The role of turbulent coherent structures on the evolving seabed

海底测深特征（称为波纹）负责水动力消散、沉积物输运和海底通量交换。虽然中、细沙滩在自然界中较为常见，但在中、细沙滩（中位粒径为0.15-0.3毫米）中常见的三维低陡度沙纹，较之高陡度粗沙沙纹，受到的关注要少得多。此外，中、细砂波的迁移可能对海滩剖面的演变起重要作用。本文将应用数值模拟方法研究波浪和水流驱动下的中细砂沙纹。大涡模拟（LES）两相流方法将用于了解在湍流分辨系统中存在流动分离涡、流动不稳定性和湍流相干结构的情况下中细沙波纹的几何形状和迁移。中细沙波纹将使用大涡模拟（LES）欧拉两相流方法进行研究。中细砂波纹的陡度是低的和反复无常的，并且在该项目中假设它们的演变是由流动不稳定性和相干结构决定的，使得从湍流相干结构的级联中分离主涡的常规范例是不够的。该工作有望为解释中细砂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，并改进传输速率和迁移的参数化。该项目将检验一种假设，即客观地将湍流相干结构的能量级联与已分辨的主涡分开的平均建模方法对于低陡度波纹变得不充分。该项目将通过扩展现有的开源模型SedFoam，创建第一个能够模拟中细砂波纹的几何形状和迁移的双相分辨率（LES）欧拉两相模型。该奖项反映了NSF的法定使命，并通过使用基金会的智力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.