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Vortex

/Vortex
Vortex2012-11-30T16:45:50+00:00

Pathlines of fluid particles around the axis (dashed line) of an ideal irrotational vortex. (See animation)

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Vortices form spontaneously in stirred fluids, including Jupiter’s atmosphere.

Vortices are a major component of vorticity (the curl of the fluid’s velocity) is very high in a core region surrounding the axis, and nearly zero in the rest of the vortex; while the pressure drops sharply as one approaches that region.

Once formed, vortices can move, stretch, twist, and interact in complex ways. A moving vortex carries with it some angular and linear momentum, energy, and mass. In a spirals.

Properties

contrail demonstrates vortex

Vorticity

A key concept in the dynamics of vortices is the vorticity, a vector that describes the local rotary motion at a point in the fluid, as would be perceived by an observer that moves along with it. Conceptually, the vorticity could be observed by placing a tiny rough ball at the point in question, free to move with the fluid, and observing how it rotates about its center. The direction of the vorticity vector would be the direction of the axis of rotation of this imaginary ball (according to the right-hand rule) while its length would be proportional to the ball’s angular velocity. Mathematically, the vorticity is defined as the curl (or rotational) of the velocity field of the fluid, usually denoted by $vec omega$ and expressed by the vector analysis formula $nabla times vec{mathit{u}}$, where $nabla$ is the nabla operator.[5]

The local rotation measured by the vorticity $vec omega$ must not be confused with the angular velocity vector of that portion of the fluid with respect to the external environment or to any fixed axis. In a vortex, in particular, $vec omega$ may be opposite to the mean angular velocity vector of the fluid relative to the vortex line.

Vorticity profiles

The vorticity in a vortex depends on how the speed v of the particles varies as the distance r from the axis. There are two important special cases:

• If the fluid rotates like a rigid body – that is, if v increases proportionally to r – a tiny ball carried by the flow would also rotate about its center as if it were part of that rigid body. In this case, $vec omega$ is the same everywhere: its direction is parallel to the spin axis, and its magnitude is twice the angular velocity of the whole fluid.
• If the particle speed v is inversely proportional to the distance r, then the imaginary test ball would not rotate over itself; it would maintain the same orientation while moving in a circle around the vortex line. In this case the vorticity $vec omega$ is zero at any point not on that line, and the flow is said to be irrotational.
 Irrotational vortex Rotational (rigid-body) vortex

Irrotational vortices

In the absence of external forces, a vortex usually evolves fairly quickly toward the irrotational flow pattern, where the flow velocity v is inversely proportional to the distance r. For that reason, irrotational vortices are also called free vortices.

For an irrotational vortex, the circulation is zero along any closed contour that does not enclose the vortex axis and has a fixed value, $Gamma$, for any contour that does enclose the axis once.[6] The tangential component of the particle velocity is then $v_{theta} = Gamma/(2 pi r)$. The angular momentum per unit mass relative to the vortex axis is therefore constant, $r v_{theta} = Gamma/(2 pi)$.

However, the ideal irrotational vortex flow is not physically realizable, since it would imply that the particle speed (and hence the force needed to keep particles in their circular paths) would grow without bound as one approaches the vortex line. Indeed, in real vortices there is always a core region surrounding the axis where the particle velocity stops increasing and then decreases to zero as r goes to zero. Within that region, the flow is no longer irrotational: the vorticity $vec omega$ becomes non-zero, with direction roughly parallel to the vortex line. The Navier-Stokes equations governing fluid flows and assumes cylindrical symmetry, for which

$v_{theta} = (1 - e^{-r^2/(4nu t)})Gamma/(2 pi r).$

In an irrotational vortex, fluid moves at different speed in adjacent streamlines, so there is friction and therefore energy loss throughout the vortex, especially near the core.

Rotational vortices

A rotational vortex – one which has non-zero vorticity away from the core – can be maintained indefinitely in that state only through the application of some extra force, that is not generated by the fluid motion itself.

For example, if a water bucket is spun at constant angular speed w about its vertical axis, the water will eventually rotate in rigid-body fashion. The particles will then move along circles, with velocity v equal to wr.[6] In that case, the free surface of the water will assume a parabolic shape.

In this situation, the rigid rotating enclosure provides an extra force, namely an extra pressure gradient in the water, directed inwards, that prevents evolution of the rigid-body flow to the irrotational state.

