河口盐淡水混合过程中盐跃层K-H涡演化机制研究

Kelvin-Helmholtz (K-H) instability, thought to be the onset of turbulence in the pycnocline of stratified flow, is the dominant mechanism for converting fluid motion from stratification to mixing. A laboratory experiment will be carried out to study the critical condition for the generation of K-H instability, and analyze the dominant mechanism for the scales of K-H billows. An efficient non-hydrostatic model will be developed by applying a dynamic layer splitting method. Based on this model, an ideal model for estuarial fresh-salt water mixing will be implemented to investigate the influences of tides and river discharges on the scales, structures and distributions of K-H billows. Moreover, the evolution of K-H billows will be further investigated by analyzing the intensity variation of vorticity and turbulence. Finally, the model will be applied to study the K-H instability along the North Passage of Yangtze River Estuary. The investigation will be focused on the spatiotemporal distribution of the K-H instability, turbulent evolution of K-H billows and the influence of K-H billows on the fresh-salt water mixing rate. Non-hydrostatic modelling of K-H billows can offer new insight into the study of estuarial fresh-salt water mixing, which is also the basis for the research of interaction between fresh-salt water mixing and sediment resuspension.

Kelvin-Helmholtz (K-H)不稳定性促进盐跃层紊动的产生，是河口水体由分层到混合转化的主要动力机制。通过河口盐淡水混合过程物理实验，确定K-H不稳定性的产生的临界条件，探讨由此导致的K-H涡尺度的主控因子；开发适用于河口盐淡水混合模拟的非静压高效计算模型，提出动态网格分离方法，提高模型计算效率；建立理想河口非静压模型，研究K-H涡尺度、结构、分布等运动特性与径潮流动力条件的响应关系，揭示河口盐跃层K-H涡强度和紊动演化的动力机制；应用于长江口北槽，明晰北槽盐洪枯季盐跃层K-H涡的时空分布及紊动演化机制，阐明K-H涡的存在对北槽盐淡水混合速率的影响机制。采用非静压模型研究盐跃层K-H涡的演化机制，是对传统河口盐淡水混合过程静压模拟研究的突破，是研究盐淡水混合与泥沙再悬浮关系的基础。

非静压模型适用于局部地形剧烈变化、盐淡水高度分层等垂向流速变化较快区域的模拟，逐渐成为河口海岸数值模型研究的热点,但也存在计算极为耗时的问题。本项目针对非静压模型PDI高效计算方法进行了理论分析，推导了双层流系统动压分布和频散关系。非静压模型模拟波浪频散性精度主要受垂向动压分布的影响，垂向层数越多，模型计算结果越趋近于理论值。PDI方法中动压插值方法的选择影响模型计算精度，线性插值由于不改变垂向动压积分值，计算精度与粗网格计算结果一致。二次抛物线插值和三次样条插值计算精度介于粗网格和细网格之间。三种插值方法中，三次样条插值计算精度最高。.通过自由表面波和内波算例，对比了PDI方法和不同插值方法对模型频散性精度的影响。发现PDI方法对非静压模型计算精度的影响较小，垂向压力网格主要影响非静压模型计算频散性的精度，PDI方法对模型数值耗散和扩散的影响较小。垂向速度网格对模型数值扩散影响较大，速度网格分辨率不足时模型产生较大数值扩散，减小了盐跃层密度梯度，使波速预测值偏小。垂向速度网格对模型数值耗散有影响，但影响较小，数值耗散主要受水平速度网格影响。.建立了长江口北槽垂向二维非静压模型，模拟了长江口北槽的一次枯季大潮过程，模型能够较准确的模拟长江口北槽径流、潮流引起的盐淡水混合过程；在北下段距离CS0 测站48-51km 范围内首次模拟出K-H 涡的存在，K-H 涡出现在涨急与涨憩之间，持续时间约为2.5h。K-H 涡的水平尺度在56-61m 之间，垂向尺度约为6-7m。K-H 不稳定性发生区域盐淡水垂向混合速率加快，水体紊动增强。