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On the relation of fine layering of a stratified water environment with vertical turbulent mass transfer
- Authors: Gerasimov V.V.1, Zatsepin A.G.1
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Affiliations:
- Shirsov Institute of Oceanology Russian Academy of Sciences
- Issue: Vol 64, No 5 (2024)
- Pages: 739-750
- Section: Физика моря
- URL: https://journal-vniispk.ru/0030-1574/article/view/281267
- DOI: https://doi.org/10.31857/S0030157424050013
- EDN: https://elibrary.ru/OGEWIB
- ID: 281267
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Abstract
The results of a laboratory experiment performed to test the fundamental mechanism of fine layering of a stratified fluid during turbulent impact are described and analyzed. A series of experimental runs were carried out with stirring of an aquatic environment with an initially linear vertical salinity gradient using oscillating vertical rods, creating a uniform turbulent impact throughout the entire thickness of the water column. At the same time, in each run regular measurements of electrical conductivity (salinity) profiles were fulfilled and calculations of the vertical salt flux (mass) were carried out. It turned out that in case of a sufficiently large density (salinity) gradient the mass flux is a decreasing function of the density gradient, and this is the main condition for the formation of a fine layering, according to the proposed mechanism. The experimental results confirmed its feasibility. The dependence of the vertical scale of the fine structure on the parameters of stratification and turbulent impact has also been established.
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About the authors
V. V. Gerasimov
Shirsov Institute of Oceanology Russian Academy of Sciences
Author for correspondence.
Email: gerasimov.vv@ocean.ru
Russian Federation, Moscow
A. G. Zatsepin
Shirsov Institute of Oceanology Russian Academy of Sciences
Email: zatsepin@ocean.ru
Russian Federation, Moscow
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Fig. 1. Schematic diagram of the experimental setup. 1 – organic glass pool with internal dimensions of 36 × 13.5 × 25 cm3; 2 – system of grids with vertical rods; 3 – rod on which the grids with rods are fixed; 4 – electric motor with an eccentric providing horizontal oscillation of the rod; 5 – aquatic environment linearly stratified by salinity; 6 – electrical conductivity microsensor for measuring the salinity profile; 7 – Expert 002 electrical conductivity sensor in the upper quasi-homogeneous layer; 8 – vertical elevator with an electric motor for moving the conductivity microsensor; PC – personal computer for collecting the measured data; S1 – salinity of the upper layer, ρ(z) = β d S/dz – vertical density gradient, β – salinity compression coefficient. The circle in the center of the tank corresponds to the outlines of the plane-parallel beam of light created by the shadow device.
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3.
Fig. 2. Two-tank system for filling with linearly stratified liquid. 1 – right tank with initial solution of water with zero salinity S10 = 0, 2 – left tank with water with salinity S20, 3 – electric motor, 4 – mixer, 5 – tap with tube connecting tanks, 6 – tap with tube connecting right tank with pool.
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4.
Fig. 3. Shadow photographs (a–d) of the successive formation and disappearance of a step structure during turbulent mixing of a liquid initially linearly stratified by salinity. The corresponding salinity profiles obtained using a microelectrode conductivity sensor are shown next to the photographs. Bright bands are layers with a large density/salinity gradient separating layers with quasi-homogeneous density/salinity. Experiment at Ri0 = 13, Re = 115.
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5.
Fig. 4. Stepped salinity profile. The principle of identifying the thickness of the upper mixed layer, determining its thickness H1 and the thicknesses of other quasi-homogeneous layers.
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6.
Fig. 5. Dependence of the logarithm of the mass flux modulus Q on the logarithm of the density gradient Gρ. Experiment with initial values Ri0 = 13, Re = 115. Zones highlighted in different shades of gray correspond to different numbers of high-gradient layers: 3, 2, and 1.
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7.
Fig. 6. Dependence of the density gradient on time (n is the serial number of the period of vertical passage of the point conductivity sensor) in the successively formed high-gradient layers L1, L2, L3 and the average density gradient L0 between the near-surface and bottom quasi-homogeneous layers.
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Fig. 7. Experiment with mixing of stratified liquid without formation of steps at Ri0 = 5, Re = 115. Photo (a) – start of mixing, (b) – after 30 minutes, (c) – after 90 minutes, (d) – after 140 minutes.
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Fig. 8. Dependence of the logarithm of the mass flux modulus Q on the logarithm of the density gradient . Experiment at Ri0 = 5, Re = 96.
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Fig. 9. Dependence of the thickness H of the intermediate quasi-homogeneous layers on the ratio of the mixing speed U to the Väisälä–Brünt frequency N in this work (points circled by squares) together with the results of similar experiments performed earlier. The straight line is a linear regression.
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