Category: Abstract/Art/Design

In fluid dynamics, a vortex (pl.: vortices or vortexes)^[1][2] is a region in a fluid in which the flow revolves around an axis line, which may be straight or curved.^[3][4] Vortices form in stirred fluids, and may be observed in smoke rings, whirlpools in the wake of a boat, and the winds surrounding a tropical cyclone, tornado or dust devil.

Vortices are a major component of turbulent flow. The distribution of velocity, vorticity (the curl of the flow velocity), as well as the concept of circulation are used to characterise vortices. In most vortices, the fluid flow velocity is greatest next to its axis and decreases in inverse proportion to the distance from the axis.

In the absence of external forces, viscous friction within the fluid tends to organise the flow into a collection of irrotational vortices, possibly superimposed to larger-scale flows, including larger-scale vortices. Once formed, vortices can move, stretch, twist, and interact in complex ways. A moving vortex carries some angular and linear momentum, energy, and mass, with it.

Overview[edit]

In the dynamics of fluid, a vortex is fluid that revolves around the axis line. This fluid might be curved or straight. Vortices form from stirred fluids: they might be observed in smoke rings, whirlpools, in the wake of a boat or the winds around a tornado or dust devil.

Vortices are an important part of turbulent flow. Vortices can otherwise be known as a circular motion of a liquid. In the cases of the absence of forces, the liquid settles. This makes the water stay still instead of moving.

When they are created, vortices can move, stretch, twist and interact in complicated ways. When a vortex is moving, sometimes, it can affect an angular position.

For an example, if a water bucket is rotated or spun constantly, it will rotate around an invisible line called the axis line. The rotation moves around in circles. In this example the rotation of the bucket creates extra force.

The reason that the vortices can change shape is the fact that they have open particle paths. This can create a moving vortex. Examples of this fact are the shapes of tornadoes and drain whirlpools.

When two or more vortices are close together they can merge to make a vortex. Vortices also hold energy in its rotation of the fluid. If the energy is never removed, it would consist of circular motion forever.

Properties[edit]

Vorticity[edit]

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 is defined to be the direction of the axis of rotation of this imaginary ball (according to the right-hand rule) while its length is twice 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 ${displaystyle {vec {omega }}}$ and expressed by the vector analysis formula ${displaystyle abla imes {vec {mathit {u}}}}$, where ${displaystyle abla }$ is the nabla operator and ${displaystyle {vec {mathit {u}}}}$ is the local flow velocity.^[5]

The local rotation measured by the vorticity ${displaystyle {vec {omega }}}$ 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, ${displaystyle {vec {omega }}}$ may be opposite to the mean angular velocity vector of the fluid relative to the vortex's axis.

Vortex types[edit]

In theory, the speed u of the particles (and, therefore, the vorticity) in a vortex may vary with the distance r from the axis in many ways. There are two important special cases, however:

• If the fluid rotates like a rigid body – that is, if the angular rotational velocity Ω is uniform, so that u increases proportionally to the distance r from the axis – a tiny ball carried by the flow would also rotate about its center as if it were part of that rigid body. In such a flow, the vorticity is the same everywhere: its direction is parallel to the rotation axis, and its magnitude is equal to twice the uniform angular velocity Ω of the fluid around the center of rotation.

  ${displaystyle {egin{aligned}{vec {Omega }}&=(0,0,Omega ),quad {vec {r}}=(x,y,0),\{vec {u}}&={vec {Omega }} imes {vec {r}}=(-Omega y,Omega x,0),\{vec {omega }}&= abla imes {vec {u}}=(0,0,2Omega )=2{vec {Omega }}.end{aligned}}}$

• If the particle speed u is inversely proportional to the distance r from the axis, then the imaginary test ball would not rotate over itself; it would maintain the same orientation while moving in a circle around the vortex axis. In this case the vorticity ${displaystyle {vec {omega }}}$ is zero at any point not on that axis, and the flow is said to be irrotational.

  ${displaystyle {egin{aligned}{vec {Omega }}&=left(0,0,alpha r^{-2} ight),quad {vec {r}}=(x,y,0),\{vec {u}}&={vec {Omega }} imes {vec {r}}=left(-alpha yr^{-2},alpha xr^{-2},0 ight),\{vec {omega }}&= abla imes {vec {u}}=0.end{aligned}}}$

Irrotational vortices[edit]

In the absence of external forces, a vortex usually evolves fairly quickly toward the irrotational flow pattern^{[citation needed]}, where the flow velocity u is inversely proportional to the distance r. 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, Γ, for any contour that does enclose the axis once.^[6] The tangential component of the particle velocity is then ${displaystyle u_{ heta }={ frac {Gamma }{2pi r}}}$. The angular momentum per unit mass relative to the vortex axis is therefore constant, ${displaystyle ru_{ heta }={ frac {Gamma }{2pi }}}$.

