The Turbulence Responsible for Horizontal Surfzone Dispersion

造成水平表面区扩散的湍流

• 批准号: 1357060
• 负责人: Matthew Spydell
• 金额: $ 23.01万
• 依托单位: University of California-San Diego Scripps Inst of Oceanography
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2014
• 资助国家: 美国
• 起止时间: 2014-03-15 至 2018-02-28
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1357060&HistoricalAwards=false
• 关键词: Turbulence; Responsible; Horizontal; Surfzone; Dispersion

Improved understanding of the surfzone (within a few 100 m of the shoreline) horizontal turbulence that causes material dispersion is the goal of this project. Poor water quality at many beaches poses health risks to beach goers and negatively impacts local economies. Recent research has indicated that surfzone horizontal eddies, which have length scales greater than the water depth, mix and disperse surfzone tracers such as pollution. Although dispersion by surfzone eddies is critical to the dilution, as well as the cross-shore exchange of material (including larvae), how to parameterize the surfzone eddy diffusivity in computer models is unknown. Exactly how the Eulerian properties of the turbulence (the eddy velocity, and the eddy space and time scales) are related to surfzone Lagrangian statistics (i.e. the dispersion and eddy diffusivity) is unknown. Also not completely understood is how the surfzone Eulerian eddy properties are related to the incident wave field. By analyzing existing observations and performing model experiments, the link between Eulerian eddy properties and dispersion will be illuminated.Intellectual Merit:The first objective of this study is to examine surfzone horizontal eddy properties in existing observations. As dispersion depends critically on the velocity c, eddy length L, and eddy time scales T, and in particular the parameter alpha = cT/L, surfzone values will be determined from space- and time-lagged velocity autocorrelation functions. First, they will be calculated for SandyDuck observations (Huntington Beach 2006 and Imperial Beach 2009 will also be considered to the extent possible) and observed surfzone eddy fields will then be characterized. Small values of alpha indicate that the eddy field changes very quickly in time and results in qualitatively different dispersion than if the eddies are long lived (large alpha). How the observed eddy properties depend on the incident wave field will be investigated and whether alongshore currents change the nature of the eddy field determined. Also, the dependence of surfzone eddies on cross-shore position will also be calculated. In particular, as surfzone eddies are only generated within the surfzone, whether or not the surfzone eddy field is different within and seaward of the surfzone will be examined.The second objective is to perform the same type of analysis done for the observations on wave resolving and wave averaged numerical simulations of observed (e.g. SandyDuck) conditions. Do the models reproduce the observed eddy velocities, length and time scales even though wave averaged and wave resolving velocity frequency-wavenumber spectra can differ significantly? This is an important consideration as dispersion depends on the bulk properties of the frequency-wavenumber spectra rather than its detailed shape. Model simulations will also allow the calculation of very reliable Lagrangian statistics as virtual drifters will be added to the model. Eulerian and Lagrangian eddy statistics will then be compared and the relevance of an existing theory which relates Eulerian to Lagrangian statistics, developed for isotropic homogeneous turbulence, for the surfzone will be determined. If this theory is applicable, the surfzone eddy diffusivity can then be inferred without explicit Lagrangian statistics as long as the Eulerian eddy statistics are known.Broader Impacts:At United States beaches in 2008, 7% of beach water samples exceed national health standards and there were over 20,000 closure and advisory days. Pollution from urban runoff often drains directly onto beaches and poses significant health risks to swimmers. Management decisions could benefit from better understanding of surfzone turbulence and dispersion. As part of educational outreach efforts, the PI will collaborate with I Love A Clean San Diego (ILACSD), a nonprofit environmental group, and assist them with the development of their teaching materials and education program. Additionally, the PI will provide guest presentations at selected ILACSD programs. Partnering with ILACSD is going to have a large impact as they have provided environmental education to 30,000 high school students (45% Title 1 or underserved) in total and they work closely with resource managers throughout San Diego.

该项目的目标是提高对导致物质分散的冲浪区（海岸线 100 m 以内）水平湍流的了解。许多海滩的水质较差，给海滩游客带来健康风险，并对当地经济产生负面影响。最近的研究表明，长度尺度大于水深的冲浪区水平涡流会混合和分散污染物等冲浪区示踪物。尽管表面带涡流的扩散对于稀释以及物质（包括幼虫）的跨岸交换至关重要，但如何在计算机模型中参数化表面带涡流扩散率尚不清楚。湍流的欧拉特性（涡流速度、涡流空间和时间尺度）与表面区拉格朗日统计（即色散和涡流扩散率）的确切关系尚不清楚。同样尚未完全了解的是表面区欧拉涡流特性与入射波场的关系。通过分析现有的观测结果并进行模型实验，将阐明欧拉涡流特性与色散之间的联系。 智力优点：本研究的第一个目标是检查现有观测中的表面带水平涡流特性。由于色散主要取决于速度 c、涡流长度 L 和涡流时间尺度 T，特别是参数 alpha = cT/L，因此表面区域值将由空间和时间滞后的速度自相关函数确定。首先，将针对 SandyDuck 观测计算它们（也将尽可能考虑亨廷顿海滩 2006 和帝国海滩 2009），然后对观测到的冲浪区涡流场进行表征。小的 alpha 值表明涡流场随时间变化非常快，并且与长期存在的涡流（大 alpha）相比会导致质量上不同的色散。将研究观测到的涡流特性如何取决于入射波场，以及沿岸流是否改变所确定的涡流场的性质。此外，还将计算海浪区涡流对跨岸位置的依赖性。特别是，由于海浪区涡流仅在海浪区内产生，因此将检查海浪区涡流场在海浪区内部和向海处是否不同。第二个目标是对观测条件（例如 SandyDuck）的波浪解析和波浪平均数值模拟进行相同类型的分析。即使波平均和波分辨速度频率波数谱可能存在显着差异，模型是否再现了观察到的涡流速度、长度和时间尺度？这是一个重要的考虑因素，因为色散取决于频率波数谱的整体特性而不是其详细形状。模型模拟还可以计算非常可靠的拉格朗日统计数据，因为虚拟漂移器将添加到模型中。然后将比较欧拉和拉格朗日涡流统计，并确定将欧拉与拉格朗日统计联系起来的现有理论（为各向同性均匀湍流而开发）与表面区的相关性。如果这一理论适用，那么只要知道欧拉涡流统计，就可以在没有明确的拉格朗日统计的情况下推断出海浪区涡流扩散率。 更广泛的影响：2008年，在美国海滩，7%的海滩水样超过国家卫生标准，并且有超过20,000个关闭和咨询日。城市径流的污染通常直接排入海滩，对游泳者造成严重的健康风险。更好地了解表面区湍流和扩散可以使管理决策受益。作为教育推广工作的一部分，PI 将与非营利环保组织“我爱清洁圣地亚哥”(ILACSD) 合作，并协助他们开发教材和教育计划。此外，PI 将在选定的 ILACSD 项目中提供客座演讲。与 ILACSD 的合作将产生巨大的影响，因为他们总共为 30,000 名高中生（45% 的 Title 1 或服务不足）提供了环境教育，并且他们与整个圣地亚哥的资源经理密切合作。