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

热对流是最重要的热传输机制之一，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的热驱动而具有高度湍流性。 在实验室实验中，流体通常从顶部和底部被刚性板限制，在该刚性板处形成热和粘性边界层（BL）。 由于通过细胞的垂直热通量通常是有限的，通过这些BL的热通量，标度律已通过考虑BL内的热能和动能耗散而导出。 虽然是湍流，但大体积的流动会形成从底部到顶部的大尺度环流结构（LSC）。据预测，在足够大的热驱动（由无量纲瑞利数Ra表示），由LSC引起的剪切，使BL湍流。 这实际上将导致热传输的显著增加，并且系统将进入一个区域，其中标度定律预计对于任意大的Ra都成立，也就是“对流的最终状态”。 这项工作的目的是调查的影响LSC的粘性BL的稳定性，从而在瑞利-贝纳德设置的热传输。我们特别感兴趣的是接近过渡到最终政权的Ra范围。我们想了解大尺度环流（热对流的湍流上层结构）如何影响BL的稳定性。 特别是，我们计划在第一个实验中增加LSC的强度与风扇，并通过这种尝试达到过渡到最终政权已经在较低的Ra。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，使用光学流速测量方法，如粒子图像流速测量或激光多普勒流速测量。

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.