Warren Lecture with Michele Guala

Michele Guala
Civil, Environmental, and Geo- Engineering, University of Minnesota
"An Open Conversation about Sand Grain and Vortex Organization in Turbulent Flows"

ABSTRACT:  Remarkable geometrical and scaling similarities emerge when a turbulent boundary layer acts on a rough or erodible surface. Both sand grains and vortices self-organize into aggregates which exhibit general ramp-like patterns and hierarchies extending up to a significant fraction of the reference outer scale, e.g., the river depth or the boundary layer height. These aggregates represent ripples and dunes in rivers, and attached-eddy coherent structures in near surface atmospheric flows, respectively.

While their advection velocity displays systematic differences, leading to a scale-dependent migration of fluvial bedforms that we do not observe for coherent structures, the dependency on the shear velocity remains a hallmark feature of these aggregates. Not only that, the shear velocity also percolates down to the scaling of the respective elemental components, governing both the velocity of the moving grains and the maximum azimuthal velocity of the vortices in the flow. Hence, this qualitative comparative analysis between bedforms and flow-structures has some quantitative ramifications in the modeling, confirming that the shear velocity is indeed a star.

Guala concludes by highlighting a second order effect, represented by a weak dependency on the outer scale observed both in the scaling of bedform migrating velocity and in the largest coherent structures of the flow. The qualitative flow structures – bedforms comparison could then be extended: in the same way attached-eddy structures may be imagined as frozen into dunes, so very-large-scale motions may end up being frozen into alternate distortions of the mean river bed.