We report first experimental data of the wall shear stress in turbulent air flow in a large-scale Rayleigh–Bénard experiment. Using a novel, nature-inspired measurement concept [C. H. Bruecker and V. Mikulich, PLoS One 12, e0179253 (2017)], we measured the mean and fluctuating part of the two components of the wall shear stress vector at the heated bottom plate at a Rayleigh number Ra = 1.58 × 10¹⁰ and a Prandtl number Pr = 0.7. The total sampling period of 1.5 h allowed us to capture the dynamics of the magnitude and the orientation of the vector over several orders of characteristic timescales of the large-scale circulation. We found the amplitude of short-term (turbulent) fluctuations to be following a highly skewed Weibull distribution, while the long-term fluctuations are dominated by the modulation effect of a quasi-regular angular precession of the outer flow around a constant mean, the timescale of which is coupled to the characteristic eddy turnover time of the global recirculation roll. Events of instantaneous negative streamwise wall shear occur when rapid twisting of the local flow happens. A mechanical model is used to explain the precession by tilting the spin moment of the large circulation roll and conservation of angular momentum. A slow angular drift of the mean orientation is observed in a phase of considerable weakening of mean wind magnitude.

中文翻译：

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大型自然对流池壁剪应力矢量的涨落

我们在大规模瑞利-贝纳德实验中报告了湍流中壁面剪应力的第一批实验数据。使用新颖的，受自然启发的测量概念[CH Bruecker和V.Mikulich，PLoS One 12，e0179253（2017）]，我们在加热的底板处测量了壁切应力矢量的两个分量的平均值和波动部分瑞利数Ra = 1.58×10 ¹⁰普朗特数Pr = 0.7。1.5 h的总采样时间使我们能够捕获大规模循环的几个特征时间尺度上向量的大小和方向的动态变化。我们发现短期（湍流）波动的幅度遵循高度偏斜的Weibull分布，而长期波动主要受围绕恒定均值的外流准规则角度旋进的调制效应的影响。其时间尺度与整体再循环辊的特征性涡流周转时间有关。当局部流动发生快速扭曲时，会发生瞬时负向流墙剪切的事件。使用机械模型通过倾斜大循环辊的旋转力矩和保持角动量来解释岁差。在平均风幅显着减弱的阶段，可以观察到平均方位角的缓慢漂移。