Wingtip vortices
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Wingtip vortices are circular patterns of rotating air left behind a
Wingtip vortices are associated with
Wingtip vortices form the primary component of wake turbulence. Depending on ambient atmospheric humidity as well as the geometry and wing loading of aircraft, water may condense or freeze in the core of the vortices, making the vortices visible.
Generation of trailing vortices
When a wing generates aerodynamic lift, it results in a region of downwash between the two vortices.[3][2]: 8.1.1 [4]
Three-dimensional lift and the occurrence of wingtip vortices can be approached with the concept of horseshoe vortex and described accurately with the Lanchester–Prandtl theory. In this view, the trailing vortex is a continuation of the wing-bound vortex inherent to the lift generation.
Effects and mitigation
Wingtip vortices are associated with
The
As a consequence, aircraft for which a high
Another method of reducing induced drag is the use of winglets, as seen on most modern airliners. Winglets increase the effective aspect ratio of the wing, changing the pattern and magnitude of the vorticity in the vortex pattern. A reduction is achieved in the kinetic energy in the circular air flow, which reduces the amount of fuel expended to perform work upon the spinning air[citation needed].
After NASA became concerned about the increasing density of air traffic potentially causing vortex related accidents at airports, an experiment by NASA Ames Research Center wind tunnel testing with a 747 model found that the configuration of the flaps could be changed on existing aircraft to break the vortex into three smaller and less disturbing vortexes. This primarily involved changing the settings of the outboard flaps, and could theoretically be retrofitted to existing aircraft.[5]
Visibility of vortices
The cores of the vortices can sometimes be visible when the water present in them condenses from gas (vapor) to liquid. This water can sometimes even freeze, forming ice particles.
Condensation of water vapor in wing tip vortices is most common on aircraft flying at high angles of attack, such as fighter aircraft in high g maneuvers, or airliners taking off and landing on humid days.
Aerodynamic condensation and freezing
The cores of vortices spin at very high speed and are regions of very low pressure. To
The phase of water (i.e., whether it assumes the form of a solid, liquid, or gas) is determined by its temperature and pressure. For example, in the case of liquid-gas transition, at each pressure there is a special "transition temperature" such that if the sample temperature is even a little above , the sample will be a gas, but, if the sample temperature is even a little below , the sample will be a liquid; see
Vortex cores are regions of low pressure. As a vortex core begins to form, the water in the air (in the region that is about to become the core) is in vapor phase, which means that the local temperature is above the local dew point. After the vortex core forms, the pressure inside it has decreased from the ambient value, and so the local dew point () has dropped from the ambient value. Thus, in and of itself, a drop in pressure would tend to keep water in vapor form: The initial dew point was already below the ambient air temperature, and the formation of the vortex has made the local dew point even lower. However, as the vortex core forms, its pressure (and so its dew point) is not the only property that is dropping: The vortex-core temperature is dropping also, and in fact it can drop by much more than the dew point does.
To
Here and are the absolute temperature and pressure at the beginning of the process (here equal to the ambient air temperature and pressure), and are the absolute temperature and pressure in the vortex core (which is the end result of the process), and the constant is about 7/5 = 1.4 for air (see here).
Thus, even though the local dew point inside the vortex cores is even lower than in the ambient air, the water vapor may nevertheless condense — if the formation of the vortex brings the local temperature below the new local dew point.[6]
For a typical transport aircraft landing at an airport, these conditions are as follows: and have values corresponding to the so-called
The temperature in the vortex core is given by the equation above as or 0.86 °C = 33.5 °F.
Next, the partial pressure of water in the vortex core drops in proportion to the drop in the total pressure (i.e., by the same percentage), to about 650 Pa = 6.5 mb. According to a dew point calculator, that partial pressure results in the local dew point of about 0.86 °C; in other words, the new local dew point is about equal to the new local temperature.
