【作者】 林振华；

【导师】 赵栋梁；

【作者基本信息】 中国海洋大学 ， 物理海洋学， 2009， 博士

【摘要】 为了更有效减少温室气体排放,阻止全球气候持续变暖,发展可持续能源供给,减低对于其他国家的能源倚赖,创造更多的就业机会,各国政府对于可再生能源的开发都非常重视。近年来在可再生能源的开发中,近岸风能的利用越来越被重视,这种能源的开发可以远离人口密集区域,减少噪声对于人类的影响,同时又能有效的利用海上风场,提供清洁能源。在我们尽可能利用这些能源的同时,又要尽量避免这种能源的开发可能带来潜在的负面影响,我们开发的目标是相关设施的建设和环境生态相和谐,为了达到这个目标,对于相关过程影响的研究是非常有必要的。由于目前欧洲仍然是海上风能利用的主要区域之一,我们以欧洲附近的波罗的海为例,从层结流体动力学的角度来考虑海上风能设施的建设对于相关生态过程的可能影响。由于波罗的海是一个典型的接近封闭的海域,受到陆地径流的影响：海洋层结效应很强,底层的海水难以得到持续更新,因而形成了一个低氧区。这个区域的氧分主要通过北海高盐度海水的入流来更新,现在计划在北海入流高密度海水通道上建设的风能发电设施可能会对于高密度的入流水体产生阻碍和混合作用,使得入流水体的密度(盐度)降低,可能导致的直接后果是使得原本可以达到海底来更新底部的入流水体无法达到原有的深度,引起底层的水体失去了更新氧分的来源,从而对于海洋生态过程产生负面的影响。因而这个过程的研究对于我们评价海上风能建筑的对于海洋环境的影响非常重要。本文尝试使用数值计算方法对于相关过程进行初步的探索研究。由于目前已有的海洋数值计算模型无法满足我们的要求,基于前人工作的基础上,本文结合了计算流体力学模型和海洋数值计算模型的优点,整合了一个可以满足层结流体绕流问题计算需求的非静力数值模型。并且通过一系列二维模型检验该模型对于二维的绕流问题和层结流体动力过程数值计算的有效性,计算结果表明该数值模型较其他模型更适用于层结流体绕流问题的研究。同实验观测数据的定性比较表明该模型可以更好地对于相关过程进行数值计算,进一步分析了不同的对流项离散方式和湍流模型对于绕流问题的影响,同实验数据的定量比较为我们今后了解这些参数对于绕流过程计算结果的影响有了一个初步的掌握。在二维模型成功验证的基础上,我们分别尝试了对于层结流体的强迫入流过程和高密度水体沿着倾斜地形下沉的过程作了三维的数值计算,定性分析了扰动,流速剪切,不同的柱体形状对于层结流体运动的影响,这些理想情形的数值算例使得我们对于相关的物理过程有了一个初步的了解。计算结果表明背景流动,剪切,扰动和障碍物的共同作用可以在柱体后侧产生一个非常强烈的混合过程,由此引起的其他效应,如生态过程,值得进一步研究。由于目前对于风能发电设施建设对于层结流体的影响没有直接的观测数据,本文采用在大贝尔特桥附近的一个观测过程来研究障碍物对于层结流体运动的影响。该桥西桥具有和海上风能发电设施相似的尺寸。在前面工作的基础上,我们尝试对于西桥桥墩附近的流场和盐度场结构进行数值模拟,期望能够通过数值计算重构桥墩附近的流场和盐度场结构,弥补直接的观测数据时间上不同步,空间上稀疏的缺点。虽然这个尝试最终失败,我们通过分析其失败的原因对于障碍物对于层结流体混合强度有了更深刻的了解,如障碍物对于层结流体的影响非常明显,其对于层结流体过程引起的混合,可以引起一个和流动方向相反的斜压梯度力,使得层结流体底层产生一个和原入流方向相反的流动,最终引起计算模型边界条件和计算区域内部产生的流动相冲突,造成计算失败。这为我们进一步研究相关过程提出了新的问题。障碍物对于层结流体运动的影响是一个非常重要的研究方向,对于评估实际海上建筑对于海洋动力过程的影响具有重要意义。本文尝试使用数值模型对于若干方面作了初步的了解,这个过程中出现的很多问题有待于以后进一步的研究。同时受限于目前数值模型对计算资源的需求,对于这个过程的参数化是有必要的,我们期望以后通过进一步的相关研究能够构建一套可以作为基准的数据集并寻求一个恰当的参数化方法,并将这种方法来评估实际海上建筑,如桥梁,石油平台,海上风力发电设施等对于海洋环境影响。更多还原

【Abstract】 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.更多还原