Double diffusive convection
Encyclopedia
Double-diffusive convection is an important fluid dynamics
topic that describes a form of convection
driven by two different density gradients which have different rates of diffusion
.
Convection in fluids is driven by density variation within them. These density variations may be caused by gradients in the composition of the fluid, or by differences in temperature (through thermal expansion
). Thermal and compositional gradients can often diffuse
with time, reducing their ability to drive the convection, and requiring that gradients in other regions of the flow exist in order for the convection to continue. A common example of double diffusive convection is in oceanography
, where heat and salt
concentrations exist with different gradients and diffuse at differing rates. An example that affects both of these variables is the input of cold freshwater from an iceberg.
Double diffusive convection is important in understanding the evolution of a number of systems that have multiple causes for density variations. These include convection in the Earth's oceans (as mentioned above), in magma chamber
s, and in the sun (where temperature and Helium diffuse at differing rates).
Two quite different types of fluid motion exist - and therefore are classified accordingly - depending on whether the stable stratification is provided by the density-affecting component with the lowest or the highest molecular diffusivity. If the stratification is provided by the component with the lower molecular diffusivity (for example in case of a stable salt-stratified ocean perturbed by a thermal gradient due to an iceberg), the stratification is called to be of "diffusive"
type (see external link below), otherwise it is of "finger" type, occurring frequently in oceanographic studies as salt-fingers. These long fingers of rising and sinking water occur when hot saline water lies over cold fresh water of a higher density. A perturbation to the surface of hot salty water results in an element of hot salty water surrounded by cold fresh water. This element loses its heat more rapidly than its salinity because the diffusion of heat is faster than of salt; this is analogous to the way in which just unstirred coffee goes cold before the sugar has diffused to the top. Because the water becomes cooler but remains salty, it becomes denser than the fluid layer beneath it. This makes the perturbation grow and causes the downward extension of a salt finger. As this finger grows, additional thermal diffusion accelerates this effect.
Fluid dynamics
In physics, fluid dynamics is a sub-discipline of fluid mechanics that deals with fluid flow—the natural science of fluids in motion. It has several subdisciplines itself, including aerodynamics and hydrodynamics...
topic that describes a form of convection
Convection
Convection is the movement of molecules within fluids and rheids. It cannot take place in solids, since neither bulk current flows nor significant diffusion can take place in solids....
driven by two different density gradients which have different rates of diffusion
Diffusion
Molecular diffusion, often called simply diffusion, is the thermal motion of all particles at temperatures above absolute zero. The rate of this movement is a function of temperature, viscosity of the fluid and the size of the particles...
.
Convection in fluids is driven by density variation within them. These density variations may be caused by gradients in the composition of the fluid, or by differences in temperature (through thermal expansion
Thermal expansion
Thermal expansion is the tendency of matter to change in volume in response to a change in temperature.When a substance is heated, its particles begin moving more and thus usually maintain a greater average separation. Materials which contract with increasing temperature are rare; this effect is...
). Thermal and compositional gradients can often diffuse
Diffusion
Molecular diffusion, often called simply diffusion, is the thermal motion of all particles at temperatures above absolute zero. The rate of this movement is a function of temperature, viscosity of the fluid and the size of the particles...
with time, reducing their ability to drive the convection, and requiring that gradients in other regions of the flow exist in order for the convection to continue. A common example of double diffusive convection is in oceanography
Oceanography
Oceanography , also called oceanology or marine science, is the branch of Earth science that studies the ocean...
, where heat and salt
Salinity
Salinity is the saltiness or dissolved salt content of a body of water. It is a general term used to describe the levels of different salts such as sodium chloride, magnesium and calcium sulfates, and bicarbonates...
concentrations exist with different gradients and diffuse at differing rates. An example that affects both of these variables is the input of cold freshwater from an iceberg.
Double diffusive convection is important in understanding the evolution of a number of systems that have multiple causes for density variations. These include convection in the Earth's oceans (as mentioned above), in magma chamber
Magma chamber
A magma chamber is a large underground pool of molten rock found beneath the surface of the Earth. The molten rock in such a chamber is under great pressure, and given enough time, that pressure can gradually fracture the rock around it creating outlets for the magma...
s, and in the sun (where temperature and Helium diffuse at differing rates).
Two quite different types of fluid motion exist - and therefore are classified accordingly - depending on whether the stable stratification is provided by the density-affecting component with the lowest or the highest molecular diffusivity. If the stratification is provided by the component with the lower molecular diffusivity (for example in case of a stable salt-stratified ocean perturbed by a thermal gradient due to an iceberg), the stratification is called to be of "diffusive"
type (see external link below), otherwise it is of "finger" type, occurring frequently in oceanographic studies as salt-fingers. These long fingers of rising and sinking water occur when hot saline water lies over cold fresh water of a higher density. A perturbation to the surface of hot salty water results in an element of hot salty water surrounded by cold fresh water. This element loses its heat more rapidly than its salinity because the diffusion of heat is faster than of salt; this is analogous to the way in which just unstirred coffee goes cold before the sugar has diffused to the top. Because the water becomes cooler but remains salty, it becomes denser than the fluid layer beneath it. This makes the perturbation grow and causes the downward extension of a salt finger. As this finger grows, additional thermal diffusion accelerates this effect.
External links
- Double Diffusive Convection
- Oceanic Double-diffusion: Introduction
- Double Diffusion in Oceanography
- Diffusive-mode Double Diffusive Convection, Stability and Density-Driven Flows
- Stockman, H.W; Li, C.; Cooper, C.; 1997. Practical application of lattice-gas and lattice Boltzmann methods to dispersion problems. InterJournal of Complex Systems, manuscript no. 90.