Editing Gravity current (section)

{{Short description|Flow that is driven by a density difference and gravity}}

In [[fluid dynamics]], a '''gravity current''' or '''density current''' is a primarily horizontal flow in a [[gravity|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 (buoyancy)|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.<ref name=gravintro>
{{Cite book
| last = Ungarish
| first = Marius
| year = 2009
| title = An Introduction to Gravity Currents and Intrusions
| publisher = Chapman and Hall/CRC
| place = New York City
| edition = 1st
| doi = 10.1201/9781584889045
| isbn = 978-0-429-14343-4}}</ref> Other examples include [[dust storm]]s, [[turbidity current]]s, [[avalanche]]s, discharge from [[wastewater]] or industrial processes into rivers, or river discharge into the ocean.<ref>{{Cite book|last=Turner|first=J. S.|url=https://books.google.com/books?id=x8NqYA97-wMC&dq=buoyancy+effects+in+fluids&pg=PR13|title=Buoyancy Effects in Fluids|date=1979|publisher=Cambridge University Press|isbn=978-0-521-29726-4|language=en}}</ref><ref name=Huppert2006>{{Cite journal
 | last = Huppert | first = H. E.
 | year = 2006
 | title = Gravity currents: a personal perspective
 | journal = [[Journal of Fluid Mechanics]]
 | volume = 554| pages = 299–322
 | doi=10.1017/S002211200600930X| doi-broken-date = 2024-11-11
 | bibcode = 2006JFM...554..299H
 | s2cid = 53073589
 }}</ref>

Gravity currents are typically much longer than they are tall. Flows that are primarily vertical are known as [[Plume (fluid dynamics)|plumes]]. As a result, it can be shown (using [[dimensional analysis]]) that vertical velocities are generally much smaller than horizontal velocities in the current;  the pressure distribution is thus approximately [[Hydrostatics|hydrostatic]], apart from near the leading edge. Gravity currents may be simulated by the [[shallow water equations]], with special dispensation for the leading edge which behaves as a discontinuity.<ref name=gravintro /> When a gravity current propagates along a plane of neutral buoyancy within a stratified ambient fluid, it is known as a [[gravity current intrusion]].