Gravity current

In fluid dynamics, a gravity current is a primarily horizontal flow in a gravitational field that is driven by a density difference, hence gravity currents also sometimes being referred to as "density currents". When a gravity current propagates along a plane of neutral buoyancy within a stratified ambient fluid, it is known as a Gravity Current Intrusion. Typically, the density difference is small enough for the Boussinesq approximation to be valid. Gravity currents can be either finite in volume, such as the release from a dam break, or continuously supplied from a source, such as in doorway or lava flows.

Gravity currents are typically of very low aspect ratio: they are much longer than they are high. It can be shown using dimensional analysis that typical vertical velocities are much smaller than typical horizontal velocities in the current, and so the pressure distribution is thus approximately hydrostatic, apart from near the leading edge. Thus gravity currents may be simulated by the shallow water equations, with special dispensation for the leading edge which behaves as a discontinuity.

A typical gravity current consists of a head and tail structure. 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 fluid which follows the head.

Immediately in the wake of the head, intense mixing occurs between the gravity current and the ambient fluid. Mixing occurs from both above and below the gravity current. Mixing from above is a result of turbulent billows (Kelvin-Helmholtz instabilities) which form in the wake of the head and engulf ambient fluid into the tail, a process referred to as "entrainment". Mixing from below is a result of the gravity current overrunning ambient fluid, literally trapping it underneath. 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.

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