there is a stable backflow on the dune leeward surface.The shape of the recirculation zone is controlled by the incoming flow conditions.The larger the incoming Reynolds number,the stronger the turbulence in the flow field.The separated stream will form a vortex wake under the strong mixing effect.The spread range of wake is proportional to the Reynolds number.The magnitude of the dune's steepness affects the height of the recirculation zone,and the ratio of the recirculation length decreases as the wave steepness increases,but this trend will gradually weaken as the wave steepness further increases.At the same time,the ratio of the horizontal distance of the dune's windward surface to the leeward surface has a certain effect on the height of the recirculation zone.(2)Under the effect of waves,the sand wave turbulence will also generate backflow vortices on the backflow surface.The main reason for this is the pressure difference caused by the waves on both sides of the trough during the wave propagation.When the wave travels in the forward direction,the vortex will play a supporting role to the overlying water,and when the wave travels in the reverse direction,the reflection of sand wave on the waves is obvious.The effect of wave height on the sand wave turbulence is mainly reflected in the height of wave peak and the intensity of the backflow vortices,the height of wave peak will increases while the wave height increases,also the intensity and the size of the backflow vortices follow the same way.The wave period also has a certain impact on the sand wave turbulence.When the wave travels in the forward direction,the longer the period,the vortex will play a more significant role to the overlying water,and when the wave travels in the reverse direction,the effect of the period on the sand wave turbulence depends on whether the peak of the reflected wave overlaps the incident wave.When both peak overlaps,the trend of vorticity diffusion towards the mainstream would be weaken,otherwise the wave surface will have larger fluctuations,also the vorticity will spread more easily to the mainstream under the fluctuating pressure.(3)When the wave and the current are in the same direction,the backflow vortices of the sand wave turbulence have higher intensity,and the morphological changes are manifested as a periodic process of "weaken-disappearance-generation-growth",however when the wave and current are reversed,the backflow vortices show a periodic process of " growth-weaken-generation-disappearance ".The hydrodynamic of the sand wave turbulence are simultaneously affected by both current and wave,the separated stream swings up and down due to fluctuating pressure,and a dynamic equilibrium is formed between the backflow vortices and the separated stream.When the wave height is large,the dynamic equilibrium will be destroyed,then the backflow vortices are strengthened,freed from the constraints of separated stream and enter the mainstream,also the vorticities will dissipate around and causing disturbance in the flow,the wave would even break when it is opposite to the current.The change of the average velocity also has a strong influence on the morphological changes of the backflow vortices,when the velocity is low,it is difficult for the backflow vortices to maintain the morphological integrity,and the vorticities will escape from the back of the main vortex in the form of a secondary vortex.When the velocity is large,the dynamic equilibrium will be formed again between the backflow vortices and the separated stream.