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# vorticity

## definition : vorticity

In continuum mechanics, the vorticity is a pseudovector field that describes the local spinning motion of a continuum near some point (the tendency of something to rotate ), as would be seen by an observer located at that point and traveling along with the flow.

Conceptually, vorticity could be determined by marking the part of continuum in a small neighborhood of the point in question, and watching their relative displacements as they move along the flow. The vorticity vector would be twice the mean angular velocity vector of those particles relative to their center of mass, oriented according to the right-hand rule. This quantity must not be confused with the angular velocity of the particles relative to some other point.

More precisely, the vorticity is a pseudovector field , defined as the curl (rotational) of the flow velocity vector. The definition can be expressed by the vector analysis formula:

:$\vec\left\{\omega\right\}\equiv \nabla \times \vec\left\{u\right\}\,,$

where ∇ is the del operator. The vorticity of a two-dimensional flow is always perpendicular to the plane of the flow, and therefore can be considered a scalar field.

The vorticity is related to the flow's circulation (line integral of the velocity) along a closed path by the (classical) Stokes' theorem. Namely, for any infinitesimal surface element C with normal direction and area dA, the circulation dΓ along the perimeter of C is the dot product ∙ (dA ) where is the vorticity at the center of C.

:$\left\{dx\over \omega_x\right\} = \left\{dy\over \omega_y\right\} = \left\{dz\over \omega_z\right\},$

where $\vec\left\{\omega\right\} = \left(\omega_x,\omega_y,\omega_z\right),$ is the vorticity vector in cartesian coordinates.

A vortex tube is the surface in the continuum formed by all vortex-lines passing through a given (reducible) closed curve in the continuum. The 'strength' of a vortex-tube (also called vortex flux) is the integral of the vorticity across a cross-section of the tube, and is the same everywhere along the tube (because vorticity has zero divergence). It is a consequence of Helmholtz's theorems (or equivalently, of Kelvin's circulation theorem) that in an inviscid fluid the 'strength' of the vortex tube is also constant with time. Viscous effects introduce frictional losses and time dependence.

In a three dimensional flow, vorticity (as measured by the volume integral of its squared magnitude) can be intensified when a vortex-line is extended — a phenomenon known as vortex stretching. This phenomenon occurs in the formation of a bath-tub vortex in out-flowing water, and the build-up of a tornado by rising air-currents.

Helicity is vorticity in motion along a third axis in a corkscrew fashion.

== Vorticity meters ==

=== Rotating-vane vorticity meter === A rotating-vane vorticity meter was invented by Russian hydraulic engineer A.Ya.Milovich (1874-1958). In 1913 he proposed a cork with four blades attached as a device qualitatively showing the magnitude of the vertical projection of the vorticity and demonstrated a motion-picture photography of float's motion on the water surface in a model of river bend.

Rotating-vane vorticity meters are commonly shown in educational films on continuum mechanics (famous examples include the NCFMF's "Vorticity" and "Fundamental Principles of Flow" by Iowa Institute of Hydraulic Research).

=== Non-rotating vorticity meters ===

==Specific sciences==

===Aeronautics=== In aerodynamics, the lift distribution over a finite wing may be approximated by assuming that each segment of the wing has a semi-infinite trailing vortex behind it. It is then possible to solve for the strength of the vortices using the criterion that there be no flow induced through the surface of the wing. This procedure is called the vortex panel method of computational fluid dynamics. The strengths of the vortices are then summed to find the total approximate circulation about the wing. According to the Kutta–Joukowski theorem, lift is the product of circulation, airspeed, and air density.

===Atmospheric sciences=== The relative vorticity is the vorticity of the air velocity field relative to the Earth. This is often modeled as a two-dimensional flow parallel to the ground, so that the relative vorticity vector is generally perpendicular to the ground, and can then be viewed as a scalar quantity, positive when the vector points upward, negative when it points downwards. Therefore, vorticity is positive when the wind turns counter-clockwise (looking down onto the Earth's surface). In the Northern Hemisphere, positive vorticity is called cyclonic rotation, and negative vorticity is anticyclonic rotation; the nomenclature is reversed in the Southern Hemisphere.

The absolute vorticity is computed from the air velocity relative to an inertial frame, and therefore includes a term due to the Earth's rotation, the Coriolis parameter.

The potential vorticity is absolute vorticity divided by the vertical spacing between levels of constant entropy (or potential temperature). The absolute vorticity of an air mass will change if the air mass is stretched (or compressed) in the z direction, but the potential vorticity is conserved in an adiabatic flow, which predominates in the atmosphere. The potential vorticity is therefore useful as an approximate tracer of air masses over the timescale of a few days, particularly when viewed on levels of constant entropy.

The barotropic vorticity equation is the simplest way for forecasting the movement of Rossby waves (that is, the troughs and ridges of 500 hPa geopotential height) over a limited amount of time (a few days). In the 1950s, the first successful programs for numerical weather forecasting utilized that equation.

In modern numerical weather forecasting models and general circulation models (GCM's), vorticity may be one of the predicted variables, in which case the corresponding time-dependent equation is a prognostic equation.

Helicity of the air motion is important in forecasting supercells and the potential for tornadic activity.

==See also== * Barotropic vorticity equation * D'Alembert's paradox * Enstrophy * Velocity potential * Vortex * Vortex tube * Vortex stretching * Vortical * Horseshoe vortex * Wingtip vortices

=== Fluid dynamics === * Biot–Savart law * Circulation * Vorticity equations * Kutta–Joukowski theorem

=== Atmospheric sciences === * Prognostic equation * Carl-Gustaf Rossby * Hans Ertel

==References==

==Bibliography== *Clancy, L.J. (1975), Aerodynamics, Pitman Publishing Limited, London * "[http://www.weather.com/glossary/v.html Weather Glossary]"' The Weather Channel Interactive, Inc.. 2004. * "[http://www.tpub.com/content/aerographer/14010/css/14010_18.htm Vorticity]". Integrated Publishing.

==Further reading== * * Ohkitani, K., "Elementary Account Of Vorticity And Related Equations". Cambridge University Press. January 30, 2005. * Chorin, Alexandre J., "Vorticity and Turbulence". Applied Mathematical Sciences, Vol 103, Springer-Verlag. March 1, 1994. * Majda, Andrew J., Andrea L. Bertozzi, "Vorticity and Incompressible Flow". Cambridge University Press; 2002. * Tritton, D. J., "Physical Fluid Dynamics". Van Nostrand Reinhold, New York. 1977. * Arfken, G., "Mathematical Methods for Physicists", 3rd ed. Academic Press, Orlando, Florida. 1985.