超重力湍流对流中的温度和速度脉动统计特性研究

Statistics of temperature and velocity fluctuations in supergravitational convective turbulence

本文通过实验研究了超重力湍流对流系统中的温度脉动和速度脉动.负尖峰信号在温度脉动时间序列中周期性地出现,这可能是由旋转坐标系下的所谓的纬向流推动冷羽流的周向运动造成的.周期τ0随着瑞利数(Ra)的增加而略微减小,并且τ0比大尺度环流的时间尺度大超过一个数量级.此外,对于不同Ra数,温度脉动能谱在2≾f≾10范围内遵循P(f)∼f^(−5),这意味着超重力湍流对流中可能存在湍流与纬向流相平衡的反向能量级串,本研究还计算了反向级串的截止频率莱茵(Rhines)频率f_(β),f_(β)比–5次方斜率能谱范围的最低频率还低半个数量级左右.本研究还发现时间平均径向速度的周向分布没有明显的峰谷,这为纬向流的存在性提供了进一步的证据.此外,我们发现该系统中的温度和速度脉动的概率密度分布可以通过Wang等人提出的经典瑞利-伯纳德(Rayleigh-Bénard)对流的理论模型(Wang等,2019,2022)较好地拟合和分析.

We report an experimental study of the temperature and velocity fluctuations in supergravitational turbulent convection system.The negative spikes of the time series of temperature fluctuations appear periodically,which may result from the azimuthal movement of the cold plumes in the rotating frame pushed by the zonal flow.The periodτ0 decreases slightly with increasing Rayleigh number(Ra)andτ0 is over one order larger than the time scale of the large-scale circulation.In addition,the temperature frequency power spectra follow P(f)∼f^(−5) for different Ra over the range 2≾f≾10,implying the possible existence of inverse cascade due to turbulence in equilibrium with zonal flow.The Rhines frequency f_(β) at which the inverse cascade ceases is around half of a decade smaller than the smallest frequency of the−5 scaling range.Further,the azimuthal distribution of the time-averaged radial velocity has no obvious peak and trough,which provides further evidence for the existence of zonal flow.Additionally,it is found that the probability density function for temperature and velocity fluctuations in the current system can be well fitted with the model(Wang et al.2019,2022)for the classical Rayleigh-Bénard convection.