The ideal irrotational vortex flow in free space 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 axis. 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 ${displaystyle {vec {omega }}}$ becomes non-zero, with direction roughly parallel to the vortex axis. The Rankine vortex is a model that assumes a rigid-body rotational flow where r is less than a fixed distance r₀, and irrotational flow outside that core regions.

In a viscous fluid, irrotational flow contains viscous dissipation everywhere, yet there are no net viscous forces, only viscous stresses.^[7] Due to the dissipation, this means that sustaining an irrotational viscous vortex requires continuous input of work at the core (for example, by steadily turning a cylinder at the core). In free space there is no energy input at the core, and thus the compact vorticity held in the core will naturally diffuse outwards, converting the core to a gradually-slowing and gradually-growing rigid-body flow, surrounded by the original irrotational flow. Such a decaying irrotational vortex has an exact solution of the viscous Navier–Stokes equations, known as a Lamb–Oseen vortex.

Rotational vortices[edit]

A rotational vortex – a vortex that rotates in the same way as a rigid body – cannot exist indefinitely in that state except through the application of some extra force, that is not generated by the fluid motion itself. It has non-zero vorticity everywhere outside the core. Rotational vortices are also called rigid-body vortices or forced vortices.

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 u 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 transition of the rigid-body flow to the irrotational state.

Vortex formation on boundaries[edit]

Vortex structures are defined by their vorticity, the local rotation rate of fluid particles. They can be formed via the phenomenon known as boundary layer separation which can occur when a fluid moves over a surface and experiences a rapid acceleration from the fluid velocity to zero due to the no-slip condition. This rapid negative acceleration creates a boundary layer which causes a local rotation of fluid at the wall (i.e. vorticity) which is referred to as the wall shear rate. The thickness of this boundary layer is proportional to ${displaystyle surd (vt)}$ (where v is the free stream fluid velocity and t is time).

If the diameter or thickness of the vessel or fluid is less than the boundary layer thickness then the boundary layer will not separate and vortices will not form. However, when the boundary layer does grow beyond this critical boundary layer thickness then separation will occur which will generate vortices.

This boundary layer separation can also occur in the presence of combatting pressure gradients (i.e. a pressure that develops downstream). This is present in curved surfaces and general geometry changes like a convex surface. A unique example of severe geometric changes is at the trailing edge of a bluff body where the fluid flow deceleration, and therefore boundary layer and vortex formation, is located.

Another form of vortex formation on a boundary is when fluid flows perpendicularly into a wall and creates a splash effect. The velocity streamlines are immediately deflected and decelerated so that the boundary layer separates and forms a toroidal vortex ring.^[8]

Vortex geometry[edit]

In a stationary vortex, the typical streamline (a line that is everywhere tangent to the flow 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 flow 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 Helmholtz's theorems, a vortex line cannot start or end in the fluid – except momentarily, in non-steady flow, while the vortex is forming or dissipating. In general, vortex lines (in particular, the axis line) are either closed loops or end at the boundary of the fluid. A whirlpool is an example of the latter, namely a vortex in a body of water whose axis ends at the free surface. A vortex tube whose vortex lines are all closed will be a closed torus-like surface.

A newly created vortex will promptly extend and bend so as to eliminate any open-ended vortex lines. For example, when an airplane engine is started, a vortex usually forms ahead of each propeller, or the turbofan of each jet engine. One end of the vortex line is attached to the engine, while the other end usually stretches out 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 vortex tubes and were stretched into spiral shapes by the non-uniform flow velocity distribution.

Pressure in a vortex[edit]

The fluid motion in a vortex creates a dynamic pressure (in addition to any hydrostatic pressure) that is lowest in the core region, closest to the axis, and increases as one moves away from it, in accordance with Bernoulli's principle. One can say that it is the gradient of this pressure that forces the fluid to follow a curved path around the axis.

In a rigid-body vortex flow of a fluid with constant density, the dynamic pressure is proportional to the square of the distance r from the axis. In a constant gravity field, the free surface of the liquid, if present, is a concave paraboloid.

In an irrotational vortex flow with constant fluid density and cylindrical symmetry, the dynamic pressure varies as P_∞ − K/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 shape formed by the free surface is called a hyperboloid, or "Gabriel's Horn" (by Evangelista Torricelli).

The core of a vortex in air is sometimes visible because water vapor condenses as the low pressure of the core causes adiabatic cooling; the funnel of a tornado is an example. When a vortex line ends at a boundary surface, the reduced pressure 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. A vortex that ends at the free surface of a body of water (like the whirlpool that often forms over a bathtub drain) may draw a column of air down the core. The forward vortex extending from a jet engine of a parked airplane can suck water and small stones into the core and then into the engine.

