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

许多过程地貌学涉及到理解由松散或不牢固的沉积物组成的河床、河坝、海滩和沙丘等地貌的发展和演变。理解这些系统动力学的基础是对泥沙运移速率进行建模和预测的能力。已设计了许多模型来预测风沙输移率，几乎所有模型都依赖于剪切速度的三次方(代表床面风切变大小的变量)。大多数剪切速度的估计都是由测量的速度剖面的斜率与von Karman常数(von Karman常数的典型值为0.4)的乘积得出的。因此，冯·卡曼常数是预测风沙输移的基础，但实验室水槽中的水动力学研究表明，在有悬浮泥沙存在的情况下，这个“常数”并不是恒定的，因为随着泥沙浓度的增加，这个“常数”会显著减小(可能小于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，还没有人试图评估风沙输送系统中冯·卡曼常数的稳定性。这项博士论文研究项目将进行一项现场实验，旨在评估冯·卡曼常数在一系列输沙条件下的变异性。结果将与一般的湍流混合理论进行比较，并将它们与水中的结果进行比较。将使用连续称重的捕集器和基于麦克风的跳跃传感器以及带有热和超声波风速计的风场来测量沙子的输送。剪切速度的估计将通过直接测量垂直和水平速度波动以及根据速度剖面得出。现场试验后，将获得足够的数据来评估和量化泥沙对风速剖面的影响，表现为von Karman常数的变化。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·卡曼常数的显著变化将导致预测运输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交通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分布的解释，并开启关于沙子输送在改变其载流中的作用的辩论。虽然这项研究只涉及泥沙运移，但其结果也将适用于河流系统。该项目应改进泥沙输送模型，并为风尘预测、沙丘稳定化、荒漠化评估以及相关的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名有前途的博士生的研究生涯，同时还将为第二名博士生提供复杂仪器系统现场实验的经验。

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.