Pressure distribution around flapped profile with starting vortex

fluid dynamics In physics, physical chemistry and engineering, fluid dynamics is a subdiscipline of fluid mechanics that describes the flow of fluids – liquids and gases. It has several subdisciplines, including (the study of air and other gases in motion ...

, the starting vortex is a

vortex In fluid dynamics, a vortex (: vortices or vortexes) is a region in a fluid in which the flow revolves around an axis line, which may be straight or curved. Vortices form in stirred fluids, and may be observed in smoke rings, whirlpools in th ...

which forms in the air adjacent to the trailing edge of an

airfoil An airfoil (American English) or aerofoil (British English) is a streamlined body that is capable of generating significantly more Lift (force), lift than Drag (physics), drag. Wings, sails and propeller blades are examples of airfoils. Foil (fl ...

as it is accelerated from rest. It leaves the airfoil (which now has an equal but opposite "bound vortex" around it), and remains (nearly) stationary in the flow. It eventually decays through the action of

viscosity Viscosity is a measure of a fluid's rate-dependent drag (physics), resistance to a change in shape or to movement of its neighboring portions relative to one another. For liquids, it corresponds to the informal concept of ''thickness''; for e ...

, but before doing so it contributes significantly to

wake turbulence Wake turbulence is a disturbance in the Atmosphere of Earth, atmosphere that forms behind an aircraft as it passes through the air. It includes several components, the most significant of which are wingtip vortices and jet-wash, the rapidly moving ...

. The initial (and quite brief) presence of a starting

as an

begins to move was predicted by early aerodynamicists, and eventually photographed. Whenever the speed or

angle of attack In fluid dynamics, angle of attack (AOA, α, or \alpha) is the angle between a Airfoil#Airfoil terminology, reference line on a body (often the chord (aircraft), chord line of an airfoil) and the vector (geometry), vector representing the relat ...

of an airfoil changes there is a corresponding amount of vorticity deposited in the wake behind the airfoil, joining the two

trailing vortices Wingtip vortices are circular patterns of rotating air left behind a wing as it generates Lift (force), lift. The name is a misnomer because the cores of the vortex, vortices are slightly inboard of the wing tips. Wingtip vortices are sometimes ...

. This vorticity is a continuum of mini-starting-vortexes. The wake behind an aircraft is a continuous sheet of weak vorticity, between the two trailing vortices, and this accounts for the changes in strength of the trailing vortices as the airspeed of the aircraft and angle of attack on the wing change during flight."This starting vortex formation occurs not only when a wing is first set into motion, but also when the circulation around the wing is subsequently changed for any reason whatever." Millikan, Clark B., ''Aerodynamics of the Airplane'', page 65 (The strength of a

cannot change within the fluid except by the dissipative action of

. Vortices either form continuous loops of constant strength, or they terminate at the boundary of the fluid - usually a solid surface such as the ground.) The starting vortex is significant to an understanding of the

Kutta condition The Kutta condition is a principle in steady-flow fluid dynamics, especially aerodynamics, that is applicable to solid bodies with sharp corners, such as the trailing edges of airfoils. It is named for German mathematician and aerodynamicist Mart ...

and its role in the circulation around any airfoil generating

lift Lift or LIFT may refer to: Physical devices * Elevator, or lift, a device used for raising and lowering people or goods ** Paternoster lift, a type of lift using a continuous chain of cars which do not stop ** Patient lift, or Hoyer lift, mobile ...

. The starting vortex has certain similarities with the "starting plume" which forms at the leading edge of a slug of fluid, when one fluid is injected into another at rest. See

plume (hydrodynamics) In hydrodynamics, a plume or a column is a vertical body of one fluid moving through another. Several effects control the motion of the fluid, including momentum (inertia), diffusion and buoyancy (density differences). Pure '' jets'' and pure ''pl ...

References

* L. J. Clancy (1975), ''Aerodynamics'', Section 4.8, Pitman Publishing Limited, London * Millikan, Clark B., ''Aerodynamics of the Airplane'', Section 1–6, eighth printing, John Wiley and Sons, Inc., (1941) New York. * Massey, B.S., ''Mechanics of Fluids'', Section 9.10, 2nd Edition. Van Nostrand Reinhold Co., London (1970) Library of Congress Catalog Card No. 67-25005

References

Notes

See also