Therefore, this is a marginal case; if the relative humidity of the ambient air were even a bit higher (with the total pressure and temperature remaining as above), then the local dew point inside the vortices would rise, while the local temperature would remain the same. Thus, the local temperature would now be lower than the local dew point, and so the water vapor inside the vortices would indeed condense. Under the right conditions, the local temperature in vortex cores may drop below the local
The water-vapor condensation mechanism in wingtip vortices is thus driven by local changes in air pressure and temperature. This is to be contrasted to what happens in another well-known case of water condensation related to airplanes: the
Formation flight
One theory on migrating bird flight states that many larger bird species fly in a V formation so that all but the leader bird can take advantage of the upwash part of the wingtip vortex of the bird ahead.[8][9]
Hazards
Wingtip vortices can pose a hazard to aircraft, especially during the
The hazardous aspects of wingtip vortices are most often discussed in the context of wake turbulence. If a light aircraft immediately follows a heavy aircraft, wake turbulence from the heavy aircraft can roll the light aircraft faster than can be resisted by use of ailerons. At low altitudes, in particular during takeoff and landing, this can lead to an upset from which recovery is not possible. ("Light" and "heavy" are relative terms, and even smaller jets have been rolled by this effect.) Air traffic controllers attempt to ensure an adequate separation between departing and arriving aircraft by issuing wake turbulence warnings to pilots.
In general, to avoid vortices an aircraft is safer if its takeoff is before the rotation point of the airplane that took off before it. However care must be taken to stay upwind (or otherwise away) from any vortices that were generated by the previous aircraft. On landing behind an airplane the aircraft should stay above the earlier one's flight path and touch down further along the runway.[11]
Glider pilots routinely practice flying in wingtip vortices when they do a maneuver called "boxing the wake". This involves descending from the higher to lower position behind a tow plane. This is followed by making a rectangular figure by holding the glider at high and low points away from the towing plane before coming back up through the vortices. (For safety this is not done below 1500 feet above the ground, and usually with an instructor present.) Given the relatively slow speeds and lightness of both aircraft the procedure is safe but does instill a sense of how strong and where the turbulence is located.[12]
Gallery
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AnEA-6 Prowlerwith condensation in the cores of its wingtip vortices and also on the top of its wings.
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Vortices can be formed at the ends of propeller blades, as seen on this DHC-5 Buffalo.
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The core of the vortex trailing from the tip of theflapof a commercial airplane with landing flap extended.
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Wingtip vortices from a Cessna 182 wind tunnel model.
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Wingtip vortices shown inC-17 Globemaster III. Also known as smoke angels.
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TheMV-22 Osprey tiltrotor has a high disk loading, producing visible blade tip vortices.
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Euler computation of a steady tip vortex. Contour colours and isosurface reveal vorticity.
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A Boeing 747 model has just passed through a stationary sheet of smoke, which is showing its trailing vortices, at the Vortex Facility at the Langley Research Center.
See also
- Aspect ratio (wing)
- Contrail
- Helmholtz's theorems
- Horseshoe vortex
- Lift-induced drag
- V formation
- Vortex
- Wake turbulence
- Crow instability
References
- ^ ISBN 978-0-273-43342-2. Retrieved 10 February 2023.
- ^ ISBN 978-1-118-45422-0. Retrieved 10 February 2023.
- ^ McLean, Doug (2005). Wingtip Devices: What They Do and How They Do It (PDF). 2005 Boeing Performance and Flight Operations Engineering Conference. p. 4.5.
The vortex cores are often referred to as "wingtip vortices," though this is a bit of a misnomer. While it is true that the cores line up fairly closely behind the wingtips, the term "wingtip vortices" implies that the wingtips are the sole sources of the vortices. Actually, as we saw in Figure 3.2, the vorticity that feeds into the cores generally comes from the entire span of the trailing edge, not just from the wingtips.
- YouTube
- ^ Corsiglia, Victor R.; Rossow, Vernon J.; Ciffone, Donald L. (1975). Experimental Study of the Effect of Span Loading on Aircraft Wakes (PDF) (Report). NASA Ames Research Center.
- ^ ISBN 978-0-7923-3376-0
- ^ NASA, Contrail Science Archived June 5, 2009, at the Wayback Machine
- ^ Wieselsberger, C. (1914). "Beitrag zur Erklärung des Winkelfluges einiger Zugvögel". Zeitschrift für Flugtechnik und Motorluftschiffahrt (in German). 5. München/Berlin: Wissenschaftliche Gesellschaft für Luftfahrt: 225–229.
- S2CID 21251564.
- ^ Butler, K.M (1993), Estimation of Wake Vortex Advection and Decay Using Meteorological Sensors and Aircraft Data (PDF), Lincoln Laboratory, MIT, p. 11
- ^ How To Avoid Wake Turbulence During Takeoff And Landing
- ^ Boxing the Wake
External links
- Video from Dryden Flight Research Centertests on wingtip vortices:
- Wingtip Vortices during a landing - Video at Youtube