The role of turbulent coherent structures on the evolving seabed

被称为波纹的海底水深特征负责水动力耗散、沉积物输送和底栖生物通量交换。尽管中细沙海滩在自然界中更为常见，但与高陡度粗沙波纹相比，中细沙海滩中常见的三维低陡度波纹（中位数粒径为0.15 ~ 0.3 mm）受到的关注要少得多。此外，中细沙波纹的迁移可能是海滩剖面演变的重要组成部分。建议的工作将应用数值模拟方法来研究波浪和水流驱动的中细沙波纹。大涡模拟（LES）两相流方法将被应用于理解在湍流分解系统中存在流动分离涡、流动不稳定性和湍流相干结构时中细波纹的几何形状和迁移。将使用大涡模拟（LES）欧拉两相方法研究中细沙波纹。中细沙波纹的陡度既低又反复无常，在这个项目中假设它们的演变是由流动不稳定性和连贯结构决定的，这样传统的将初级漩涡从湍流连贯结构的级联中分离出来的范式是不充分的。这项工作有望为解释中细沙波纹的几何形状和迁移提供新的理解，提供一个能够模拟低陡度波纹的开源数值模拟模型，并为传输速率和迁移提供改进的参数化信息。该项目将测试这样一个假设，即雷诺平均建模方法客观上将湍流相干结构的能量级联与已分解的初级涡分离开来，这一方法不适用于低陡度波纹。该项目将通过扩展现有的开源模型SedFoam，创建第一个湍流分辨（LES）欧拉两相模型，能够模拟中细沙波纹的几何形状和迁移。该奖项反映了美国国家科学基金会的法定使命，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。

Seabed bathymetric features, known as ripples, are responsible for hydrodynamic dissipation, sediment transport, and benthic flux exchange. The three-dimensional low steepness ripples commonly observed in medium and fine sand beaches (median grain size of 0.15-0.3 mm) have received much less attention compared to the high steepness coarse sand ripples, even though the medium and fine sand beaches are more common in nature. Furthermore, the migration of medium and fine sand ripples may contribute to a significant part of the beach profile evolution. The proposed work will apply numerical modeling methods to study medium and fine sand ripples driven by waves and currents. The Large Eddy Simulation (LES) two-phase flow methodology will be applied to understand the geometry and migration of medium-fine ripples in the presence of flow separation vortices, flow instabilities, and turbulent coherent structures in a turbulence-resolving system.Medium and fine sand ripples will be studied using a large-eddy simulation (LES) Eulerian two-phase methodology. The steepness of medium-fine sand ripples is both low and capricious, and it is hypothesized in this project that their evolution is dictated by flow instabilities and coherent structures such that the conventional paradigm which separates the primary vortices from the cascade of turbulent coherent structures is inadequate. The proposed work is expected to gain a new understanding of explaining the geometry and migration of medium and fine sand ripples, deliver an open-source numerical simulation model capable of simulating low steepness ripples, and inform improved parameterizations on transport rate and migration. The project will test the hypothesis that the Reynolds-averaged modeling approach that objectively separates the energy cascade of turbulent coherent structures from the resolved primary vortices becomes inadequate for low steepness ripples. This project will create the first turbulence-resolving (LES) Eulerian two-phase model capable of simulating the geometry and migration of medium and fine sand ripples by extending the existing open-source model, SedFoam.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.