Disturbed fluid motion occurring in the region following an object moving through a fluid.
Two forms of wake turbulence are significant in flight. The first is the presence of vortices in the wake of aircraft. A vortex is defined as a region within a body of fluid in which the fluid elements have an angular velocity. Wake vortices are regions of spiraling or circulating fluid left behind in a medium after an object producing lift, or experiencing changes in lift, passes through the medium. Wake vortices created by aircraft pose hazards to following aircraft. This hazard of spiraling air limits the spacing between aircraft takeoffs or landings at many airports.
A second form of wake turbulence is the turbulence in the wake of mountains or tall buildings that is encountered by low-flying aircraft under windy conditions.
Every aircraft leaves behind a region of disturbed air, called its wake. The disturbances that are due to the flow accelerated by the propeller or jet engines are called prop-wash or thrust-stream turbulence. The largest disturbances are due to the effects of lift. When an aircraft generates lift on its wings, the pressure below the wings is lower than the pressure above. At the wingtips, the flow from below rolls up and over the wings. This rotation forms a vortex, which is a region resembling a long, thin tube, where the air moves in a spiral path. A vortex generally has a small core region of high rotation speed, surrounded by a much larger region of slower-moving fluid. Thus each aircraft typically leaves behind a pair of vortices, rotating in opposite directions and also moving downward at speeds of a few hundred feet per minute. The strength of these vortices is a function of the aircraft’s weight, speed, wing shape and aspect ratio, and acceleration or deceleration. Peak tangential velocities encountered in a vortex can be in excess of 300 feet per second.
In addition to the strong wingtip vortices, there are several other vortices, and often a continuous sheet of vortices trailing behind aircraft. These vortices originate wherever there is a change in the distribution of lift across the aircraft. Leading-edge vortices are seen over swept wings and tails. Inboard vortex sheets are seen behind most wings and rotor blades. When lift changes suddenly, as during takeoff or a sharp maneuver, a starting vortex is left behind, with its core perpendicular to orientation of the trailing vortex. Sufficiently far behind the aircraft, all these vortices are swept up into the wingtip vortices.
Wake vortices originate when the aircraft rotates off the runway at takeoff and end when the aircraft touches down on the runway. The vortices generated near the runway can persist near the ground, generally spaced a little less than a wingspan apart and generally within a height of about one wingspan from the ground. They can then drift with the wind and cross over to adjacent runways.
An aircraft’s wake vortices persist in the air for several minutes after the aircraft has passed. Because large airliners travel at nearly 600 miles per hour, persistence of vortices for three minutes means that strong vortices can be encountered as far as 30 miles behind the airliner in the upper atmosphere.
Helicopter rotors, whose tip speeds can exceed the speed of sound, also generate very strong vortices, which persist for substantial periods. Interaction of these vortices with the ground can kick up clouds of dust and small stones, posing hazards to people standing on unprepared surfaces when a helicopter hovers close to the ground. Pilot training manuals generally recommend that people remain at least three rotor diameters away from helicopter rotors to avoid this hazard.
Aircraft encountering another aircraft’s wake vortices are in danger of rolling out of control. Strong upward or downward air motion may be encountered as many as 50 feet from the central core of a vortex. The danger is greatest for small aircraft with short wings following a large, heavy aircraft with a clean configuration, that is, with a minimum of flaps and other controls deflected, flying at a slow speed. General guidance for avoiding the wake vortex hazard includes: flying above the flight path of the airplane ahead, touching down on the runway beyond the touchdown point of the previous aircraft, spacing takeoffs on the same runway by three minutes, maintaining a vertical separation of at least 1,000 feet between airplanes.
Because wake vortices limit the spacing between aircraft in flight, there is strong interest in the aviation community to find ways to alleviate their hazards. One method involves the alteration of wingtip shapes to generate several vortices that interfere with each other. Others include blowing air out of the wings near the tips and deflecting various small control surfaces. A phenomenon called the Crow instability has been observed, in which the counterrotating pair of tip vortices left behind by an aircraft develop sudden bursts and dissipate shortly thereafter. Some research efforts attempt to accelerate the instability by suitably modifying the vortices. Other approaches to the alleviation of wake vortex hazards include the placement of sensing devices near airports that identify vortices drifting onto active runways and warn approaching aircraft. Researchers also attempt to place sensors on aircraft that sense such vortices in the aircraft’s flight path.
Although aircraft are streamlined, mountains and buildings rarely are, and winds blowing across them cause large regions of turbulence downstream. Theodore von Kármán analyzed the flow patterns behind cylinders and described a phenomenon that became known as Kármán’s vortex street, a series of vortices, of opposite directions, left behind alternately from each side of the cylinder. Such patterns can be observed in the clouds moving across islands and mountains. Aircraft flying into such conditions can encounter strong turbulence.
Anderson, J. D. Fundamentals of Aerodynamics. 3d ed. New York: McGraw-Hill, 2001. An undergraduate-level engineering text on the history and methods of aerodynamics, with a primary focus on low-speed aerodynamics. Crow, S. C., and E. R. Bate. “Lifespan of Trailing Vortices in a Turbulent Atmosphere.” Journal of Aircraft 13 (1976): 476-482. An authoritative technical paper on the processes by which trailing vortices in the wake of an aircraft ultimately destroy themselves; it presents what has come to be called the Crow instability mechanism. Shrager, J. J. A Summary of Helicopter Vorticity and Wake Turbulence Publications with an Annotated Bibliography. Washington, D.C.: Federal Aviation Administration, 1974. A classic discussion of the safety problems caused by helicopter wakes.