Vortex geometry

In a stationary vortex, the typical streamline (a line that is everywhere tangent to the velocity vector) is a closed loop surrounding the axis; and each vortex line (a line that is everywhere tangent to the vorticity vector) is roughly parallel to the axis. A surface that is everywhere tangent to both velocity and vorticity is called a vortex tube. In general, vortex tubes are nested around the axis of rotation. The axis itself is one of the vortex lines, a limiting case of a vortex tube with zero diameter.

According to jet engine. One end of the vortex line is attached to the engine, while the other end usually stretches outs and bends until it reaches the ground.

When vortices are made visible by smoke or ink trails, they may seem to have spiral pathlines or streamlines. However, this appearance is often an illusion and the fluid particles are moving in closed paths. The spiral streaks that are taken to be streamlines are in fact clouds of the marker fluid that originally spanned several streamlines and were stretched into spiral shapes by the non-uniform velocity distribution. This is the case, for example, of the spiral arms of hurricanes.

Pressure in a vortex

The fluid motion in a vortex creates a dynamic Bernoulli’s Principle. One can say that it is the gradient of this pressure that forces the fluid to curve around the axis.

In a rigid-body vortex flow of a fluid with constant paraboloid.

In an irrotational vortex flow with constant fluid density and cylindrical symmetry, the dynamic pressure varies like PK/r2, where P is the limiting pressure infinitely far from the axis. This formula provides another constraint for the extent of the core, since the pressure cannot be negative. The free surface (if present) dips sharply near the axis line, with depth inversely proportional to r2.

The core of a vortex in air is sometimes visible because of a plume of water vapor caused by condensation in the low pressure and low temperature of the core; the spout of a tornado is a classic example. When a vortex line ends at a boundary surface, the reduced pressure at may also draw matter from that surface into the core. For example, a dust devil is a column of dust picked up by the core of an air vortex attached to the ground. By the same token, a vortex in a body of water that ends at the free surface (like the whirlpool that often forms over a bathtub drain) may draw a column of air down the core. The forward vortex extending from an engine of a parked airplane can suck water and small stones into the core and then into the engine.

Evolution

Vortex created by the passage of an aircraft wing, revealed by colored smoke

Vortices need not be steady-state features; they can move about and change their shape.

In a moving vortex, the particle paths are no longer closed, but are open loopy curves similar to cycloids.

A vortex flow may also be combined with a radial or axial flow pattern. In that case the streamlines and pathlines are not closed curves but spirals or solenoidal.

As long as the effects of viscosity and diffusion are negligible, the fluid in a moving vortex is carried along with it. In particular, the fluid in the core (and matter trapped by it) tends to remain in the core as the vortex moves about. This is a consequence of guns.

Two or more vortices that are approximately parallel and circulating in the same direction will attract and eventually merge to form a single vortex, whose propeller blades. On the other hand, two parallel vortices with opposite circulations (such as the two wingtip vortices of an airplane) tend to remain separate.

Vortices contain substantial energy in the circular motion of the fluid. In an ideal fluid this energy can never be dissipated and the vortex would persist forever. However, real fluids exhibit viscosity and this dissipates energy very slowly from the core of the vortex. It is only through dissipation of a vortex due to viscosity that a vortex line can end in the fluid, rather than at the boundary of the fluid.

Two-dimensional modeling

When the particle velocities are constrained to be parallel to a fixed plane, one can ignore the space dimension perpendicular to that plane, and model the flow as a two-dimensional velocity field on that plane. Then the vorticity vector $vec omega$ is always perpendicular to that plane, and can be treated as a scalar. This assumption is sometimes made in meteorology, when studying large-scale phenomena like hurricanes.

The behavior of vortices in such contexts is qualitatively different in many ways; for example, it does not allow the stretching of vortices that is often seen in three dimensions.

Further examples

North pole vortex on the planet Saturn

References

Notes

1. 3-540-53713-9.
2. ^ Kida, Shigeo (2001). “Life, Structure, and Dynamical Role of Vortical Motion in Turbulence”. IUTAM Symposium on Tubes, Sheets and Singularities in Fluid Dynamics. Zakopane, Poland.
3. ^ The Oxford English Dictionary
4. ^ The Merriam Webster Collegiate Dictionary
5. http://www.princeton.edu/~gkv/geoturb/turbch.pdf. Retrieved 2012-09-26.
6. ^ Clancy 1975, sub-section 7.5