Slipstream
 | This article includes a list of generalreferences,butit lacks sufficient correspondinginline citations.(June 2011) |
Aslipstreamis a region behind a moving object in which awakeoffluid(typically air or water) is moving atvelocitiescomparable to that of the moving object, relative to the ambient fluid through which the object is moving.[1]The term slipstream also applies to the similar region adjacent to an object with a fluid moving around it. "Slipstreaming" or "drafting"works because of the relative motion of the fluid in the slipstream.
Overview
[edit]A slipstream created byturbulent flowhas a slightly lower pressure than the ambient fluid around the object. When the flow islaminar,the pressure behind the object is higher than the surrounding fluid. The shape of an object determines how strong the effect is. In general, the moreaerodynamican object is, the smaller and weaker its slipstream will be. For example, a box-like front (relative to the object's motion) will collide with the medium's particles at a high rate, transferring more momentum from the object to the fluid than a more aerodynamic object. A bullet-like profile will cause lessturbulenceand create a morelaminar flow.
A tapered rear will permit the particles of the medium to rejoin more easily and quickly than a truncated rear. This reduces lower-pressure effect in the slipstream, but also increasesskin friction(in engineering designs, these effects must be balanced).
Slipstreaming
[edit] | This articleneeds additional citations forverification.(June 2011) |
The term "slipstreaming" describes an object travelling inside the slipstream of another object (most often objects moving through the air though not necessarily flying). If an object is following another object, moving at the same speed, the rear object will require less power to maintain its speed than if it were moving independently. This technique, also calleddraftingcan be used by bicyclists.
- Following in the slipstream of another motor vehicle, or "drafting", allows for significantly improvedfuel efficiencydue to reduced atmosphericdrag.Truckconvoys are a common example, travelling highways in a single-file queue several vehicles long. In tests, this has been shown to produce significant fuel savings.[2]Auto racingdrivers also draft in order to conserve fuel, the better to gain competitive advantage by reducing the frequency of fuel stops or, more often, to reach a higher speed before pulling out to attempt to overtake another driver for example, a driver tries to overtake the leading driver so he follows the rear of the leading driver, the rear driver will gain slipstream causing the whole vehicle to gain more speed than the leading driver.
- A related effect used forliftrather than drag reduction isvortex surfingfor airborne objects. The extended formations (V formation) or "skeins"in which manymigratorybirds (especiallygeese) fly enable the birds (except, of course, thebirdat the front) to use vortex surfing to take advantage of one another'svortices.[3]Other birds (for examplecormorants) that typically fly in close formation, even on short journeys, are probably also exploiting this effect. Usingwingtip devicesto reduceinduced dragcaused bywingtip vorticeshas been tested for aircraft, and could save 10%–29% fuel.[4][5][6]
Spiral slipstream
[edit]Spiral slipstream,also known aspropwash,prop wash,orspiraling slipstream,is aspiral-shaped slipstream formed behind a rotatingpropelleron anaircraft.The most noticeable effect resulting from the formation of a spiral slipstream is the tendency toyawnose-left at low speed and fullthrottle(in centerlinetractoraircraft with a clockwise-rotating propeller.) This effect is caused by the slipstream acting upon the tailfinof the aircraft: The slipstream causes the air to rotate around the longitudinal 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 stabilizer) 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 theHawker Hurricanefighter fromWorld War II.
Propeller slipstream causes increased lift by increasing the airspeed over part of the wings. It also reduces thestall speedof the aircraft by energizing the flow over the wings.[7]: 61
See also
[edit]
References
[edit]- Specific references
- ^Recent studies of Train Slipstreams by Johnson, Dalley, and Temple
- ^Konvoi – Development and examination of the application of electronically coupled truck convoys on highways Aachen University study 2012Archived2014-04-14 at theWayback Machine
- ^Drag Reduction from Formation Flight. Flying Aircraft in Bird-Like Formations Could Significantly Increase Range;Defense Technical Information Center; April 2002; Retrieved February 27, 2008
- ^NASA SKY SURFING FOR FUEL ECONOMYArchived2011-04-18 at theWayback Machine
- ^Cooney, Michael (October 11, 2012)."Air Force lab tests out" aircraft surfing "technique to save fuel".Network World.
- ^Drinnon, Roger (11 October 2012)."'Vortex surfing' could be revolutionary ".US Air Force.Archived fromthe originalon 12 December 2012.Retrieved23 November2012.
- ^Davies, David P. (1971).Handling the Big Jets: An Explanation of the Significant Differences in Flying Qualities Between Jet Transport Aeroplanes and Piston Engined Transport Aeroplanes, Together with Some Other Aspects of Jet Transport Handling(3rd ed.). Air Registration Board.ISBN0903083019.
- General references