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
Highlights
Since the flow mechanisms in thermally driven convection are of importance for many technical applications, such as the steel casting (Burr and Müller 2001), crystal growth (Cramer et al 2013) and the ventilation of passenger compartments in trains or airplanes (Zhang et al 2020), and for a better understanding of geo- and astrophysical settings (Schumacher and Sreenivasan 2020), the detailed analysis of this type of flow has become increasingly important in the last decades
The experiments were performed in the horizontal mid plane and in a horizontal plane closer to the cooling plate at the top of a Rayleigh–Bénard cell with an aspect ratio of = 25
In an additional preliminary experiment the initial growth of the flow structures was shown by means of temperature measurements via the color signal of thermochromic liquid crystals (TLCs) in the top measurement plane
Summary
Since the flow mechanisms in thermally driven convection are of importance for many technical applications, such as the steel casting (Burr and Müller 2001), crystal growth (Cramer et al 2013) and the ventilation of passenger compartments in trains or airplanes (Zhang et al 2020), and for a better understanding of geo- and astrophysical settings (Schumacher and Sreenivasan 2020), the detailed analysis of this type of flow has become increasingly important in the last decades. In order to study the flow mechanisms by means of laboratory experiments or numerical simulations, the so-called Rayleigh–Bénard setup has established as an appropriate model. This model consists of a fluid volume enclosed by adiabatic side walls, which is uniformly heated from below and cooled from above. Applying Rayleigh–Bénard cells with flat surfaces, the boundary conditions of complex technical and natural systems with thermal buoyancy as the main flow driving force are simplified. This model allows to investigate typical characteristics of thermally driven flows and is suited to gain deeper insights into these systems. The flow inside this setup, i.e. Rayleigh–Bénard convection, has already been analyzed comprehensively (Bodenschatz et al 2000)
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