Wake: Wikis

  
  

Note: Many of our articles have direct quotes from sources you can cite, within the Wikipedia article! This article doesn't yet, but we're working on it! See more info or our list of citable articles.

Encyclopedia

From Wikipedia, the free encyclopedia

The wake of a slow moving boat
Wake of a boat crossing an alpine lake

A wake is the region of recirculating flow immediately behind a moving solid body, caused by the flow of surrounding fluid around the body.

Contents

Fluid dynamics

In fluid dynamics, a wake is the region of disturbed flow (usually turbulent) downstream of a solid body moving through a fluid, caused by the flow of the fluid around the body. In incompressible fluids (liquids) such as water, a bow wake is created when a watercraft moves through the medium; as the medium cannot be compressed, it must be displaced instead, resulting in a wave. As with all wave forms, it spreads outward from the source until its energy is overcome or lost, usually by friction or dispersion.

The formation of these waves in liquids is analogous to the generation of shockwaves in compressible flow, such as those generated by rockets and aircraft traveling supersonically through air (see also Lighthill equation). The non-dimensional parameter of interest is the Froude number.

For a blunt body in subsonic external flow, for example the Apollo or Orion capsules during descent and landing, the wake is massively separated and there exists a reverse flow region behind the body, in which the flow is actually moving toward the body. This phenomenon is often observed in wind tunnel testing of aircraft, and is especially important when parachute systems are involved, because unless the parachute lines extend the canopy beyond the reverse flow region, the chute can fail to inflate and thus collapse. Parachutes deployed into wakes suffer dynamic pressure deficits which reduce their expected drag forces. High-fidelity computational fluid dynamics simulations are often undertaken to model wake flows, although such modeling has uncertainties associated with turbulence modeling (for example RANS versus LES implementations), in addition to unsteady flow effects. Example applications include rocket stage separation and aircraft store separation.

Wake pattern of a boat

Wake from a small motorboat with an outboard motor

Waterfowls and boats moving across the surface of water produce a wake pattern, first explained mathematically by Lord Kelvin and known today as the Kelvin wake pattern. This pattern consists of two wake lines that form the arms of a V, with the source of the wake at the point. Each wake line is offset from the path of the wake source by around 17° and is made up with feathery wavelets that are angled at roughly 53° to the path. The interior of the V is filled with transverse curved waves, each of which is an arc of a circle centered at a point lying on the path at a distance twice that of the arc to the wake source. This pattern is independent of the speed and size of the wake source over a significant range of values. The angles in this pattern are not intrinsic properties of water; Any isentropic and incompressible liquid with low viscosity will exhibit the same phenomenon. This phenomenon has nothing to do with turbulence. Everything discussed here is based on the linear theory of an ideal fluid.

This pattern follows from the Dispersion relation of deep water waves, which is often written as,

\omega = \sqrt{g k},

where g is the strength of the gravity field and "deep" means that the depth is greater than half of the wavelength. This formula has two implications: first, the speed of the wave scales with the wavelength and second, the group velocity of a deep water wave is half of its phase velocity.

As a surface object moves along its path at a constant velocity v, it continuously generates a series of small disturbances corresponding to waves with a wide spectrum of wavelengths. Those waves with the longest wavelengths have phase speeds above v and simply dissipate into the surrounding water without being easily observed. Only the waves with phase speeds at or below v get amplified through the process of constructive interference and form visible shock waves.

In a medium like air, where the dispersion relation is linear, i.e.

 \omega = c k,\,

the phase velocity c is the same for all wavelengths and the group velocity has the same value as well. The angle θ of the shock wave thus follows from simple trigonometry and can be written as,

\theta = \arcsin \left( \frac{c}{v} \right).

This angle is dependent on v, and the shock wave only forms when v > c.

In deep water, however, shock waves always form even from slow-moving sources because waves with short enough wavelengths move still more slowly. These shock waves also manifest themselves at sharper angles than one would naively expect because it is group velocity that dictates the area of constructive interference and, in deep water, the group velocity is only half of the phase velocity.

The trigonometrical coincidence that contributes to the Kelvin wake pattern.

By a simple accident in trigonometry, all shock waves that should have had angles between 33° and 72° get compressed into a narrow band of wake with angles between 15° and 19°, resulting in the two arms of the V in the Kelvin wake pattern. This can be seen easily in the diagram on the left. Here, we consider waves generated at point C by the source which has now moved to point A. These waves would have formed a shock wave at the line AB, with the angle CAB = 62° because the phase velocity of the wave has been chosen to be \sin \left( 62^\circ \right) = 0.883 of the boat velocity. But the group velocity is only half of the phase velocity, so the wake actually forms along the line AD, where D is the mid-point on the segment BC, and the wake angle CAD turns out to be 19°. The wavefronts of the wavelets in the wake coming from the wave components in our example still maintain an angle of 62° to the AC line. In reality, all the waves with would-be-shock-wave-angles between 33° and 72° contribute to the same narrow wake band and the wavelets exhibit an angle of 53°, which is roughly the average of 33° and 72°.

The wave components with would-be-shock-wave-angles between 73° and 90° dominate the interior of the V. Again, the waves that should have joined together and formed a wall similar to the phenomenon in sonic boom end up half-way between the point of generation and the current location of the wake source. This explains the curvature of the arcs.

Those very short waves with would-be-shock-wave-angles below 33° lack a mechanism to reinforce their amplitudes through constructive interference and are usually perceived by the naked eyes as small ripples on top of the interior transverse waves.

