The 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.
