Turbulent sediment suspension absent mean flow-induced shear

1233842 cowen由于河流冲刷过程的重要性，对泥沙悬浮和输移的研究历史悠久。在河流和风沙或雪的情况下，悬浮过程是由边界上的流体流动（即河床上的流动）引起的湍流控制的。然而，泥沙悬浮流动经常受到远离边界产生的湍流的影响，这种类型的泥沙悬浮过程很少被研究。这种例子在环境和工业流动中广泛存在。以冲蚀带为例，冲蚀带是海滩的一个区域，它交替被海浪覆盖和裸露，是海岸侵蚀的主要地点。在浪涌上升过程中，湍流水平以破碎波平流和孔塌缩产生的湍流为主，而边界产生的湍流相对较小。实验室研究的一大优势是能够将不同的物理过程从更复杂的多物理现实中分离出来。建议使用一个独特的湍流设备进行基础实验研究项目，该设备是在以前的NSF项目上开发的，它通过随机发射在8 × 8网格中组装的一系列射流来产生湍流。这些喷流产生强烈的湍流，平均气流很少。湍流从阵列向沙床传播，在那里仅仅由于湍流而发生悬浮。这有助于仔细研究纯粹由湍流引起的泥沙悬浮。定量成像（QI）技术将用于表征导致悬浮和波纹形成的湍流和沉积物颗粒运动的细节。QI测量将用于确定导致沉积物悬浮的瞬时湍流结构，并专门研究流体对沉积物运动和沉积物对流体运动的作用。将构建湍流应力在空间和时间上的整体平均图像，从而允许发展泥沙悬浮过程的参数化，以便将其包含在泥沙悬浮和输运的计算模型中。美国沿海地区人口密集，对当地经济至关重要，具有重要的战略意义。调查显示，美国近90%的沙质海岸正在被侵蚀——在东海岸，侵蚀速度约为每年一米。对冲积带沉积物输运率的实地测量已观察到每米海岸线每秒可达10公斤；这些明显高于在更远的海区观察到的海浪。上升的水平是由于边界产生的湍流、平均流和强环境湍流水平与破波过程平流的复杂相互作用。冲积带泥沙输运只是一系列重要流动中的一种，在这些流动中，远离边界产生的湍流支配着泥沙悬浮和动力学。在没有平均水流的情况下，对湍流悬浮的理解是开发更好的泥沙悬浮模型的关键基石。这将大大提高我们模拟工业和环境侵蚀和沉积过程的能力。该项目将为有兴趣从事教师职业的有才华的女性提供博士学位，她将与pi一起为考虑从事科学和工程职业的高中女性主持一个基于探究式的环境运输过程项目。

1233842CowenThe study of sediment suspension and transport has a long history, due to the importance of scour processes in rivers. In the case of rivers and wind-blown sand or snow, the suspension processes are controlled by the turbulence induced by the fluid flow over the boundary - that is, the flow over the bed. However, sediment-suspending flows are often subject to turbulence that was generated away from the boundary, and this type of sediment suspension process has seen very little research. Examples are widely found in environmental and industrial flows. Consider the specific example of the swash zone, the region of the beach that is alternately covered and uncovered by wave run-up and the principal location of coastal erosion. During wave up-rush, turbulence levels are dominated by turbulence advected from breaking waves and generated by bore collapse, while boundary-generated turbulence is relatively small. A great advantage of laboratory research is the ability to isolate distinct physical processes from the more complex multi-physics reality. It is proposed to carry out a fundamental experimental research program using a unique turbulence facility, developed on a previous NSF project, which generates turbulence by randomly firing an array of jets assembled in an eight-by-eight grid. These jets generate strong turbulence, with very little mean flow. The turbulence propagates away from the array and towards a bed of sand, where suspension due solely to the turbulence occurs. This facilitates the careful study of sediment suspension purely by turbulence. Quantitative imaging (QI) techniques will be used to characterize the details of the turbulence and the sediment grain motions that lead to suspension and ripple formation. The QI measurements will be used to determine the instantaneous turbulence structures responsible for sediment suspension and to specifically investigate the role of both the fluid on the sediment motions and the sediment on the fluid motions. An ensemble mean picture of the turbulent stresses, in space and time, will be constructed, allowing the development of parameterizations of the sediment suspension process appropriate for inclusion in computational models of sediment suspension and transport.Coastal areas in the United States are heavily populated, vital to local economies, and of strategic importance. Surveys reveal that nearly 90% of U.S. sandy coasts are eroding - on the East Coast at rates on the order of one meter per year. Field measurements of swash zone sediment transport rates as large as 10 kilograms per second per meter of shoreline have been observed; these are significantly higher than those observed farther seawards in the surf zone. The elevated levels are due to a complex interaction of boundary-generated turbulence, mean currents, and strong ambient turbulence levels advected with the wave breaking processes. Swash zone sediment transport represents just one of a range of important flows in which turbulence generated away from the boundary dominates sediment suspension and dynamics. An understanding of turbulent suspension in the absence of mean flows is a critical building block on which better models of sediment suspension can be developed. These will significantly improve our ability to model industrial and environmental erosive and depositional processes. The project will lead to a Ph.D. for a talented woman interested in a faculty career who, along with the PIs, will host an inquiry-based project on environmental transport processes for high-school women considering careers in science and engineering.