Doctoral Dissertation Research: Evaluating the von Karman Constant in Sediment-Laden Air Flow

博士论文研究：评估充满沉积物的气流中的冯卡门常数

• 批准号: 0727775
• 负责人: Douglas Sherman
• 金额: --
• 依托单位: Texas A&M Research Foundation
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2007
• 资助国家: 美国
• 起止时间: 2007-07-15 至 2010-06-30
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0727775&HistoricalAwards=false
• 关键词: Doctoral; Dissertation; Research; Evaluating; von

Much of process geomorphology is concerned with understanding the development and evolution of landforms like bedforms, channel bars, beaches, and dunes that are comprised of unconsolidated or poorly consolidated sediments. Fundamental to understanding the dynamics of these systems is the ability to model and predict sediment transport rates. Numerous models have been designed to predict aeolian sediment transport rates, almost all of which depend on the third power of shear velocity (a variable representing the magnitude of wind shear on the bed surface). Most estimates of shear velocity are derived as the product of the slope of a measured velocity profile and the von Karman constant (the typical value of von Karman constant is 0.4). The von Karman constant therefore is fundamental for the prediction of aeolian transport, but hydrodynamic research in laboratory flumes indicates that this "constant" is not constant in the presence of suspended sediments, because those sediments decrease it significantly (perhaps to less than 0.2) as the sediment concentration increases. Such a change would cause order-of-magnitude errors in sediment transport rate predictions. Surprisingly, there have been no attempts to evaluate the stability of the von Karman constant in aeolian sand transport systems. This doctoral dissertation research project will conduct a field experiment designed to assess the variability of the von Karman constant under a range of sand transport conditions. The results will be compared with general turbulent mixing theories, and they will be compared with results obtained in water. Sand transport will be measured with continuous-weighing traps and with microphone-based saltation sensors and the wind field with thermal and ultrasonic anemometers. Estimates of shear velocity will be derived from direct measurement of vertical and horizontal velocity fluctuations and from velocity profiles. After the field experiment, enough data will be acquired to evaluate and quantify sediment influences on the wind velocity profile as manifested in changes in the von Karman constant.This project will evaluate and quantify sediment influences on turbulence from perspectives of different physical fluid dynamic theories. Substantial variability in the von Karman constant will cause disproportionate errors in predicted transport rates. This potentially means that all transport studies based on wind profile analysis are flawed to an unknown extent. This project therefore has the potential to fundamentally change the interpretation of sediment-laden wind velocity profiles and to open debate about the role of sand transport in the alteration of its carrier flow. Although this research is concerned with sand transport only, the results also will be applicable to fluvial systems. The project should lead to improved sediment transport models and a sounder basis for wind-blown dust prediction, dune stabilization, desertification assessment, and related environmental planning challenges. As a Doctoral Dissertation Research Improvement award, this project will help launch the research career of a promising doctoral student while also providing a second doctoral student with experience in field experimentation with complex instrumentation systems.

过程地貌学的大部分内容是关于了解地貌的发展和演变，如由未固结或固结不良的沉积物组成的底形、河坝、海滩和沙丘。 了解这些系统的动力学的基础是建立模型和预测泥沙输移率的能力。 已经设计了许多模型来预测风成泥沙输运率，几乎所有的模型都依赖于剪切速度的三次方（一个代表床面上风切变大小的变量）。 剪切速度的大多数估计值都是作为测量速度剖面的斜率和冯卡门常数（冯卡门常数的典型值为0.4）的乘积得出的。 因此，冯卡门常数是预测风成输运的基础，但在实验室水槽中的水动力研究表明，这个“常数”在悬浮沉积物的存在下并不恒定，因为这些沉积物会随着沉积物浓度的增加而显着降低（可能低于0.2）。 这种变化将在泥沙输移率预测中造成数量级的误差。 令人惊讶的是，一直没有尝试评估风成沙输运系统中的冯卡门常数的稳定性。 本博士论文研究项目将进行一项实地实验，旨在评估在一系列沙输运条件下冯卡门常数的变化。 结果将与一般的湍流混合理论进行比较，并与在水中获得的结果进行比较。 将使用连续称重陷阱和基于麦克风的跃移传感器以及使用热风速计和超声波风速计的风场来测量沙粒输送。 剪切速度的估计将来自垂直和水平速度波动的直接测量和速度分布。 本项目将从不同的物理流体动力学理论的角度来评估和量化泥沙对湍流的影响。 冯卡门常数的显著变化将导致预测输运速率的不成比例的误差。这可能意味着所有基于风廓线分析的运输研究都存在未知程度的缺陷。 因此，该项目有可能从根本上改变对含沙风速剖面的解释，并就沙输送在改变其载体流中的作用展开辩论。 虽然这项研究只涉及泥沙输运，但其结果也将适用于河流系统。 该项目应能改进沉积物迁移模型，并为风沙预测、沙丘稳定、荒漠化评估和相关环境规划挑战奠定更坚实的基础。作为博士论文研究改进奖，该项目将有助于启动一个有前途的博士生的研究生涯，同时也提供了第二个博士生与复杂的仪器系统现场实验的经验。