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Tail structure and bed friction velocity distribution of gravity currents propagating over an array of obstacles

Published online by Cambridge University Press:  30 January 2012

Talia Tokyay ,
George Constantinescu  and
Eckart Meiburg
Talia Tokyay
Affiliation:
Department of Civil and Environmental Engineering, IIHR-Hydroscience & Engineering, The University of Iowa, Iowa City, IA 52242, USA Department of Civil and Environmental Engineering, University of Illinois Urbana-Champaign, Urbana, IL 61801, USA
George Constantinescu*
Affiliation:
Department of Civil and Environmental Engineering, IIHR-Hydroscience & Engineering, The University of Iowa, Iowa City, IA 52242, USA
Eckart Meiburg
Affiliation:
Department of Mechanical Engineering, University of California at Santa Barbara, Santa Barbara, CA 93106-5070, USA
*
†Email address for correspondence: sconstan@engineering.uiowa.edu
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Abstract

The bed friction velocity distribution and sediment entrainment potential of Boussinesq compositional gravity currents propagating over a series of obstacles and over a smooth surface, respectively, are analysed based on high-resolution, three-dimensional large-eddy simulations. The investigation focuses on the parameter regime for which currents with a high volume of release go through an extended slumping phase with approximately constant front velocity (Tokyay, Constantinescu & Meiburg, J. Fluid Mech., vol. 672, 2011, 570–605). Under these conditions, a quasi-steady regime is reached between consecutive obstacles that is similar to the steady regime observed for constant-density channel flows over bottom obstacles. At a given location, this quasi-steady regime is reached in the tail of the current after the passage of the front and the associated hydraulic jumps reflected from the first few downstream obstacles. A double-averaging procedure is employed to characterize the global changes in the structure of the tail region between currents with a high volume of release propagating over smooth surfaces and over obstacles. Reynolds-number-induced scale effects on the flow and turbulence structure within the tail region are discussed in some detail. The presence of this quasi-steady regime is significant, since the simulations with obstacles show that most of the sediment is entrained by the tail of the current, rather than by its front. A detailed analysis of the effects of the obstacle shape on the quasi-steady mean flow and turbulence structure is presented, which provides insight into why gravity currents over dunes can entrain more sediment than gravity currents over ribs of comparable size. Finally, the bed friction velocity distributions and the potential to entrain sediment are compared for a compositional current with a high volume of release during the slumping phase, and a current with a low volume of release for which transition to the buoyancy–inertia phase occurs a short time after the release of the lock gate.

JFM classification

Geophysical and Geological Flows: Gravity currents Turbulent Flows: Turbulence simulation
Type
Papers
Information
Journal of Fluid Mechanics , Volume 694 , 10 March 2012 , pp. 252 - 291
Copyright
Copyright © Cambridge University Press 2012

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