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Dynamics of the head of gravity currents

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Abstract

The present work experimentally investigates the dynamics of unsteady gravity currents produced by lock-release of a saline mixture into a fresh water tank. Seven different experimental runs were performed by varying the density of the saline mixture in the lock and the bed roughness. Experiments were conducted in a Perspex flume, of horizontal bed and rectangular cross section, and recorded with a CCD camera. An image analysis technique was applied to visualize and characterize the current allowing thus the understanding of its general dynamics and, more specifically, of the current head dynamics. The temporal evolution of both head length and mass shows repeated stretching and breaking cycles: during the stretching phase, the head length and mass grow until reaching a limit, then the head becomes unstable and breaks. In the instants of break, the head aspect ratio shows a limit of 0.2 and the mass of the head is of the order of the initial mass in the lock. The average period of the herein called breaking events is seen to increase with bed roughness and the spatial periodicity of these events is seen to be approximately constant between runs. The rate of growth of the mass at the head is taken as a measure to assess entrainment and it is observed to occur at all stages of the current development. Entrainment rate at the head decreases in time suggesting this as a phenomenon ruled by local buoyancy and the similarity between runs shows independence from the initial reduced gravity and bed roughness.

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Acknowledgments

This research was supported by the European Fund for Economic and Regional Development (FEDER) through the Program Operational Factors of Competitiveness (COMPETE) and National Funds through the Portuguese Foundation for Science and Technology (FCT) with the research project PTDC/ECM/099752/2008, FCOMP 01 0124 FEDER 009735 and the research grant SFRH/BD/48705/2008. Acknowledgments are due to Alexandre Sousa for helping in data interpretations. The authors would like to thank the reviewers for the comments and suggestions that certainly improved the content of the manuscript.

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  1. Mário J. Franca

    Present address: Laboratoire de Constructions Hydrauliques, École Polytechnique Fédérale de Lausanne, Lausanne, Switzerland

Authors and Affiliations

  1. Department of Civil Engineering and IMAR-CMA, University of Coimbra, Coimbra, Portugal

    Helena I. S. Nogueira

  2. Department of Civil Engineering, University of Rome, “Roma Tre”, Rome, Italy

    Claudia Adduce

  3. Department of Hydraulics and Environment, National Laboratory of Civil Engineering, Lisbon, Portugal

    Elsa Alves

  4. Department of Civil Engineering and IMAR-CMA, New University of Lisbon, Lisbon, Portugal

    Mário J. Franca

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  1. Helena I. S. Nogueira
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Correspondence to Helena I. S. Nogueira.

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Nogueira, H.I.S., Adduce, C., Alves, E. et al. Dynamics of the head of gravity currents. Environ Fluid Mech 14, 519–540 (2014). https://doi.org/10.1007/s10652-013-9315-2

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