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，在许多地球物理和天体物理系统以及工业应用中发挥着重要的作用。这种自然对流系统由于其强烈的热驱动而具有高度的湍流性。在实验室实验中，流体通常从上到下受刚性板的约束，在板上形成热边界层和粘性边界层。由于通过边界层的垂直热通量通常受通过边界层的热通量的限制，因此考虑边界层内部的热耗散和动能耗散，得到了标度律。在整体中的流动，虽然是湍流的，但发展了大尺度的环流结构(LSC)，从底部到顶部的程度。据预测，在足够大的热驱动下(用无量纲瑞利数Ra表示)，LSC引起的切变使边界层湍流。事实上，这将导致热量传输的显著增加，系统将进入一个区域，在该区域中，标度定律有望对任意大的Ra成立，也就是“对流的终极状态”。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Rayleigh-Benard装置中的热传输产生影响。我们对接近向终极政权过渡的Ra-Range特别感兴趣。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，我们计划在第一次实验中增加带风扇的LSC的强度，并通过此尝试达到已经在较低Ra时的终极制度的过渡。在第二个实验中，我们想要用光学测速方法，如粒子图像测速或激光多普勒测速，直接研究剪切对加热平板上方边界层的影响。

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.