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Gravity currents produced by lock exchange
Published online by Cambridge University Press: 13 December 2004
Abstract
The dynamics of gravity currents are believed to be strongly influenced by dissipation due to turbulence and mixing between the current and the surrounding ambient fluid. This paper describes new theory and experiments on gravity currents produced by lock exchange which suggest that dissipation is unimportant when the Reynolds number is sufficiently high. Although there is mixing, the amount of energy dissipated is small, reducing the current speed by a few percent from the energy-conserving value. Benjamin (J. Fluid Mech. vol. 31, 1968, p. 209) suggests that dissipation is an essential ingredient in gravity current dynamics. We show that dissipation is not important at high Reynolds number, and provide an alternative theory that predicts the current speed and depth based on energy-conserving flow that is in good agreement with experiments. We predict that in a deep ambient the front Froude number is 1, rather than the previously accepted value of $\sqrt 2$. New experiments are reported for this case that support the new theoretical value.
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- © 2004 Cambridge University Press
Shin et al. supplementary movie
Figure 9. Gravity current produced by lock exchange: full-depth release, vertical lock gate. The upper panel shows a side view of the experiment. Initially, dense fluid (salt solution), marked with dye, is separated from less dense fluid (fresh water) by a vertical lock gate. When the gate is removed the dense fluid flows along the bottom boundary as a gravity current, and the fresh water flows along the surface forming a second gravity current. The total depth of the flow is 0.2 m and the initial reduced gravity of the two fluids is 29.7 mm/s. The effective depth of the current, determined by integrating the width-average density, is superimposed as the red curve. The side view also shows the initial lock position (the vertical yellow line) and the other lines correspond to the depths of the energy-conserving solution (blue) and of the current with maximum speed and dissipation (the two green lines, one each for the dense and light currents). The lower panel shows the effective depth in false colour plotted in the x-t plane. The lock gate is at x=0, and the propagation of the fronts of the dense current (x>0) and the light current (x<0) from this position are shown. The fact that the edges of the coloured regions are straight shows that the currents travel at constant (and almost equal) speeds. The horizontal line indicates the time corresponding to the image in the upper panel.
Shin et al. supplementary movie
Figure 9. Gravity current produced by lock exchange: full-depth release, vertical lock gate. The upper panel shows a side view of the experiment. Initially, dense fluid (salt solution), marked with dye, is separated from less dense fluid (fresh water) by a vertical lock gate. When the gate is removed the dense fluid flows along the bottom boundary as a gravity current, and the fresh water flows along the surface forming a second gravity current. The total depth of the flow is 0.2 m and the initial reduced gravity of the two fluids is 29.7 mm/s. The effective depth of the current, determined by integrating the width-average density, is superimposed as the red curve. The side view also shows the initial lock position (the vertical yellow line) and the other lines correspond to the depths of the energy-conserving solution (blue) and of the current with maximum speed and dissipation (the two green lines, one each for the dense and light currents). The lower panel shows the effective depth in false colour plotted in the x-t plane. The lock gate is at x=0, and the propagation of the fronts of the dense current (x>0) and the light current (x<0) from this position are shown. The fact that the edges of the coloured regions are straight shows that the currents travel at constant (and almost equal) speeds. The horizontal line indicates the time corresponding to the image in the upper panel.
Shin et al. supplementary movie
Figure 18. Gravity current produced by lock exchange: partial-depth release. The upper panel shows a side view of the experiment. Initially, dense fluid (salt solution), marked with dye, is separated from less dense fluid (fresh water) by a vertical lock gate. When the gate is removed the dense fluid flows along the bottom boundary as a gravity current, and the fresh water flows along the surface forming a second gravity current. The total depth of the flow is 0.2 m and the initial reduced gravity of the two fluids is 29 mm/s. The effective depth of the current, determined by integrating the width-average density, is superimposed as the red curve. The side view also shows the initial lock position (the vertical yellow line) and the other lines correspond to the depths of the energy-conserving solution (blue) and of the current with maximum speed and dissipation (the two green lines, one each for the dense and light currents).
Shin et al. supplementary movie
Figure 18. Gravity current produced by lock exchange: partial-depth release. The upper panel shows a side view of the experiment. Initially, dense fluid (salt solution), marked with dye, is separated from less dense fluid (fresh water) by a vertical lock gate. When the gate is removed the dense fluid flows along the bottom boundary as a gravity current, and the fresh water flows along the surface forming a second gravity current. The total depth of the flow is 0.2 m and the initial reduced gravity of the two fluids is 29 mm/s. The effective depth of the current, determined by integrating the width-average density, is superimposed as the red curve. The side view also shows the initial lock position (the vertical yellow line) and the other lines correspond to the depths of the energy-conserving solution (blue) and of the current with maximum speed and dissipation (the two green lines, one each for the dense and light currents).
Shin et al. supplementary movie
Figure 10. Gravity current produced by lock exchange: full-depth release, tilted lock gate. The upper panel shows a side view of the experiment. Initially, dense fluid (salt solution), marked with dye, is separated from less dense fluid (fresh water) by a tilted lock gate. When the gate is removed the dense fluid flows along the bottom boundary as a gravity current, and the fresh water flows along the surface forming a second gravity current. The total depth of the flow is 0.2 m and the initial reduced gravity of the two fluids is 29.1 mm/s. The effective depth of the current, determined by integrating the width-average density, is superimposed as the red curve. The side view also shows the initial lock position (the vertical yellow line) and the other lines correspond to the depths of the energy-conserving solution (blue) and of the current with maximum speed and dissipation (the two green lines, one each for the dense and light currents). The lower panel shows the effective depth in false colour plotted in the x-t plane. The lock gate is at x=0, and the propagation of the fronts of the dense current (x>0) and the light current (x<0) from this position are shown. The fact that the edges of the coloured regions are straight shows that the currents travel at constant (and almost equal) speeds. The horizontal line indicates the time corresponding to the image in the upper panel.
Shin et al. supplementary movie
Figure 10. Gravity current produced by lock exchange: full-depth release, tilted lock gate. The upper panel shows a side view of the experiment. Initially, dense fluid (salt solution), marked with dye, is separated from less dense fluid (fresh water) by a tilted lock gate. When the gate is removed the dense fluid flows along the bottom boundary as a gravity current, and the fresh water flows along the surface forming a second gravity current. The total depth of the flow is 0.2 m and the initial reduced gravity of the two fluids is 29.1 mm/s. The effective depth of the current, determined by integrating the width-average density, is superimposed as the red curve. The side view also shows the initial lock position (the vertical yellow line) and the other lines correspond to the depths of the energy-conserving solution (blue) and of the current with maximum speed and dissipation (the two green lines, one each for the dense and light currents). The lower panel shows the effective depth in false colour plotted in the x-t plane. The lock gate is at x=0, and the propagation of the fronts of the dense current (x>0) and the light current (x<0) from this position are shown. The fact that the edges of the coloured regions are straight shows that the currents travel at constant (and almost equal) speeds. The horizontal line indicates the time corresponding to the image in the upper panel.
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