CONVECTION & BUOYANT FORCE

By Ross Nuechterlein

Convection

Convection is the movement of molecules within fluids and is one of the major modes of heat and mass transfer in liquids and gas.

Buoyancy

Buoyancy is an upward acting force exerted by a fluid , that opposes an object's weight.

Buoyancy = weight of displaced fluid.

Convection CellsA change in Temperature, Salinity or Density causes a fluid parcel to have vertical tendencies until a great enough force is achieved. Until then:Transport takes place horizontally till the downward force becomes greater than the rising force below.

Types of Convection

Natural Convection Gravitational Convection

Forced Convection Granular Convection Thermo magnetic

convection.

Natural Convection or Free Convection

Occurs due to temperature differences affecting density resulting in buoyancy (more or less dense)

This leads to bulk fluid movement. More rapid movement between two fluids

of large density differences. Larger acceleration occurs through

greater mediums

Gravitational Convection

Induced by buoyancy variations resulting from material properties other than temperature.

Variable salinity in water is a frequent cause of ocean convection.

Dependant on the effects of gravity and does not occur in micro gravity environments

Wind Driven Convection Cells

Circulation

Solar radiation warms the oceans near the Equator, driving warm water toward the poles.

Cold polar water replaces displaced warm water at the equator creating circulation.

Density differences also drive ocean convection due to varying salinity. This is known as Thermohaline convection.

In some cases warm saline water may sink being replaced by cooler fresher water which rises. This reverses the transport of heat.

Deep Ocean circulation is important because it determines the vertical stratification of the ocean and because it also modulates climate.

This circulation is caused by the convection processes discussed.

References

Stewart, R. H., 2002. Introduction to Physical Oceanography.

Open ocean convection: observations, theories, models.  http://www.math.nyu.edu/caos_teaching/ocean_dynamics/reading/marshall_schott99.pdf