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An experimental study of flow regimes of a gravity current over a Cape in a stratified environment

Abstract

Properties of the flow generated by a buoyancy source are investigated by a number of laboratory experiments. Experiments are carried out in a tank with a Cape to simulate the features of a gravity current when moving and separating over a delta Cape as in a real marine flow. Of the many experiments performed, 63 are recorded and analyzed in detail. Most of the experiments are carried out under 0.01 < Fr < 3 and 0.03 < Ro < 0.45 (where Fr and Ro are the flow Froude and Rossby numbers respectively). Dimensionless parameters of the simulations of the flow in a laboratory are comparable with those of the real flow in the Caspian Sea. Based on the behavior of flow upstream of the Cape, three regimes can occur in these experiments: laminar, eddy, and laminar-eddy regimes. The eddy regime is found for values of the Rossby number less than 0.05. For the moving flow on the bottom slope, an empirical relation between CNof, the Nof speed, and u, the current speed, is found from which CNof is predicted from u using oceanographic data. Based on the Rossby number of the flow in the Caspian Sea, the laminar-eddy regime is more likely to occur. The flow shows a different behavior when moving over the Cape which is categorized by the Cape upstream behavior of the flow. Under  = 0.02 m/s2, f = 0.72 s−1 (T = 17 s) Ro = 0.17, and Fr = 0.24, one cyclone eddy and one anticyclone eddy are formed similar to those seen in nature (here the Caspian Sea). In the eddy regime, the cascade process occurs with a timescale of longevity tL < 6T for each eddy. The results indicate that the geometry of the Cape and the features of the flow (, Fr, and Ro) upstream can be effective in the shape, size, and location of eddy formation. The experimental results also show that the radius of the eddy is about two times larger than the Rossby deformation radius of the flow in upstream of the Cape, while having a timescale tf between T/2 and 2T. In nature, the eddy formation and development time scale are about 1 to 2 years with 10 and 15 months for Seddy (cyclone eddy) and Anseddy (anticyclone eddy) respectively. Because of the ability of an eddy to transport and spread pollutants such as oil in the southern Caspian Sea, this work can also be important for the marine dispersion estimations.

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Acknowledgments

The experiments were performed in the GFD laboratory in the Institute of Geophysics at the University of Tehran. This work has been supported by a grant funded from the University of Tehran (number: 6202018/1/03). We thank Dr. Maryam Shieh for the help in the estimations of some parameters of the eddy in the Caspian Sea using numerical simulations.

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Correspondence to Abbasali Aliakbari Bidokhti.

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Responsible Editor: Alejandro Orfila

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Babagoli Matikolaei, J., Aliakbari Bidokhti, A. An experimental study of flow regimes of a gravity current over a Cape in a stratified environment. Ocean Dynamics 69, 769–786 (2019) doi:10.1007/s10236-019-01272-4

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Keywords

  • Density driven flow
  • Rotating fluid
  • Eddy shedding
  • SefidRud Delta Cape
  • Cyclonic and anticyclonic eddies