Flight–crash events in turbulence
Flight–crash events in turbulence
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DOI：
10.1073/pnas.1321682111
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发表时间：
2014-05
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影响因子：
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通讯作者：
Haitao Xu;A. Pumir;G. Falkovich;E. Bodenschatz;M. Shats;H. Xia;N. Francois;G. Boffetta
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文献类型：
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作者：
Haitao Xu;A. Pumir;G. Falkovich;E. Bodenschatz;M. Shats;H. Xia;N. Francois;G. Boffetta
不可逆性是自然系统进化的一个基本方面，在描述非平衡系统的任何尝试中，量化其表现都是一个挑战。在流体湍流的情况下，一个系统远离平衡的象征性例子，我们表明，单个流体粒子的运动提供了时间不可逆性的清晰表现。也就是说，我们观察到流体粒子失去动能的速度往往快于获得动能的速度。这一点在罕见的“飞行坠毁”事件中得到了最好的体现，在这种事件中，快速移动的粒子突然减速到流体运动缓慢的区域。值得注意的是，这些事件的统计特征在不可逆性程度和湍流强度之间建立了定量关系。湍流的统计性质在本质上与处于或接近热平衡的系统的统计性质不同，这是因为能量在不同的尺度之间的流动，在不同的尺度上提供能量和耗散能量。我们通过实验和数值研究在湍流中运动的小流体粒子的能量波动来阐明这种差异。我们演示了详细平衡的基本属性是如何被打破的，因此向前和向后转换的概率对于湍流来说是不相等的。在物理方面，我们发现在大量的流动配置中，流体元素减速比加速快，这是在密集交通中驾驶的一个众所周知的特征。罕见的“飞行坠毁”事件的统计特征，与快速粒子减速有关，提供了一种量化湍流不可逆性的方法。也就是说，我们发现沿轨迹的能量波动的第三矩，非量纲化的能量通量，在二维和三维空间中都表现出显著的幂律作为雷诺数的函数。这建立了系统的不可逆性与活动尺度范围之间的关系。我们推测，这里所描述的详细平衡的破坏是其他远离平衡的系统的一般特征，显示出广泛的空间尺度。
Significance Irreversibility is a fundamental aspect of the evolution of natural systems, and quantifying its manifestations is a challenge in any attempt to describe nonequilibrium systems. In the case of fluid turbulence, an emblematic example of a system very far from equilibrium, we show that the motion of a single fluid particle provides a clear manifestation of time irreversibility. Namely, we observe that fluid particles tend to lose kinetic energy faster than they gain it. This is best seen by the presence of rare “flight–crash” events, where fast moving particles suddenly decelerate into a region where fluid motion is slow. Remarkably, the statistical signature of these events establishes a quantitative relation between the degree of irreversibility and turbulence intensity. The statistical properties of turbulence differ in an essential way from those of systems in or near thermal equilibrium because of the flux of energy between vastly different scales at which energy is supplied and at which it is dissipated. We elucidate this difference by studying experimentally and numerically the fluctuations of the energy of a small fluid particle moving in a turbulent fluid. We demonstrate how the fundamental property of detailed balance is broken, so that the probabilities of forward and backward transitions are not equal for turbulence. In physical terms, we found that in a large set of flow configurations, fluid elements decelerate faster than accelerate, a feature known all too well from driving in dense traffic. The statistical signature of rare “flight–crash” events, associated with fast particle deceleration, provides a way to quantify irreversibility in a turbulent flow. Namely, we find that the third moment of the power fluctuations along a trajectory, nondimensionalized by the energy flux, displays a remarkable power law as a function of the Reynolds number, both in two and in three spatial dimensions. This establishes a relation between the irreversibility of the system and the range of active scales. We speculate that the breakdown of the detailed balance characterized here is a general feature of other systems very far from equilibrium, displaying a wide range of spatial scales.