Recreation

"No wake zones" may prohibit wakes in marinas, near moorings and within some distance of shore[1] in order to facilitate recreation by other boats, and reduce the damage wakes cause.

Wakes are occasionally used recreationally. Swimmers, people riding personal watercraft, and aquatic mammals such as dolphins, can ride the leading edge of a wake. In the sport of wakeboarding the wake is used as a jump. The wake is also used to propel a surfer in the sport of wakesurfing. In the sport of water polo, the ball carrier can swim while advancing the ball, propelled ahead with the wake created by alternating armstrokes, a technique known as dribbling.

See also

References

  1. ^ BoatWakes.org, Table of distances

External links


1911 encyclopedia

Up to date as of January 14, 2010

From LoveToKnow 1911

'WAKE (A.S.' wacan, to "wake" or "watch"), a term now restricted to the Irish custom of an all-night "waking" or watching round a corpse before burial, but anciently used in the wider sense of a vigil kept as an annual church celebration in commemoration of the completion or dedication of the parish church. This strictly religious wake consisted in an all-night service of prayer and meditation in the church. These services, popularly known as "wakes," were officially termed Vigiliae by the church, and appear to have existed from the earliest days .of Anglo-Saxon Christianity. Tents and booths were set up in the churchyard before the dawn which heralded in a day devoted to feasting, dancing and sports, each parish keeping the morrow of its vigil as a holiday. Wakes soon degenerated into fairs; people from neighbouring parishes journeyed over to join in the merry-making, and as early as Edgar's reign (958-975) the revelry and drunkenness had become a scandal. The vigiliae usually fell on Sundays or saints' days, those being the days oftenest chosen for church dedications, and thus the abuse was the more scandalous. In 1445 Henry VI. attempted to suppress markets and fairs on Sundays and holy days. In 1536 an Act of Convocation ordered that the yearly "wake" should be held in every parish on the same day, viz. the first Sunday in October, but this regulation was disregarded. Wakes are specially mentioned in the Book of Sports of James I. and Charles I. among the feasts which should be observed.

Side by side with these church wakes there existed from the earliest times the custom of "waking" a corpse. The custom, as far as England was concerned, seems to have been older than Christianity, and to have been at first essentially Celtic. Doubtless it had a superstitious origin, the fear of evil spirits hurting or even removing the body, aided perhaps by the practical desire to keep away rats and other vermin. The Anglo-Saxons called the custom lich-wake or like-wake (A.S. lic, a corpse). With the introduction of Christianity the offering of prayer was added to the mere vigil, which until then had been characterized by formal mourning chants and recitals of the life story of the dead. As a rule the corpse, with a plate of salt on its breast, was placed under the table, on which was liquor for the watchers. These private wakes soon tended to become drinking orgies, and during the reign of Edward III. the provincial synod held in London proclaimed by its 10th canon the object of wakes to be the offering of prayer for the dead, and ordered that in future none but near relatives and friends of the deceased should attend. The penalty for disobedience was excommunication. With the Reformation and the consequent disuse of prayers for the dead the custom of "waking" in England became obsolete and died out. Many countries and peoples have been found to have a custom equivalent to "waking," which, however, must be distinguished from the funeral feasts pure and simple.

For detailed accounts of Irish wakes see Brand's Antiquities of Great Britain (W. C. Hazlitt's edition, 1905) under "Irish Wakes."


<< William Wake

Edward Gibbon Wakefield >>


Wiktionary

Up to date as of January 15, 2010
(Redirected to wake article)

Definition from Wiktionary, a free dictionary

Contents

English

Pronunciation

Etymology 1

Verb

Infinitive
to wake

Third person singular
wakes

Simple past
woke or waked

Past participle
woken or waked

Present participle
waking

to wake (third-person singular simple present wakes, present participle waking, simple past woke or waked, past participle woken or waked)

  1. (intransitive) (often followed by up) To stop sleeping.
    I woke up at 4 am this morning.
  2. (transitive) (often followed by up) To make somebody stop sleeping.
  3. to lay out a body prior to burial in order to allow family and friends to pay their last respects.
Translations
The translations below need to be checked and inserted above into the appropriate translation tables, removing any numbers. Numbers do not necessarily match those in definitions. See instructions at Help:How to check translations.

Etymology 2

Old English wacu

Noun

Singular
wake

Plural
wakes

wake (plural wakes)

  1. A period after a person's death before the body is buried, in some cultures accompanied by a party.
Synonyms
  • death watch
See also
Translations

Etymology 3

Probably Middle Low German from Old Norse vǫk (a hole in the ice) ( > Danish våge, Icelandic vök).

Noun

Singular
wake

Plural
wakes

wake (plural wakes)

  1. The path left behind a ship on the surface of the water.
  2. The turbulent air left behind a flying aircraft.
Translations
See also

Etymology 4

Noun

Singular
wake

Plural
wakes

wake (plural wakes)

  1. A number of vultures assembled together.
See also

Related terms

Anagrams


Dutch

Pronunciation

  • IPA: /'wakə/

Noun

wake f

  1. a gathering to remember a dead person

Torres Strait Creole

Etymology

From Meriam wakey.

Noun

wake

  1. (eastern dialect) upper leg

Synonyms

  • dokap (western dialect)

Wikispecies

Up to date as of January 23, 2010
(Redirected to David Burton Wake article)

From Wikispecies

U.S. herpetologist (1936- )


Simple English

Wake could mean several things:

  • A wake is a gathering for a person who has died
  • Wake is another name for a wave in water
  • Wake can mean changing from being asleep to awake







Got something to say? Make a comment.
Your name
Your email address
Message