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Bed characteristics and Stratification effects in Turbulent Oscillatory Boundary Layers

Marcelo Garcia, University of Illinois at Urbana-Champaign

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Marcelo Garcia, Dimitrios Fytanidis

The proposed work is the first computational effort to examine the effects of bed porosity and different stratification levels on the maximum bed shear stress phase difference compared to the maximum free-stream velocity value. Turbulence characteristics and the effect of the turbulent flow structures will be examined using advanced interactive visualization and data analysis techniques.

It will also be among the first studies that will study the mixing layer and momentum exchange between the free-stream oscillatory flow and the pore-scale flow under oscillatory conditions. Our 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.

Wave-boundary layer flows play an important role in coastal engineering and coastal ecosystems. However, current state-of-the-art models fail to accurately predict the complex mechanics of the turbulence, sediment and momentum exchange between seabed and free-stream flow under oscillatory flow conditions. Recent experimental and numerical studies in our laboratory suggest the presence of phase-lag between the time instance when the maximum bed shear stress occurs with respect to the maximum free-stream velocity. This effect is dominant in the so-called transitional/intermittent-turbulent flow regime over hydrodynamically smooth and transitionally rough beds.

This observation is extremely important for the field of environmental fluid mechanics and coastal sediment transport. Yet, the oceanic or coastal bed is characterized by high levels of inhomogeneity, different levels of roughness, bedforms, and porosity. Preliminary numerical results suggest that the bed porosity can change significantly the shear/free-stream velocity maxima phase difference independently of the hydrodynamic regime that characterizes the bed (smooth, transitional, rough).

We hypothesize that this is due to the enhanced momentum transfer between the turbulent free stream flow and the porous flow inside the bed. In addition, the role of stratification effects on the phase difference is unexplored in the current literature. Strong thermal or sediment induced stratification can cause turbulence damping that may shift the phase of the wave periods when the maximum bed shear stress occurs. It is likely that the turbulent spots, which are arrowhead-shaped turbulent flow structures associated with local bed shear stress peaks and strong turbulent bursting, can still exist in highly energetic flows under high stratified conditions.