Evolution[edit]

Vortices need not be steady-state features; they can move and change shape. In a moving vortex, the particle paths are not closed, but are open, loopy curves like helices and cycloids. A vortex flow might also be combined with a radial or axial flow pattern. In that case the streamlines and pathlines are not closed curves but spirals or helices, respectively. This is the case in tornadoes and in drain whirlpools. A vortex with helical streamlines is said to be 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 Helmholtz's second theorem. Thus vortices (unlike surface waves and pressure waves) can transport mass, energy and momentum over considerable distances compared to their size, with surprisingly little dispersion. This effect is demonstrated by smoke rings and exploited in vortex ring toys and 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 circulation will equal the sum of the circulations of the constituent vortices. For example, an airplane wing that is developing lift will create a sheet of small vortices at its trailing edge. These small vortices merge to form a single wingtip vortex, less than one wing chord downstream of that edge. This phenomenon also occurs with other active airfoils, such as 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.

Further examples[edit]

• In the hydrodynamic interpretation of the behaviour of electromagnetic fields, the acceleration of electric f

NeoGAF

Type of site	Internet forum
Available in	English
Owner	NeoGaf LLC (100% — Tyler Malka)
Created by	Jim Cordeira
URL	neogaf.com
Commercial	Yes
Registration	Optional[α]
Launched	1999; 25 years ago (1999) (as Gaming-Age Forums)
Current status	Active
Written in	PHP, MySQL (powered by XenForo 2.2)

NeoGAF is an Internet forum primarily dedicated to the discussion of video games. Founded as an adjunct to a video game news site under the name Gaming-Age Forums, on April 4, 2006 it changed its name to NeoGAF and became independently hosted and administered.

In 2017, site owner Tyler "Evilore" Malka was accused of sexual harassment. The allegations resulted in moderator resignations, mass exodus off the site and later site policy changes. Former members and moderators would later launch the new forum ResetEra.

History[edit]

NeoGAF began as "The Gaming-Age Forums", a forum for gaming website Gaming-Age. As Gaming-Age outgrew its hosting, IGN took over hosting of Gaming-Age's forums. After IGN ceased hosting of GAF in mid-2001, GAF moved to ezboard, and the administration of GAF became more estranged from Gaming Age.^[1]

As the Gaming-Age staff became gradually more divorced from the day-to-day operation of GAF, problems with the new Gamesquad hosting cropped up. As software bugs in vBulletin 2, the version GAF was using at the time, continued to worsen, the Gamesquad hosting became increasingly more impractical, until the forums' database became corrupted, forcing a move to new hosting in order to change software and salvage what was left of the forums' database. In the spring of 2004, a fundraiser was held to move GAF to new hosting. On June 6, 2004, GAF took its newest form (known as NeoGAF to long-time posters) and moved to new hosting and new software, vBulletin 3.

On April 4, 2006, the forums were relaunched as NeoGAF, the former in-moniker, by its administrators. NeoGAF also features its own front page, an upfront admission that the forum's audience had drifted from that of its birthing news site, but yet mandated a single portal to represent the forum's members.

In an interview with VG247 in 2013, Tyler Malka claimed that he was offered $5 million to sell the website, turning down the offer.^[2] One year later he stated in a forum post that the offer doubled, later saying he also turned down the deal.^[3]

Sexual misconduct allegations against Malka and moderators exodus[edit]

On October 21, 2017, film director Ima Leupp described in a Facebook post, as part of the #MeToo phenomenon, a trip she took with NeoGAF owner Tyler "Evilore" Malka two years before. She said that while she and Malka were drinking together in a New Orleans hotel room in April 2015, she became very sick. While cleaning up in the shower, she said he approached her "fully naked" from behind without her consent.^{[4][5][6][7][8]}

Following the sexual harassment scandal, half of the site's moderation staff resigned, and many users posted "suicide threads" wherein they demanded to be banned from the forum. The website went offline soon after.^[9][10] Afterwards, NeoGAF was restored, suspending the off-topic sections of the board, and announcing that politics would henceforth be a prohibited subject of discussion and that moderation would become anonymous.^[11]

Reception[edit]

Industry response[edit]

Members of the video games industry have been known to be members of the website, such as David Jaffe and Cliff Bleszinski.^[12]

In 2007, in a thread discussing the resignation of Peter Moore from Microsoft, one user making fun of Microsoft's vice president of global marketing Jeff Bell received a personal message asking them "And your contribution to society is ... what?" The account was later found to be Jeff Bell's.^[13] Malka later said he saw a shift on the forums with people in the games industry being more careful of what they post.^[14]

