寬德效應

寬德效應英語:;Coandă又譯為康德、康達、柯恩達),亦稱附壁作用流體(水流或气流)有離開本來的流動方向,改為隨著凸出的物體流動之傾向,並使周圍流體逸入此一噴流中。由於流體移動方向改變,使得周圍產生壓力較低的區域,此稱為寬德效應。[1][2]這種作用是以羅馬尼亞發明家安利·寬德()為名。安利·寬德發明的一架飛機(寬德-1910)曾經因這種效應墜毀,之後他便致力這方面的研究。[3][4]

旋轉的乒乓球因為康德效應而悬浮在空中:球停留在氣流的下緣,氣壓梯度力與重力相抵銷,而讓球維持平衡高度。

發現

1800年,湯瑪士·楊格在一場皇家學會的演講中,提出了對康德效應的描述:

使蠟燭火焰往風管的氣流靠攏的側壓,與氣流通過障礙物時產生彎折的壓力,很可能是一樣的原理所造成。如果把一股氣流在水面吹起的漣漪做標記,並且在氣流的側邊放置一個凸面體,可以發現氣流往凸面體的方向彎折了;如果凸面體是可以自由移動的話,還能發現它稍微往氣流移動。

一百年後,亨利·康德在他的Coandă-1910型飛機實驗中應用了這個效應。

示範實驗

康德效應可以用小氣流柱和乒乓球來示範。將吸塵器的管口往上傾斜一個角度,使氣流從乒乓球上表面附近通過,當氣流強度適中時,乒乓球的重力和其所受氣壓梯度力將可以平衡、讓球浮在半空中。一個對康德效應常見的誤解是水龍頭的水通過湯匙背面時,湯匙會被拉進水流中的這個現象也屬於康德效應。[5]與乒乓球的例子不同之處,在於水流和湯匙是在不同的相態中(液態和氣態),逸入水流的空氣其實相當稀少。湯匙之所以會往水流靠攏主要是表面張力的作用。[6]康德效應的例子還有:在一根點燃的蠟燭前放置罐子,對罐子吹氣,氣流可以將罐子後方的蠟燭吹熄。

在空气动力学中的应用

附壁作用是大部分飛機機翼的主要運作原理。附壁作用的突然消失是飛機失速的主要原因。

部分飛機特別使用引擎吹出的氣流來增加附壁作用,用以提高昇力。美國波音的YC-14 及前蘇聯安-72都是把噴射發動機裝在機翼上方的前面,配合襟翼,吹出的氣流可以提高低速時機翼的升力。波音C-17环球霸王III亦有透過附壁作用增加升力,但所產生的升力較少。

直昇機的「無尾旋翼」(NOTAR-No Tail Rotor)技術,亦是透過吹出空氣在機尾引起附壁作用,造成推力平衡主旋翼产生的反扭矩

參考文獻

  1. Tritton, D.J., Physical Fluid Dynamics, Van Nostrand Reinhold, 1977 (reprinted 1980), Section 22.7, The Coandă Effect.
  2. http://www.merriam-webster.com/dictionary/Coanda%20effect
  3. "The Coanda effect is a phenomenon that was first observed in 1910 by a mathematician and engineer named Henri Coandă. He discovered that when air was ejected from a rectangular nozzle, it would attach itself to an inclined flat plate connected to the nozzle exit. Emphasizing the need for a sharp angle between the nozzle and the flat plate, Coandă then applied the principle to a series of deflecting surfaces, each at a sharp angle to the previous one, and succeeded in turning flows through angles as large as 180. He stated that "when a jet of fluid is passed over a curved surface, it bends to follow the surface, entraining large amounts of air as it does so," and this phenomenon has become known as the Coandă Effect. On Some Recent Applications of the Coanda Effect Caroline Lubert International Journal of Acoustics and Vibration, Vol. 16, No. 3, 2011 http://www.iiav.org/ijav/content/volumes/16_2011_1739941303237209/vol_3/237_firstpage_856831320254369.pdf
  4. Coandă effect. (2013). Columbia Electronic Encyclopedia, 6th Edition. Digital version available here: http://www.answers.com/topic/coanda-effect archiveurl=https://web.archive.org/web/20120118131611/http://www.answers.com/topic/coanda-effect archivedate=2012-01-18
  5. "Mechanics of Fluids, 4th edition 1979, Van Nostrand Reinhold Company, New York, ISBN 0-442-30245-2, Fig, 3.12
  6. "Understanding Aerodynamics Arguing from the Real Physics" Doug McLean, 2013, John Wiley & Sons Ltd. Chichester, ISBN 978-1-119-96751-4, Figure 7.3.6
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