Slipstream

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This article is about the physical phenomenon. For other uses, see Slipstream (disambiguation).

A slipstream is a region of reduced pressure produced behind an object as it moves through a fluid medium (usually air) or as that medium moves around an object. It is caused because the progress of the object forces the particles of the medium (air molecules, etc) apart and they are unable to reform immediately behind the object.

A region of reduced pressure behind the object only occurs when one has turbulent flow. When the flow is laminar, the pressure behind the object is actually higher than in the bulk (but this is balanced by a high pressure region in front of the object). In laminar flow, however, there is a region behind the object where the fluid is moving forward compared to the bulk of the fluid, and this could be called a slipstream.

The shape of an object determines how strong the effect is. A box-like front (relative to the object's motion) will force the medium's particles further apart than a bullet-like one.^[vague] A bullet-like profile will also cause less turbulence and be more likely to permit laminar flow. A tapered rear will permit the particles of the medium to rejoin more easily and quickly than a truncated rear. So a bullet (which has a flat rear) will produce a stronger slipstream than a tear drop, while a cube will produce a stronger one than either.

The term "slipstreaming" is most often used in relation to objects moving through air, though not necessarily flying. If a following object, moving at the same speed, can position itself within the slipstream, it will require less power to maintain its speed than if it were moving independently, because the first object reduces the amount of air resistance experienced by the following object. Alternatively, the following object will be able to move faster than it could in open air. Using this principle is called slipstreaming.

[edit] Slipstreaming/Drafting

Main article: Drafting (racing)

Slipstreaming is important in a number of contexts, including:

Cycling: in fast bicycle races, competitors attempt to 'draft' or use one another's slipstream, braking to overtake the leader only at the last possible moment. In recreational cycling, on the other hand, members of a group can take turns at the leading position, enabling one another to rest a little. In a group of cooperative cyclists with sufficient group-riding skill, stronger members can spend more time leading, to give weaker riders more rest, enabling riders of different strengths to ride together, at least on relatively flat routes. On hilly routes, the benefit of drafting is relatively less on climbs, when airspeeds are slower and the cyclist's primary effort is working against gravity. The flat or hilly nature of a route has consequences for both racing and recreational cycling, with the different types of routes favoring different types of cyclists. See: drafting. See also: peloton.
Bird flight, especially during migration: the extended formations or "skeins" in which many migratory birds (especially geese) fly enable the birds (except, of course, the bird at the front) to take advantage of one another's slipstream. Other birds (for example cormorants) that typically fly in close formation even on short journeys are probably also exploiting the slipstream effect.
Automobile transport: Following another motor vehicle and using care to stay in its slipstream allows for significantly improved fuel efficiency, mostly due to reduced atmospheric drag. Such practice is frequently referred to as drafting. This can be commonly seen in the instance of truck convoys traveling in a single-file queue several vehicles long on highways. One other example is auto racing drivers following each other closely in order to conserve fuel, the better to gain competitive advantage by reducing the frequency of fuel stops made during the course of the race or, more often, using the principle to drive at a faster speed before pulling out to attempt to overtake another driver on straights.

[edit] Spiral slipstream

Spiral slipstream (also known as spiraling slipstream, propwash in the US, or just slipstream in the UK) is a spiral-shaped slipstream formed behind a rotating propeller on an aircraft. The most noticeable effect resulting from the formation of a spiral slipstream is the tendency to yaw nose-left at low speed and full throttle. This effect is caused by the slipstream acting upon the tail fin of the aircraft: the slipstream causes the air to rotate around the forward-aft axis of the aircraft, and this air flow exerts a force on the tail fin, pushing it to the right. To counteract this, some aircraft have the front of the fin (vertical stablizer) slightly offset from the centreline so as to provide an opposing force that cancels out the one produced by the slipstream, albeit only at one particular (usually cruising) speed, an example being the Hawker Hurricane fighter from World War II.