Experimental investigation of the effect of coherent secondary structures upon a tip vortex

当较大体积内的一些流体被引起旋转时，就会发生涡流，并且涡流是流体力学中最重要的现象之一。涡流的大小可以从微米（如蚊子拍打翅膀周围形成的涡流）到数百公里（如飓风），在大量工程应用中非常重要。涡流在飞机工业中特别受关注，因为飞机阻力的一部分是由翼尖形成的大涡流的结果。当发动机推动飞机前进时，发动机的一部分动力除了搅动飞机后面的空气外没有其他好处。如果翼尖涡流的强度可以降低，飞机的阻力就会降低，从而减少燃油的燃烧量。这将导致减少碳排放和运营成本。飞机尾流中的大涡流也对其他飞机构成危险，因此它们之间必须保持最小距离。由于涡流在起飞和着陆等低速时最强，这种尾流危险是限制跑道容量的原因。如果涡流能够被破坏并被迫更快地分解和消散，机场将能够容纳更多的航班，而不需要额外的跑道。通过在机翼前方的气流中引入紊流（或随机扰动），可以大大降低翼尖涡流的强度和稳定性。这些扰动与涡旋相互作用，将能量从涡旋中输送出去，并降低其强度。然而，在风洞试验中，通常是通过在机翼前放置一系列重杆来引入扰动;在真实的飞机上，这是不可能的。相反，在这项研究中，小的和精心设计的“凸块”将战略性地放置在机翼表面上，以产生类似的扰动。由于凸块的几何形状和位置有无限多种可能的组合，首先有必要研究涡流以及它如何受到较小扰动的影响。开始，尽管这些流动的工程重要性，它仍然不清楚是否有自然发生的扰动内的漩涡。此外，涡流在许多方面类似于流过平壁的流动。虽然类似的扰动自然发生在壁流中，并在其发展中起着至关重要的作用，但很少有人注意到扰动在涡流中所起的作用。如果能证明非常小的扰动能对大涡产生影响，这本身就是一个极其重要的结果：许多流动模拟计算机代码假定非常小的扰动和非常大的涡之间的直接相互作用是不可能的。最后，所有的漩涡--不管它们是如何产生的，它们有多强，或者它们可能有多“受干扰”--似乎都以完全相同的方式进化。虽然这种相似性已经被注意到，但仍然不清楚为什么会发生。一旦更好地理解了涡流及其对扰动的反应方式，这种理解将用于智能地开发机翼表面，以减少飞机阻力（降低成本和碳排放）并增加机场容量。

A vortex occurs when some of the fluid within a larger volume is caused to rotate, and is one of the most important phenomena in fluid mechanics. Vortices may range in size from microns (such as those formed around the beating wings of a mosquito) to hundreds of kilometers (such as hurricanes), and are of great importance in a large number of engineering applications. Vortex flows are of particular interest in the aircraft industry, since part of an aircraft's drag is the result of the formation of large vortices by the wing tips. As the engines propel the aircraft forward, part of the engine power is used to no other benefit than stirring the air behind the aircraft. If the wing tip vortex could be reduced in strength, the aircraft drag would decrease, reducing the amount of fuel burned. This would lead to both decreased carbon emissions and operating costs. The large vortices in the aircraft wake also pose a danger to other aircraft, so a minimum distance must be kept between them. Since the vortices are strongest at low speed such as during takeoff and landing, this wake hazard is what limits runway capacity. If the vortex could be destabilized and forced to break up and dissipate more quickly, airports would be able to accommodate a larger number of flights without needing additional runways. The strength and stability of a wing-tip vortex can be greatly reduced by introducing turbulence (or random disturbances) into the flow ahead of the wing. These disturbances interact with the vortex, transport energy away from it and reduce its strength. However, in wind tunnel tests, the disturbances are usually introduced by placing a series of heavy bars ahead of the wing; on a real aircraft, this would not be possible. Instead, in this study, small and carefully designed 'bumps' will be strategically placed on the surface of the wing in order to generate similar disturbances.Since there are infinitely many possible combinations of bump geometries and locations, it is first necessary to study the vortex and how it is affected by smaller disturbances. To begin with, despite the engineering importance of these flows, it still isn't clear whether or not there are naturally-occurring disturbances inside a vortex. Also, vortices are in many ways analogous to the flow over flat walls. Though similar disturbances occur naturally in wall flows and play a vital role in their development, very little attention has been given to the role played by the disturbances in vortex flows. If it can be shown that very small disturbances can have an effect on large vortices, this in itself would be an extremely important result: many flow simulation computer codes assume that the direct interaction between very small disturbances and very large vortices is impossible. Finally, all vortices- regardless of how they were generated, how strong they are or how 'disturbed' they may be- appear to evolve in exactly the same way. While this similarity has already been noticed, it is still not clear why it happens. Once the vortex and the way it responds to disturbances is better understood, this understanding will be used to intelligently develop a wing surface which can reduce aircraft drag (cutting down both cost and carbon emissions) and increase airport capacity.