The influence of the large scale circulation on the transition to the "Ultimate state of thermal convection"

大尺度环流对“热对流终极状态”转变的影响

• 批准号: 316170110
• 负责人: Dr. Stephan Weiss
• 金额: --
• 依托单位: Max-Planck-Institut für Dynamik und Selbstorganisation (MPIDS)
• 依托单位国家: 德国
• 项目类别: Priority Programmes
• 财政年份: 2016
• 资助国家: 德国
• 起止时间: 2015-12-31 至 2021-12-31
• 项目状态: 已结题
• 来源: https://gepris.dfg.de/gepris/projekt/316170110?language=en
• 关键词: influence; large; scale; circulation; transition

Thermal convection is one of the most important heat transport mechanism and plays a major role in many geo- and astrophysical systems, as well as for industrial applications. Such natural convection systems are highly turbulent due to their strong thermal driving. In laboratory experiments, a fluid is usually confined by rigid plates from the top and the bottom, at which thermal and viscous boundary layers (BL) develop. Since the vertical heat flux through the cell is usually limited by the heat flux through these BL, scaling laws have been derived by considering thermal and kinetic energy dissipation inside the BL. The flow in the bulk, although turbulent, develops large scale circulations structures (LSC), that extent from the bottom to the top. It is predicted, that at sufficiently large thermal driving (expressed by the dimensionless Rayleigh number Ra), the shear caused by the LSC, renders the BL turbulent. This in fact would cause a significant increase in the heat transport, and the system would enter a regime, where scaling laws are expected to hold for arbitrary large Ra, aka the "ultimate state of convection". The objective of this work is to investigate the influence of the LSC on the stability of the viscous BL and thus on the heat transport in a Rayleigh-Benard setup. We are especially interested in the Ra-range close to the transition to the ultimate regime. We want to understand how the large scale circulation (the turbulent superstructure of thermal convection) affects the stability of the BL. In particular, we plan in a first experiment to increase the strength of the LSC with a fan and by this try to reach the transition to the ultimate regime already at lower Ra. In a second experiment, we want to directly investigate the influence of a shear on the boundary layer above a heated plate, with optical vlocimetry methods such as particle image velocimetry or laser Doppler velocimetry.

热对流是最重要的热传递机制之一，在许多地球和天体物理系统以及工业应用中起着重要作用。这种自然对流系统由于其强烈的热驱动而具有高度湍流性。在实验室实验中，流体通常从上到下受到刚性板的约束，在刚性板上形成热边界层和粘性边界层。由于通过单体的垂直热通量通常受到通过这些BL的热通量的限制，因此考虑BL内部的热能和动能耗散，推导出了标度定律。体内的流动虽然是湍流，但形成了从底部到顶部的大规模循环结构（LSC）。预测在足够大的热驱动（用无因次瑞利数Ra表示）下，由LSC引起的剪切使BL产生湍流。事实上，这将导致热传递的显著增加，并且系统将进入一种状态，在这种状态下，尺度定律预计将适用于任意大的Ra，即“对流的最终状态”。这项工作的目的是研究LSC对粘性边界层稳定性的影响，从而对瑞利-贝纳德装置中的热传输产生影响。我们特别感兴趣的是接近过渡到最终政权的ra范围。我们想要了解大尺度环流（热对流的湍流上层结构）如何影响边界层的稳定性。特别是，我们计划在第一个实验中用风扇增加LSC的强度，并试图通过这种方式达到已经在较低Ra下的最终状态的过渡。在第二个实验中，我们想直接研究剪切对加热板上方边界层的影响，使用光学速度测量方法，如粒子图像速度测量或激光多普勒速度测量。