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'{{other uses}} [[Image:Fjord.surface.wave.boat.jpeg|thumb|280px|Kelvin wake pattern generated by a small boat.]] A '''wake''' is the region of recirculating flow immediately behind a moving or stationary solid body, caused by the flow of surrounding fluid around the body. ==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 wave|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 form]]s, it spreads outward from the source until its [[energy]] is overcome or lost, usually by [[friction]] or [[dispersion (water waves)|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 supersonic through air (see also [[Lighthill equation]]). The non-dimensional parameter of interest is the [[Froude number]]. For a blunt body in [[subsonic flight|subsonic]] external flow, for example the [[Apollo program|Apollo]] or [[Orion Multi-Purpose Crew Vehicle|Orion]] capsules during descent and landing, the wake is massively [[flow separation|separated]] and behind the body is a reverse flow region where the flow is 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 (physics)|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 [[Reynolds-averaged Navier-Stokes equations|RANS]] versus [[Large eddy simulation|LES]] implementations), in addition to unsteady flow effects. Example applications include rocket stage separation and aircraft store separation. <gallery> Image:Wave cloud.jpg|Wave cloud pattern in the wake of the [[Île Amsterdam]] (lower left, at the "tip" of the triangular formation of clouds) in the southern [[Indian Ocean]]. Image:Cloud Wakes from Juan Fernandez Islands.jpg|Cloud wakes from the [[Juan Fernández Islands]]. </gallery> ==Wake pattern of a boat== 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 19° 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 [[Airy wave theory|dispersion relation of deep water waves]], which is often written as, :<math>\omega = \sqrt{g k},</math> where <math>g</math> 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 <math>v</math>, 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 <math>v</math> and simply dissipate into the surrounding water without being easily observed. Only the waves with phase speeds at or below <math>v</math> 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. :<math> \omega = c k,\, </math> the [[phase velocity]] ''c'' is the same for all wavelengths and the [[group velocity]] has the same value as well. The angle <math>\theta</math> of the [[shock wave]] thus follows from simple trigonometry and can be written as, :<math>\theta = \arcsin \left( \frac{c}{v} \right).</math> This angle is dependent on <math>v</math>, and the shock wave only forms when <math>v > c</math>. 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]]. By a simple accident in geometry, 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° with the strongest constructive interference occurring at the outer edge, 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 <math>\sin \left( 62^\circ \right)</math> = 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 midpoint 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. <gallery> Image:Aktersvall1.jpg|Wake from a small motorboat with an [[outboard motor]]. Image:070107-Konigssee-dramaticwake.jpg|Wake of a boat crossing an alpine lake. Image:wake.avon.gorge.2boats.arp.750pix.jpg|The wakes of two slow-moving boats. The nearer boat has made a striking series of ruler-straight waves. </gallery> ==Other effects== The above describes an ideal wake, where the body's means of propulsion has no other effect on the water. In practice the wave pattern between the V-shaped wavefronts is usually mixed with the effects of propeller backwash and eddying behind the boat's (usually square-ended) stern. [[File:Surfer about to catch the wake from a ship.png|thumbnail|Germany's only consistent surf spot is a giant wake from a ship.]] ==Recreation== <!--The angle of the wake of a body moving steady in a deep fluid is 2arcsin(1/3) <ref>{{cite web|url=http://www.maths.cam.ac.uk/undergrad/|title=Undergraduate Mathematics at the University of Cambridge |date=2007-09-19}}</ref> (approximately 39 degrees). :: No. The angle varies with speed. I have seen plenty of boat and ship and swimming bird wakes. See talk page.--> "No wake zones" may prohibit wakes in [[marinas]], near moorings and within some distance of shore<ref>BoatWakes.org, [http://boatwakes.homestead.com/files/wakesc.htm#distances Table of distances]</ref> in order to facilitate recreation by other boats, and reduce the damage wakes cause. Powered [[narrowboat]]s on British canals are not permitted to create a breaking wash (a wake large enough to create a breaking wave) along the banks, as this erodes them. This rule normally restricts these vessels to 4 statute miles per hour or less. 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 in [[crawl stroke]], a technique known as [[dribbling]]. <gallery> Image:Sunseeker_Wake.JPG|[[Sunseeker]] wake on the [[Indian River (Florida)|Indian River]] looking at the [[17th Street Bridge]] Image:Wake (Kilwater) behind a ferry.jpg|Wake behind a [[ferry]] in the [[Baltic Sea]] Image:Starr 021108-0070 returning from Molokini - wake (November 2002).jpg|Wake of a boat in the [[Hawaii|Hawaiian Islands]] File:Large engine boat wake.jpg|Wake of a ferryboat just off British Columbia, Canada. </gallery> SSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSS PPPPPPPPPPPPPPPPPPPPPPPPPPPP == See also == *[[Wake turbulence]] *[[Wakeboarding]] *[[Slipstream]] *[[Wakesurfing]] ==References== {{reflist|2}} ==External links== {{commons category|Wakes (fluids)}} *[http://www.wikiwaves.org/index.php/Ship_Kelvin_Wake Water Waves Wiki] *[http://BoatWakes.info Erosion caused by boat wakes] [[Category:Fluid dynamics]] [[Category:Water waves]]'