Gravity current

Résumé

In fluid dynamics, a gravity current or density current is a primarily horizontal flow in a gravitational field that is driven by a density difference in a fluid or fluids and is constrained to flow horizontally by, for instance, a ceiling. Typically, the density difference is small enough for the Boussinesq approximation to be valid. Gravity currents can be thought of as either finite in volume, such as the pyroclastic flow from a volcano eruption, or continuously supplied from a source, such as warm air leaving the open doorway of a house in winter. Other examples include dust storms, turbidity currents, avalanches, discharge from wastewater or industrial processes into rivers, or river discharge into the ocean. Gravity currents are typically much longer than they are tall. Flows that are primarily vertical are known as plumes. As a result, it can be shown (using dimensional analysis) that vertical velocities are generally much smaller than horizontal velocities in the current; t

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https://en.wikipedia.org/wiki/Gravity_current

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Visualization of the internal flow properties and the material exchange interface in an entraining viscous Newtonian gravity current

Christophe Ancey, Belinda Margaret Bates

In order to simulate a simple entraining geophysical flow, a viscous Newtonian gravity current is released from a reservoir by a dam-break and flows along a rigid horizontal bed until it meets a layer of entrainable material of finite depth, identical to the current. The goal is to examine the entrainment mechanisms by observing the interaction between the incoming flow and the loose bed. The sole parameter varied is the initial volume of the gravity current, thus altering its height and velocity. The gravity current plunges or spills into the entrainable bed and the velocity of the flow front becomes linear with time. The bed material is directly affected: motion is generated in the fluid far downstream of, and in that lying beneath the encroaching front. Shear bands are identified, separating horizontal flow downstream from flow with a strong vertical component close to the step. Downstream of the step the flow is horizontal and stratified, with no slip on the bottom boundary and very low shear near the surface. Between these two regions may lie transitional zones with linear velocity profiles, separated by horizontal bands of high shear; the number of transitional zones in the cross-section varies with the initial volume of the dam-break.

Springer Verlag2014

Chargement

Laboratory-scale dam-break study of gravity currents with basal entrainment: PIV measurements of a viscous newtonian fluid over a horizontal bed of the same ﬂuid.

Christophe Ancey, Belinda Margaret Bates

Geophysical gravity flows such as avalanches and debris flows belong to a special class of hazardous environmental event, in which a mixture of solids and fluids (e.g. debris and mud, snow and air) flow as a liquid and may run out much further than expected over a slope less steep than the critical angle of repose. Exchange of material between the overriding flow and a loose bed layer underneath is known to affect the characteristics of such a flow however it is not well understood how this plays a part, nor the mechanism of such an exchange, due to the difficulties of seeing inside a flowing mass in the field and experiments. Research has been carried out on an entraining viscous gravity current in an laboratory flume using Particle Image Velocimetry. The aim was to investigate the flow properties in a vertical slice of fluid in the downstream direction, far from the side-walls, when a flowing layer overrides a basal layer of the same material. The parameter varied here is the volume of fluid released from the reservoir, which affects the height and velocity of the encroaching fluid current. An interface was identified between the overriding fluid and the initially stationary bed fluid. PIV was used to show the evolution of the velocity and shear fields throughout the system compared to the location of this interface and how this changes with the volume of fluid released. The overriding fluid displaces the bed fluid, either by plunging or spilling into the erodible layer depending on the initial volume, thus inciting the stationary material downstream and below to move. Initially, a large amount of bed material is suddenly mobilized by the front of overriding fluid and little deposition occurs. Shortly after this we see the clear development of a depositional region (with low velocities, predominantly in the downwards direction) and an eroding region (with high, almost uniformly horizontal velocities) within the front of material that enters the bed, separated by a region of high shear. For larger released volumes, multiple shear bands develop, thus showing distinct regions with different rates of material deposition.

2012

Chargement

Geophysical gravity-driven flows -- including avalanches, debris flows, pyroclastic flows and submarine turbidity currents -- are multiphase natural hazards that flow under the influence of gravity. Despite their differences, they share much of the same physics, having the potential to pick up material from beneath, a process called basal entrainment during which the flow may increase in volume and velocity manyfold. Due to their complexity and unpredictability there are still many unanswered questions about their mechanics, so that many of the theoretical models in use are based on insufficient data sets and may not apply to a general case. Here, basal entrainment by geophysical gravity-driven flows is studied by isolating the process in idealised laboratory experiments. The avalanche is simplified and controlled in such a way that any changes can be confidently attributed to the entrainment process alone. Further, the methods available in the laboratory allow the continuous, passive study of entrainment, so that for the first time full data sets of internal measurements are obtained, from experiments ranging from simple to complex. The data obtained is easily exploited for confirmation of the mathematical models developed in this thesis. Experiments which simulated entraining avalanches as Newtonian dam-breaks along a horizontal flume and as viscoplastic dam-breaks along an inclined flume both showed an increase in front position, dependent on the amount of material available, amongst other changes. The experimental results allow the development of a theoretical thin-film model in both cases which is solved numerically and compares favourably with the data obtained. The Newtonian model reproduced the flow characteristics excellently and the viscoplastic model successfully simulated the effects of entrainable material. The possibility of performing similar experiments using a granular suspension is also investigated, with promising results. This work shows that the effect of entrainment on gravity-driven flows can be quantified and modelled mathematically as a non-local transport process. This has implications for hazard modelling: if the quantity of available loose material is known, and its characteristics are similar to those of the flowing avalanche, the avalanche and the entrainable bed can be modelled as a continuous flow over a rigid base. Thus it is suggested that the models developed be tested in more realistic cases, e.g. in the case of an avalanche entraining material with different characteristics, or in a more complex geometry, in order to better mimic what happens in nature.

EPFL2015

Chargement

Laboratory-scale dam-break study of gravity currents with basal entrainment: PIV measurements of a viscous newtonian fluid over a horizontal bed of the same ﬂuid.

Christophe Ancey, Belinda Margaret Bates

2012

EPFL2015