Direct Numerical Simulation of Turbulence and Sediment Transport in Oscillatory Boundary Layer Flows

Marcelo Garcia, University of Illinois at Urbana-Champaign

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Paul Fischer, University of Illinois at Urbana-Champaign

Oscillatory boundary layer flows play an important role on coastal and offshore engineering and the sediment transport mechanics in coastal environment. However, most of the current state-of-the-art models fail to accurately predict the interaction of oscillatory flow with sediment transport, highlighting the existing knowledge gaps regarding the complex interactions between the oceanic flow, the coastal bottom and sedimentation processes. A recent experimental study in our laboratory suggests the presence of phase lag between the time instance when the maximum bed shear stress occurs with respect to the maximum free-stream velocity in transitional oscillatory boundary layer flows. This work is the first in the literature which supports the existence of phase lag, while in most of the previous published work the phase difference between bed shear and free-stream maxima is expressed as "phase-lead"—although phase lag does exist in some of their results. However, due to the limitation of the applied point-wise experimental technique, it was not possible to associate the bed shear phase lag with the development of three dimensional turbulent structures; usually referred as turbulent coherent structures. Also, despite the advances in the development of measurement methods there is no available experimental technique that could solely give all the required spatial and temporal information to estimate the three-dimensional turbulent structures in oscillatory flow conditions. The present work will be the first computational effort to simulate the maximum bed shear stress phase lag with respect to the maximum free-stream velocity value and the effect of turbulent structures and bed roughness on the bed-shear/velocity maxima phase difference. In addition, it will be the first numerical study that will perform quadrant analysis of the turbulent oscillatory flow over porous bed and will examine the mixing layer and momentum exchange between the pore-scale flow and the oscillatory free stream flow. Such an analysis pushes the limits of the existing literature of turbulence-resolving numerical studies in terms of the computational resources and the high performance computing facilities it requires, and thus, it can be materialized only in a petascale supercomputer such as Blue Waters. The proposed work combines the expertise of Prof. Marcelo García's group from Ven Te Chow Hydrosystems Laboratory of the CEE Department and Prof. Paul Fischer's group from the CS and MEng Departments with the leading-edge petascale computing resources of Blue Waters available at Illinois and aims to become one of the most comprehensive studies on the effect of turbulent structures on the oscillatory boundary layer flows, the bed shear/free-stream velocity phase shift and the sediment transport in the literature; elucidating the complex interaction between flow, bed and sediment transport.

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