Collaborative Research: Physics-Based Modeling of Bridge Foundation Scour: Numerical Simulations and Experiments

该项目由美国国家科学基金会（NSF）和陆军研究办公室（ARO）联合资助，旨在开发和验证第一个能够准确再现桥梁基础冲刷的3D非定常数值模型。本工作的基本前提是由地基诱导的非定常相干涡引起的波动水动力驱动泥沙输运和冲刷，需要正确建模。现有的方法无法捕捉到问题固有的不稳定物理特性，因为它们要么依赖于定性描述，要么依赖于经验相关性，要么使用统计平稳的计算模型。为了克服这些缺点，圣安东尼福尔斯实验室（SAFL）、弗吉尼亚理工大学（VT）和美国陆军工程兵团（USACE） WES设施之间建立了研究伙伴关系。目标是将三维相干结构解决湍流结流数值模拟的最新发展与最先进的实验室能力和仪器相结合，从而允许同时测量瞬时流量和压力以及相应的冲刷孔的空间和时间发展。将建立一种新的欧拉河床输运模型，该模型采用拉格朗日思想将近河床波动水动力的影响考虑到Exner方程中。初步工作已经证明，该模型能够重现冲刷孔的高度动态演化，包括复杂河床的形成。本项目将完成以下任务：A)水动力模型的实验验证；b)在大范围的桥墩直径、泥沙大小和流动特性下，同时监测冲刷孔演化、桥墩瞬时压力和泥沙颗粒的流动结构的实验，其中一些实验具有近原型条件的代表性；c)进一步发展和验证新的非定常河床输运模型。实验室实验将在13个基于桥墩直径的雷诺数（范围从4x104到6.7x105）下进行，而数值模型将探索这些以及更大范围的雷诺数。该项目将在合作的努力下，推进能够在实际流动条件下预测冲刷的计算模型的开发，并提高我们对这种现象的认识和理解，包括升级效应。该项目的成果将通过提供一个强大的计算工具来研究和制定桥梁冲刷问题的缓解策略，从而造福社会。桥梁冲刷问题导致的桥梁破坏比近期历史上所有其他原因都要多，并有可能严重损害国家的交通基础设施。数值模型还将通过提供一种可用于解决各种河流恢复问题的工具，显著增强我们的研究基础设施。在这方面，该模型可以应用于为河流恢复研究制定改进的标准，这些研究考虑了影响河流栖息地质量的巨石周围的流动结构和其他障碍物。通过与设在SAFL的美国国家科学基金会国家地表动力学中心的跨学科互动，这项工作对流恢复和推广活动的潜在影响将大大促进。

This project is jointly funded by NSF and the Army Research Office (ARO) and seeks to develop and validate the first 3D, unsteady, numerical model capable of accurately reproducing bridge foundation scour. The basic premise of this work is that fluctuating hydrodynamic forces due to the foundation-induced unsteady coherent vortices drive sediment transport and scour and need to be modeled correctly. Available methods are incapable of capturing the inherently unsteady physics of the problem as they either rely on qualitative descriptions, empirical correlations or employ statistically stationary computational models. To overcome these shortcomings, a research partnership is established among St. Anthony Falls Laboratory (SAFL), Virginia Tech (VT), and the US Army Corps of Engineers (USACE) WES facility. The objective is to integrate the latest developments in 3D coherent-structure resolving numerical modeling of turbulent junction flows with state-of-the-art laboratory capabilities and instrumentation, which permit simultaneous measurements of instantaneous flow quantities and pressures with the corresponding spatial and temporal development of the scour hole. A novel Eulerian model of bedload transport will be developed, which employs Lagrangian ideas to account for the effect of near-bed fluctuating hydrodynamic forces into Exner's equation. Preliminary work has demonstrated the ability of this model to reproduce the highly dynamic evolution of the scour hole including the formation of complex bedforms. The following tasks will be accomplished in this project: a) experimental validation of the hydrodynamic model, b) experiments for monitoring the flow structures simultaneously with the scour hole evolution, instantaneous pressures on the pier and on sediment particles over a wide range of pier diameters, sediment sizes, and flow characteristics, with some of them representative of near-prototype conditions, and c) further development and validation of the new unsteady model of bedload transport. The laboratory experiments will be carried out at thirteen pier-diameter based Reynolds numbers (ranging from 4x104 to 6.7x105), while the numerical model will explore these as well as a wider range of Reynolds numbers. This project will advance, in a collaborative effort, the development of a computational model capable of scour prediction in practical flow conditions, as well as advance our knowledge and understanding of the phenomenon, including upscaling effects. The outcomes of the project will benefit society by providing a powerful computational tool that can be used to study and develop mitigation strategies for the bridge scour problem, which has resulted in more bridge failures than all other causes in recent history and has the potential to seriously impair the nation's transportation infrastructure. The numerical model will also enhance significantly our research infrastructure by providing a tool that can be used to tackle a wide range of stream restoration problems. In this regard the model can be applied to develop improved criteria for stream restoration studies that account for the flow structures around boulders and other obstructions affecting stream habitat quality. The potential impact of this work to stream restoration and outreach activities will be greatly facilitated through cross-disciplinary interactions with the NSF National Center for Earth Surface Dynamics housed at SAFL.