Surfzone Energy Cascades

在许多碎波带环境和广泛的条件下，都观察到了速度为0.1 m/s、周期约为30分钟的旋转流。这些低频碎波带涡旋为跨海岸扩散和海岸与内大陆架之间的物质交换提供了一种机制。这项研究测试的假设，低频碎波带电流可能是能量转移的结果，从破碎波。现有的数据从一个广泛的二维阵列的波和电流传感器将进行分析，以调查的逆能量级联到较低的频率。由于现有数据可分辨的空间尺度的限制，将通过在碎浪区部署沿岸波浪和海流传感器阵列来收集新数据。传感器部署将在2020年进行的多机构资助的实地项目期间进行，利用重要的基础设施和辅助测量。除了了解这些低频振荡对碎波带和沿海海洋之间物质交换的贡献外，该研究还将解决未来科学家和专业工程师在现场数据收集和分析方面的暴露需求。该项目将为学生和年轻的沿海专业人员提供现场数据收集经验和为期一周的现场观察分析研讨会。此外，该研究将成为麻省理工学院-WHOI博士生的论文的一部分，由PI建议。有几个假设的低频冲浪区运动的产生，包括不稳定的平均电流（剪切波），入射波场（波群）的缓慢调制，和非线性能量转移（破碎波）。在现有的现场数据集，结合新的观察波条件的范围广泛，将允许这些机制的分离和隔离。碎波区观测的初步分析表明，低频流场的一些统计数据与二维逆能量级联的预期是一致的。根据初步结果，提出了两个假设，并将作为本项目的一部分进行调查：（1）在高频端部注入碎波带的几米空间尺度的涡能（50至300 mHz）破碎风浪通过二维逆级联转换为低频（4 mHz）碎波区电流，（2）涡度拟能（涡度平方的积分）通过二维前向叶栅从破碎波产生的涡度转换为小尺度、高频率的运动。逆叶栅和前叶栅相交的空间尺度是注入涡度的空间尺度的指示。该奖项反映了NSF的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

Rotational currents with order 0.1 m/s velocities and periods of about 30 minutes have been observed in many surf zone environments and for a wide range of conditions. These low-frequency surf zone eddies provide a mechanism for cross-shore dispersion and exchange of material between the shore and the inner continental shelf. This study tests the hypothesis that low-frequency surf zone currents may be the result of energy transfer from breaking waves. Existing data from an extensive two-dimensional array of wave and current sensors will be analyzed to investigate the inverse energy cascade to lower frequencies. Due to limitations in the spatial scales resolvable by the existing data, new data will be collected through the deployment of an alongshore array of wave and current sensors in the surf zone. The sensor deployment will occur during a multi-agency-funded field project to take place in 2020, leveraging significant infrastructure and ancillary measurements. In addition to understanding the contribution of these low frequency oscillations to material exchange between the surf zone and the coastal ocean, the study will also address the need for exposure of future scientists and professional engineers in field data collection and analysis. The project will provide students and young coastal professionals field data collection experiences and 1-week long workshops in analyzing field observations. In addition, the research will form part of the dissertation of an MIT-WHOI PhD student advised by the PIs.There are several hypotheses for the generation of low-frequency surf zone motions, including instabilities of mean currents (shear waves), slow modulations of the incident wave field (wave groups), and nonlinear energy transfers (breaking waves). The wide range of observed wave conditions in existing field data sets, combined with new observations will allow separation and isolation of these mechanisms. Preliminary analyses of surf zone observations suggest that some statistics of the low-frequency current field are consistent with the expectations of a two-dimensional inverse energy cascade. Based on preliminary results, two hypotheses have been formulated and will be investigated as part of this project: (1) vortical energy with spatial scales of a few meters injected into the surf zone at the ends of high frequency (50 to 300 mHz) breaking wind waves is transferred to low-frequency ( 4 mHz) surf zone currents via a two-dimensional inverse cascade, and (2) enstrophy (the integral of vorticity squared) is transferred from breaking-wave-generated vorticity to smaller-scale, higher-frequency motions via a two-dimensional forward cascade. The spatial scale for which the inverse and forward cascades intersect is an indication of the spatial scale of the injected vorticity.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.