论文标题

纤维跟踪速度计,用于湍流的两点统计

Fiber Tracking Velocimetry for two-point statistics of turbulence

论文作者

Brizzolara, Stefano, Rosti, Marco Edoardo, Olivieri, Stefano, Brandt, Luca, Holzner, Markus, Mazzino, Andrea

论文摘要

我们提出并验证一种新型的实验技术来测量湍流的两点统计。它包括在流动中散布刚性纤维并在时间上跟踪其位置和方向,因此被命名为``纤维跟踪速度表''(FTV)。通过选择不同的纤维长度,即在量表的惯性或耗散范围内,可以通过简单地测量其两端的纤维速度来准确探测到所选长度处的湍流波动的统计,并沿着横向到纤维方向投射。通过完全分辨的直接数值模拟和实验,我们表明这些基于纤维的横向速度增量在统计学上与(未渗透的)流动横向速度增量相等。此外,我们表明湍流耗散速率可以准确地测量足够短的纤维。该技术已针对具有极好一致的流动示踪剂的标准粒子跟踪速度计(PTV)测试。我们的技术克服了PTV的众所周知的问题,可以可靠地探测两点统计数据,因为湍流的快速相对扩散阻止了颗粒之间的相互距离在感兴趣的长度上保持恒定。这个问题使得在固定的分离距离方面很难获得融合的统计数据,对于开放域中的天然流动来说,更具戏剧性。一个突出的例子是海洋电流,在现场测量中使用的漂流器(即示踪物粒子对应物)迅速分散,但与此同时,它们的数量必须受到限制以节省成本。受我们的实验室实验的启发,我们提出了一对连接的漂流者,作为解决问题的可行选择。

We propose and validate a novel experimental technique to measure two-point statistics of turbulent flows. It consists in spreading rigid fibers in the flow and tracking their position and orientation in time and therefore been named ``Fiber Tracking Velocimetry'' (FTV). By choosing different fiber lengths, i.e. within the inertial or dissipative range of scales, the statistics of turbulence fluctuations at the selected lengthscale can be probed accurately by simply measuring the fiber velocity at its two ends, and projecting it along the transverse-to-the-fiber direction. By means of fully-resolved direct numerical simulations and experiments, we show that these fiber-based transverse velocity increments are statistically equivalent to the (unperturbed) flow transverse velocity increments. Moreover, we show that the turbulent energy dissipation rate can be accurately measured exploiting sufficiently short fibers. The technique has been tested against standard Particle Tracking Velocimetry (PTV) of flow tracers with excellent agreement. Our technique overcomes the well-known problem of PTV to probe two-point statistics reliably because of the fast relative diffusion in turbulence that prevents the mutual distance between particles to remain constant at the lengthscale of interest. This problem, making it difficult to obtain converged statistics for a fixed separation distance, is even more dramatic for natural flows in open domains. A prominent example are oceanic currents, where drifters (i.e.~the tracer-particle counterpart used in field measurements) disperse quickly, but at the same time their number has to be limited to save costs. Inspired by our laboratory experiments, we propose pairs of connected drifters as a viable option to solve the issue.

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