In a 2009 thread post on NeoGAF dedicated to the game Scribblenauts, user "Feep" relayed the experience of discovering during E3 that he was able to go back in time with a time machine to collect a dinosaur in order to defeat an army of robot zombies that could not be defeated with regular weapons.^[15][16][17] The story, memorialized as "Post 217", led to the games artist Edison Yan creating a desktop wallpaper image of the story, in appreciation of the positive fan response to the game, and the terms "Post Two One Seven", "Feep", and "Neogaf" were included as summonable objects in the game.^[18] Scribblenauts' director Jeremiah Slaczka credited the word-of-mouth popularity of "Post 217" for part of the game's success at E3, and noted that he had contacted Feep to gain his permission to include "Feep" (appearing as a robot zombie) within the game.^[19]

Describing the development struggles of 2017's Rime, Tequila Works co-founder Raúl Rubio Munárriz said that reading the forum's reactions reduced him to tears for two days and that if he had read them early on in development, the game would have been cancelled. "Partly because I just don't understand the cruelty, but more importantly because I could see those years over those two days, and I began to understand that maybe people can love something so much that they can hate it."^[20]

Criticism[edit]

One of the biggest critics of NeoGAF was game designer and former Silicon Knights president Denis Dyack. In June 2008, he issued a challenge to forum users. He asked users to say whether they were for or against the then upcoming Silicon Knights game Too Human. Once the game was released, if the game received negative reception, Dyack would have "Owned by GAF" under his forum name. If positive reception, users who voted against the game would have had "Owned by Too Human."^[21] Dyack would later go on the 1UP Yours podcast, explaining his challenge was an experiment to expose the lack of accountability on online forums, adding that NeoGAF would crumble if it doesn't reform itself.^[22] He was later permanently banned from the site in August that same year after calling it the worst online forum.^[23][24]

Circa 2017, NeoGAF was criticized for biased moderation and banning political dissidents. Tyler "Evilore" Malka made a post admitting biased moderation by at least one former member of the moderation staff. Malka stated that the ex-moderator banned hundreds of members without justification. Malka also made the declaration that discussion should be encouraged with different points of view, saying that people on the website have been "driven out, character assassinated, labeled traitor for not sounding angry enough, or for not being entirely on board with ostracizing someone else for the same reasons."^[25]

In the media[edit]

In 2007, the website partnered up with The Get-Well Gamers Foundation to launch a donation campaign to bring video games to children in hospitals. NeoGAF raised $5,600 in cash and inventory donations over the October to December period.^[26]

An exchange on the forum inspired members to start the development of Dudebro II in 2010. The game was intended to be a satirical take on the machismo found in some modern titles and was planned to feature Jon St. John, the voice of Duke Nukem, as the lead.^[27] The team's last statement, in late 2017, disassociated the game from NeoGAF due to the sexual harassment controversy, claiming a new and unrelated team would be created to continue development.^[28]

In June 2015, a Reddit sub-community devoted towards mocking NeoGAF became one of five communities shut down by the site.^[29][30] Reddit argued the ban hit groups "that allow their communities to use the subreddit as a platform to harass individuals when moderators don’t take action."^[31]

Vice News noted the site was one of the largest drivers of traffic to Hillary Clinton's website during the 2016 U.S. presidential election.^[32]

ResetEra[edit]

Several former NeoGAF members established ResetEra on October 24, 2017, after NeoGAF's owner, Tyler Malka faced sexual misconduct accusations.^[33][34]

From November 2017 to March 2018, ResetEra hosted Q&A sessions with Insomniac Games, Chucklefish, LizardCube, DotEmu, and Tom Happ and Dan Adelman, the creators of Axiom Verge.^[35]

ResetEra moderators enacted the site's first game ban by barring all promotion of the game Hogwarts Legacy, citing a "far-right" YouTube channel run by then-lead designer Troy Leavitt, and comments criticized for transphobia made by author J. K. Rowling.^[36] In January 2023, the ban was extended to include any discussion of the game.^[37][38]

In October 2021, ResetEra was purchased for 4.5 million dollars by MOBA Network,^[39] a Swedish company that focuses on managing web-based forums for video games and esports. In a statement, MOBA Network claimed that it wants to "increase advertising revenue through a higher share of direct sales, implementation of new ad formats, and a long-term product development strategy."^[40]

ResetEra is cited for hat tips by PCGamesN and other publications.^[41] Game developers and journalists have visited the forums, including journalist Geoff Keighley; Cory Barlog, Creative Director at Santa Monica Studio; James Stevenson, the Community Director for Insomniac Games; and Thomas Mahler, director of Moon Studios; among others.^[42][43][44]

See also[edit]

• List of Internet forums

Notes[edit]

References[edit]