Gravity Currents in the Laboratory, Atmosphere, and Ocean

A gravity or current, is the flow of one fluid within another caused by the density difference between the fluids. The difference in specific weight that provides the driving force may be due to dissolved or suspended material or to temperature differences. Gravity currents are primarily horizontal, occurring as either top or bottom boundary currents, or as intrusions at some intermediate level. The fluids are usually miscible and the mixing that results can play an important part in the dynamics of the flow. 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. In the atmosphere, thunderstorm outflows and sea-breeze fronts are gravity currents of relatively cold dense air. Atmospheric-suspension gravity currents include avalanches of airborne snow particles, also fiery avalanches and base surges formed from gases and solids issuing from volcanic eruptions. Gravity currents have important applications in aircraft safety, atmo­ spheric pollution, entomology and pest control, and especially in dense-gas technology. Industrial accidents in which gravity currents may be formed include the spread of a dense gas from an accidental release. In the ocean, gravity currents are driven by salinity and temperature inhomogeneities, or as turbidity currents whose density derives from sus­ pended mud or silt. Lines of foam and debris on the ocean surface may indicate the front of a gravity frequently brought about by tidal processes. The latter also affect the behavior of gravity currents such as river plumes at the surface and salt wedges on a river bed. The important problems related to oil spillage on the sea have been the subject of a review paper in this series (Hoult 1972).