The Role of Relative Submergence on Flow-Obstacle Interaction: Implications to Sediment Transport

University Of Iowa, Iowa City IA

Investigators

Abstract

1033732 Buchholz Bed-mounted obstacles, or discrete roughness elements extending into a flowing fluid, are ubiquitous in a broad range of natural and engineered systems. This simple geometry creates a very complex, three-dimensional and unsteady flow field that can significantly increase bed shear stresses and heat transfer, and affect sediment fluxes in fluvial environments. While it is apparent that the dominant flow patterns are qualitatively quite robust under a broad range of parameters, there is still much that is not well understood about the dynamics of these structures, the resulting distribution of bed stresses surrounding the body, and the turbulent transport mechanisms that govern these stresses, especially in shallow flows. In steep mountain streams, naturally-occurring obstacles or boulders are of comparable length scale with flow depth and river width, which introduces a new and primary parameter: the relative submergence H/D (where H is the flow depth and D is the obstacle characteristic dimension). These naturally-occurring obstacles typically have H/D<1, and under these conditions the sediment depositional patterns are strikingly different than at large H/D. This work seeks to characterize the effects of relative submergence on bed shear stress distributions, and mean flow and sediment depositional patterns surrounding the obstacle, and to elucidate the physical mechanisms governing these processes. These objectives will be achieved in a parametric study of a spherical obstacle on a rough bed in shallow open-channel flow. Particle image velocimetry will be used to obtain quantitative measurements of the velocity and vorticity fields, and turbulent stresses surrounding the obstacle. Spectral analysis and conditional averaging of the flow will help to elucidate the fluid-dynamic mechanisms responsible for the observed stresses. For the same flow conditions, sediment deposition patterns around the obstacle and corresponding sediment trapping efficiencies will be characterized. The flow and sediment studies will be combined to provide insight on aggregate measures of bed shear stress that can be used to predict sediment transport characteristics in more complex riverine environments involving large obstacles. Intellectual Merit: This investigation addresses a highly relevant region of the parameter space which has so far received little attention, and will provide fundamental insights into the physical mechanisms governing recent observations about sediment transport in fluvial systems. Therefore, this will lead to a new array of powerful tools to more effectively manage the nation's rivers. Fundamentally, the work will provide a clearer picture of the complex interactions between flow structure dynamics, turbulent stresses, and sediment movement in the vicinity of an obstacle, which can have implications far beyond the prototype class of problems on which this work is based. Despite many decades of research on flow around obstacles, the proposed work has great potential due to the integration of researchers in fluid dynamics and sediment transport, and the use of advanced measurement methods. Broader Impacts: Due to the ubiquity of flow over wall-mounted obstacles, the technical impacts of this work have the potential to affect a wide range of applications in the areas of river management, aerodynamics, urban planning, industrial plant design, heat transfer, and many others. The project also provides an invaluable multi-disciplinary educational opportunity for graduate and undergraduate students studying environmental hydraulics and fluid dynamics, and therefore provides an ideal environment for the training of the next generation of scientists. Community outreach in schools and community colleges will support the theme "Science Based Restoration Approaches for Rivers".