Abstract

Large-scale mean patterns in Rayleigh–Bénard convection, also referred to as turbulent superstructures, have mainly been studied by means of numerical simulations so far, but experimental investigations are still rare. However, the analysis of turbulent superstructures, which are of great importance due to their effect on the local transport of heat and momentum, require both numerical and experimental data. Therefore, within the scope of this study measurements were performed in the horizontal mid plane and in a horizontal plane closer to the top of a Rayleigh–Bénard cell with an aspect ratio of \(\varGamma =l/h=25\), thereby showing the initial formation of turbulent superstructures and their long-time rearrangement. The turbulent superstructures are investigated experimentally by noninvasive simultaneous measurements of temperature and velocity fields, using the color signal of thermochromic liquid crystals (TLCs) for the evaluation of the temperature and their temporal displacement for the determination of all three velocity components in the measurement planes via stereoscopic particle image velocimetry (stereo-PIV). Applying this measuring technique it is demonstrated that the time-averaging of instantaneous temperature and velocity fields uncovers the turbulent superstructures in both fields. Furthermore, the combination of the temperature and velocity data is used to characterize the local heat flux quantified by the local Nusselt number, which confirms that the turbulent superstructures strongly enhance the heat transfer in Rayleigh–Bénard convection.

Graphic abstract

中文翻译：

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通过非侵入式同时测量温度和速度场对Rayleigh-Bénard对流中湍流上部结构进行长期实验研究

摘要

迄今为止，瑞利-贝纳德对流中的大型均值模式（也称为湍流上层结构）主要通过数值模拟的方法进行了研究，但实验研究仍然很少。然而，湍流上层建筑的分析由于其对热量和动量的局部传递的影响而非常重要，需要数值和实验数据。因此，在本研究的范围内，在水平中平面和更接近Rayleigh-Bénard单元顶部的水平面中执行测量，其长宽比为\（\ varGamma = l / h = 25 \），从而显示出湍流上部结构的初始形成及其长时间的重排。通过对温度和速度场进行无创同步测量，对湍流的上部结构进行了实验研究，使用热致变色液晶（TLC）的彩色信号评估温度及其时间位移，从而确定测量平面中所有三个速度分量。立体粒子图像测速（stereo-PIV）。应用这种测量技术证明，瞬时温度和速度场的时间平均揭示了两个场中的湍流上部结构。此外，温度和速度数据的组合用于表征由局部Nusselt数量化的局部热通量，

图形摘要