Numerical Study Of Obstable's Influence On Stratified Flows

For the purpose of reducing the emission of Greenhouse gases to prevent the global climate from gettting warmer,developing the strategy of sustainable energy supply, de-creasing the dependence on energy import,creating more job opportunities,the govern-ment all over the world put an emphasis on developing renewable energy supply. Among all kinds of renewable energy,offshore wind farm is one of the potential source. The use of this kind of energy could make use of wind energy over the sea,produce clean energy,while at the same time reducing the noise's influence on humen beings.While we try to make full use of such kind of energy,we should avoid the potential nega-tive influence on ecosystem and environment.To achieve this goal,the related scientific research is quite necessary.Since European region is still one of the main region in the world in utilizing offshore wind energy.We take Baltic Sea as an example to get an idea the potential impact of the offshore wind farm construction's influence on marine ecosystem.Baltic Sea is a nearly enclosed coastal sea with relatively strong stratification because the river runoff is one main source of water supply.The water mass in the middle and bot-tom part is easy to become a hypoxia region for the reason that strong stratification hinders ventilation.The main source of ventilation is through the inflow from the North Sea,which has higher salinity and could enter the bottom part of the Baltic Sea. The planned constructions of offshore wind farm on the North Sea inflow path might in-crease the mixing of the inflow with ambient water mass,lower the salinity(and hence density) of the inflow,one possible result of this process is that after the construction the inflow could not reach the depth it originally could,thus fail to ventilate the lower water and cause an negative impact on marine ecosystem.Thus it is very important to carry out scientific research to evaluate the influence of wind farm's construction on marine ecosystem.The current work tries to get some basic idea of the related process using nu-merical models.Because the available ocean models could not afford to do such kind of study. The current work tries to develop one numerical code based on the current availabe resources,merging the merits of the computational fluid mechanics models and current ocean models.The new code could calculate the flow around obstables and inherently is a non-hydrostatic code without any simplification derived from dimen-sional analysis.A series of two dimensional numerical studies are carried out to test the validity of the model for flow around obstacle and stratified flow dynamics,from which we could see the current code is suitable for the current research compared to other existing numerical codes.We also test the code's sensitivity on the different dis-cretization methods of advection and choice of turbulence models,and get a preliminary quanlitative impression on the influence of these model configurations on numerical result.Following the success of the 2D numerical validation,we attemp to do some 3D numerical study of the process of forced stratified inflow and downwelling of high salinity water mass along the slope bottom to investigate the influence of pertur-bation,shear,the shape of obstacles on the movement of stratified fluid.These ide-alize test cases give us a first comprehension about these physical processes.From the numerical results,we could see that the combination effects of background cur-rents,shear,perturbation and obstacle could contribute a strong mixing process be-hind the obstacle,which could strongly effect other processes,e.g.,the ecosystem dy-namics,further related research is necessary.Lacking direct observation data with regard to the contruction of offshore wind farms,the current work tries to use one observation data around Great Belt Fixed Link(GBFL) to study the obstacle's influence on stratified flows.The west bridge pier and the structure of offshore wind farms have similiar magnitude.We try to use the cur-rent numerical code to reconstruct the flow field and salinity field around the bridge piers,to overcome the deficiency of observation data.Though we fail in this explo-ration,we gain more understanding about the related processes.The numerical results show that,the piers could induce a very strong mixing downstream of the bridge,which could induce a counter baroclinic gradient force with regard to the inflow. The numer-ical experimentation fails for the reason of the contradiction of the inflow direction and the mixing induced internal flow direction,which pose new questions for further research.The influence of obstacles on stratified fluid is a very important research topic,which is of great value to evaluate the influence of offshore construction on marine environ-ment and ecosystem.We try to use numerical method to do some preliminary study on related aspects and find that more further research is necessary. Resctricted to current computational resources,parameterezation is necessary for realistic study.We expect through further work to get a parameterization method based on a set of reliable benchmark data and use it to evaluate the offshore constructions,e.g.,bridge piers,oil platform,offshore wind farm's influence on marine environment and ecosystem.