Abstract
We present an experimental investigation of steady particle-driven gravity currents with Reynolds numbers in the range 500–1600, and with the ratio of the initial current speed to the fall speed of the particles, S = u 0 /u fall , in the range 5 < S < 160. We identify three regimes: (i) For S < 10, the particles settle close to the source at a velocity corresponding to their fall speed, consistent with the observation of sedimenting fronts in classical settling column experiments. (ii) In the range 10 < S < 40, a steady gravity current develops within the tank. The experiments show that the depth of the gravity current gradually decreases away from the source and dye added to the source liquid appears above the gravity current along its entire length, suggesting that there is a sedimentation front, so that the volume and momentum fluxes of the current gradually decrease with distance from the source. We find that as S increases, the descent speed of the sedimentation front decreases relative to the fall speed of the particles, and the run-out length of the gravity current increases. We note that the density of the interstitial fluid corresponds to the density of the ambient fluid, so that any reduction in buoyancy of the gravity current is attributed to the sedimentation of particles on the floor of the tank and we do not observe lofting of the interstitial fluid. (iii) For 40 < S < 160, the gravity currents reach the end of our experimental tank and we no longer observe a sedimentation front. For these experiments, it appears that the entrainment at the top of the current begins to match the sedimentation and so the current depth does not change significantly over the scale of the tank, but a larger scale experimental system would be needed to explore the full run-out behaviour for these larger values of S. For the intermediate case, 10 < S < 40, we develop a model for the conservation of volume, momentum and buoyancy fluxes in the current, accounting for the sedimentation front and the release of fluid at the top surface of the gravity current, and we compare this with our new experimental data.
Highlights
Continuous particle-driven gravity currents frequently occur in nature and industry.In nature, examples include pyroclastic flows issuing from volcanoes, sustained 889 A20-2M
We develop a quantitative model for the conservation of the fluxes of volume, momentum and buoyancy. This model accounts for a possible sedimentation front, and, by comparison with our experimental data, we propose that the effective speed of this front decreases as the ratio of the current speed to the fall speed of the particles, S, increases
We have studied the dynamics of steady particle-laden gravity currents
Summary
Many authors have explored the dynamics of finite-volume gravity currents produced by the release of a fixed volume of fluid from a lock gate Models of these flows often assume that the flow maintains a constant volume (Bonnecaze et al 1993; Huppert 1998; Simpson 1999; Huppert 2006; Ungarish 2009). Depending on the magnitude of the density difference between host fluid and ambient fluid, and the size and density of the particles, the lofting interstitial fluid may lift particles up from the gravity current (Steel et al 2017) This complication is beyond the scope of the present study. We present a series of experiments in which steady, particle-driven gravity currents develop from the collapse of turbulent multiphase fountains This source condition allows for the supply of a continuous. We consider the implications of our model for the dynamics of particleladen gravity currents in industry and nature
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