Editing Gravity current (section)

== Structure and propagation ==
{{More citations needed section|date=June 2019|talk=Reversions_and_reorganization}}
Although gravity currents represent the flow of fluid of one density over/under another, discussion is usually focused on the fluid that is propagating. Gravity currents can originate either from finite volume flows or from continuous flows. In the latter case, the fluid in the head is constantly replaced and the gravity current can therefore propagate, in theory, forever. Propagation of a continuous flow can be thought of as the same as that of the tail (or body) of a very long finite volume. Gravity flows are described as consisting of two parts, a head and a tail. The head, which is the leading edge of the gravity current, is a region in which relatively large volumes of ambient [[fluid]] are displaced. The tail is the bulk of flow that follows the head. Flow characteristics can be characterized by the [[Froude number|Froude]] and [[Reynolds number|Reynolds]] numbers, which represent the ratio of flow speed to gravity (buoyancy) and viscosity, respectively.<ref name=Huppert2006 />

[[File:Gravity current obstacle.webm|thumb|Gravity current from a lock-exchange release interacting with a submerged obstacle mounted at the bottom.  Result from a 3D computational fluid dynamics (CFD) simulation.  Red surface is a density iso-surface between the dense gravity-current fluid and the light ambient fluid.  Before the impact, notice the billows structure and the lobe-and-cleft structure at the front.  ]]

Propagation of the head usually occurs in three phases. In the first phase, the gravity current propagation is turbulent. The flow displays billowing patterns known as [[Kelvin-Helmholtz instabilities]], which form in the wake of the head and engulf ambient fluid into the tail: a process referred to as "entrainment". Direct mixing also occurs at the front of the head through lobes and cleft structures which form on the surface of the head. According to one paradigm, the leading edge of a gravity current 'controls' the flow behind it: it provides a boundary condition for the flow. In this phase the propagation rate of the current is approximately constant with time. For many flows of interest, the leading edge moves at a Froude number of about 1; estimates of the exact value vary between about 0.7 and 1.4.<ref name=HS>
{{Cite journal
 | last1 = Huppert | first1 = H. E.
 | last2= Simpson | first2= J. E.
 | year = 1980
 | title = The slumping of gravity currents
 | journal = [[Journal of Fluid Mechanics]]
 | volume = 99| issue = 4| pages = 785–799
 | doi=10.1017/S0022112080000894
|bibcode = 1980JFM....99..785H | s2cid = 55580871
 }}</ref>
As the driving fluid depletes as a result of the current spreading into the environment, the driving head decreases until the flow becomes laminar. In this phase, there is only very little mixing and the billowing structure of the flow disappears. From this phase onward the propagation rate decreases with time and the current gradually slows down. Finally, as the current spreads even further, it becomes so thin that viscous forces between the intruding fluid and the ambient and boundaries govern the flow. In this phase no more mixing occurs and the propagation rate slows down even more.<ref name="HS"/><ref>
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 |year=1969
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 |editor-last=Hoult |editor-first=D. P
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}}</ref>

The spread of a gravity current depends on the boundary conditions, and two cases are usually distinguished depending on whether the initial release is of the same width as the environment or not. In the case where the widths are the same, one obtains what is usually referred to as a "lock-exchange" or a "corridor" flow. This refers to the flow spreading along walls on both sides and effectively keeping a constant width whilst it propagates. In this case the flow is effectively two-dimensional. Experiments on variations of this flow have been made with lock-exchange flows propagating in narrowing/expanding environments. Effectively, a narrowing environment will result in the depth of the head increasing as the current advances and thereby its rate of propagation increasing with time, whilst in an expanding environment the opposite will occur.<!--Those were some experiments by P. F. Linden I think--> In the other case, the flow spreads radially from the source forming an "axisymmetric" flow. The angle of spread depends on the release conditions. In the case of a point release, an extremely rare event in nature, the spread is perfectly axisymmetric, in all other cases the current will form a sector.

When a gravity current encounters a solid boundary, it can either overcome the boundary, by flowing around or over it, or be reflected by it. The actual outcome of the collision depends primarily on the height and width of the obstacle. If the obstacle is shallow (part) of the gravity current will overcome the obstacle by flowing over it. Similarly, if the width of the obstacle is small, the gravity current will flow around it, just like a river flows around a boulder. If the obstacle cannot be overcome, provided propagation is in the turbulent phase, the gravity current will first surge vertically up (or down depending on the density contrast) along the obstacle, a process known as "sloshing". Sloshing induces a lot of mixing between the ambient and the current and this forms an accumulation of lighter fluid against the obstacle. As more and more fluid accumulates against the obstacle, this starts to propagate in the opposite direction to the initial current, effectively resulting in a second gravity current flowing on top of the original gravity current. This reflection process is a common feature of doorway flows (see below), where a gravity current flows into a finite-size space. In this case the flow repeatedly collides with the end walls of the space, causing a series of currents travelling back and forth between opposite walls. This process has been described in detail by Lane-Serff.<ref>
{{Cite thesis
 |last=Lane-Serff |first=G. F.
 |year=1989
 |title=Heat Flow and Air Movement in Buildings
 |degree=PhD
 |publisher=[[University of Cambridge]]
 }}</ref>