I am interested in gravity currents, or " density currents", the flow of one liquid within another caused by the density difference between the fluids. The difference that provides the driving force may be due to dissolved or suspended material or to temperature differences. Since gravity currents are formed in many different natural situations and may also be man-made, knowledge of their properties is of importance in many scientific disciplines.
Gravity currents have been studied in :
Atmosphere
,
Earth Sciences
,
Oceans
and in
Laboratory Experiments
.
This gravity current in the atmosphere is the front of an outflow of
cold air from a thunderstorm. It is about 1000 metres high and advancing
at 25 metres per second.
The front of this gravity current can be seen by suspended dust, but usually such fronts in the atmosphere are completely invisible, and their sudden wind changes present a serious hazard to aircraft. The most dangerous form of thunderstorm outflow is called a microburst . This is so small and appears so unexpectedly that it is difficult to give warning of its presence. The very large changes in wind that it can produce may only last for 2 to 4 minutes, but 18 aircraft accidents over the world have been solely attributed to microsbursts.
A microburst is surrounded by a ring vortex, spreading along the ground.
This sudden changes in wind may be lethal to aircraft approaching, or taking
off, at comparatively low speeds.
Another gravity current of cold air is produced by the sea breeze, a
result of the temperature difference between sea and land. As well as producing
welcome cool air inland, a sea breeze front is capable of carrying pollution
inland from Los Angeles, a distance of 40 miles.
Many different forms of mass transport which occur under gravity have been studied in geological sciences. Many of these are dangerous to human life and property, varying from land slides and rock falls to destructive mud flows.
Snow avalanches are familiar to most people, and much effort has gone
into trying to understand how they are formed and how they behave. Avalanches
of airborne powder-show, as shown here, are formed of fluidised snow particles.
They can have travel at 100 m s-1 and may be 100 m deep .
Volcanic eruptions can produce many different kinds of gravity current,
which depend on the nature of the magma, the molten rock beneath the ground.
If the magma is viscous and contains much dissolved gas, an explosive eruption
of fragmented glowing material may flow down the mountain side as a pyroclastic
gravity current .
This photo of a pyroclastic gravity current on Mt. Ngaurahoe, New Zealand,
was taken 90 seconds after the eruption during which the glowing material
was emitted at the summit of the mountain.
Basaltic lava flows.
Basaltic lava is erupted as a runny liquid , at a temperature of about
1200 C glowing reddish-yellow. Here is a basaltic lava flow on the island
of Surtsey, Iceland; it is flowing steadily from the lava pond higher up
the mountain, down into the sea.
Gravity currents appear in estuaries, where the fresh water of the river
meets the salt water of the ocean. The diagram shows how a gravity current
of saline water moves up the river bed as a "saline wedge".
In rivers that are not too fast or turbulent, the surface front of the fresh water can be seen. The front sometimes form a colour change in the water, but more often can be seen from floating debris which collects along the line of converging flow.
Sometimes these fronts have multiple lines as shown in this picture
taken at Trondheim in Norway.
Much of the dynamics of gravity currents has been discovered from laboratory
experiments in which salt water has been released from behind a lock gate
into a large tank of fresh water.
Simultaneous side and top views are shown of a 1% salt solution gravity current moving from left to right. Shadowgraph technique is used to display some of the internal structure of the flow, in which a beam of parallel light is deflected on a screen by density differences.
Features than can be seen here are;
Gravity currents and bores, or solitary waves.
Here a gravity current (blue) is causing a bore, or solitary wave, to form and advance in the shallow layer (pink) of fluid. This phenomenon has also been observed in the atmosphere, when a sea-breeze front reacts with an evening inversion layer.
Another effect that has been observed in the atmosphere, has also been investigated by means of laboratory experiments:
The collision of two gravity currents
When two thunderstorm outflows, or an outflow and a sea-breeze front
collide observations seem to show that , after the collision, the two fronts
cross and continue to move on almost unaffected.
Laboratory experiments suggest a simple explanation. The diagram shows what happens: after the head-on collision between two exactly equal gravity currents , all the energy is transferred to two equal bores which are reflected back along the layer laid down by the gravity currents. If the two gravity currents are unequal then the less dense one will move above the denser one, but most of the energy is still transferred to the two bores.