Complete measurement of helicity and its dynamics in vortex tubes
Complete measurement of helicity and its dynamics in vortex tubes
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DOI：
10.1126/science.aam6897
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发表时间：
2017-08
期刊：
影响因子：
56.9
通讯作者：
Martin W. Scheeler;Wim M. van Rees;Hridesh Kedia;D. Kleckner;W. Irvine
中科院分区：
文献类型：
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作者：
Martin W. Scheeler;Wim M. van Rees;Hridesh Kedia;D. Kleckner;W. Irvine
扭转和扭动时连接流体 螺旋度是一种螺旋形运动的度量，通过流体中扭转、扭动和连接的量来描述。对于理想流体来说，总螺旋度是守恒的，但即使是粘度很小的实际流体，螺旋度如何变化一直是一个悬而未决的问题。舍勒等人。通过使用一组水翼来跟踪链接、扭曲和扭动，提供真实流体中总螺旋度的完整测量（参见 Moffatt 的 Perspective）。他们表明，扭转会消除总螺旋性，而扭转和连接则保留它。这提供了对龙卷风发生、大气流动和湍流形成的基本见解。科学，本期第 14 页。 487;另见 p. 448 真实流体中的总螺旋度通过扭转运动而消散，而链接和扭动使螺旋度保持不变。螺旋度是流体流动中涡旋缠绕的拓扑量度，是无粘性流体中的守恒量，但可以通过实际流动中的粘度来消散。尽管螺旋度与一系列流动相关，但人们对实际流体中的螺旋度仍然知之甚少，因为整个数量很难测量。我们测量了水中薄芯涡流管的总螺旋度。对于被第二个涡旋拉伸或压缩的螺旋涡旋，我们发现总螺旋度守恒。对于孤立的螺旋涡，我们观察到在粘性耗散扭曲螺旋度后，向恒定螺旋度状态的演化和维持。我们的结果表明，即使在粘性流体中，螺旋度也可以保持恒定，并为理解和操纵实际流动中的螺旋度提供了改进的基础。
Linking fluids as they twist and writhe Helicity is a measure of cork-screw-like motion described by the amount of twisting, writhing, and linking in a fluid. Total helicity is conserved for ideal fluids, but how helicity changes in real fluids with even tiny amounts of viscosity has been an open question. Scheeler et al. provide a complete measurement of total helicity in a real fluid by using a set of hydrofoils to track linking, twisting, and writhing (see the Perspective by Moffatt). They show that twisting dissipates total helicity, whereas writhing and linking conserve it. This provides a fundamental insight into tornadogenesis, atmospheric flows, and the formation of turbulence. Science, this issue p. 487; see also p. 448 Total helicity in a real fluid is dissipated through twisting motions, whereas linking and writhing keeps helicity conserved. Helicity, a topological measure of the intertwining of vortices in a fluid flow, is a conserved quantity in inviscid fluids but can be dissipated by viscosity in real flows. Despite its relevance across a range of flows, helicity in real fluids remains poorly understood because the entire quantity is challenging to measure. We measured the total helicity of thin-core vortex tubes in water. For helical vortices that are stretched or compressed by a second vortex, we found conservation of total helicity. For an isolated helical vortex, we observed evolution toward and maintenance of a constant helicity state after the dissipation of twist helicity by viscosity. Our results show that helicity can remain constant even in a viscous fluid and provide an improved basis for understanding and manipulating helicity in real flows.