City Research Online

Abstract

We report first experimental data of the wall shear stress in turbulent air flow in a large-scale Rayleigh-Bénard experiment. Using a novel, nature-inspired measurement concept (Bruecker and Mikulich 2017, PLoS ONE 12, e0179253), we measured the mean and fluctuating part of the two components of the wall shear stress vector at the heated bottom plate at a Rayleigh number Ra=1.58e10 and a Prandtl number Pr=0.7. The total sampling period of 1,5 hours allowed to capture the dynamics of the magnitude and the orientation of the vector over several orders of characteristic time-scales of the large-scale circulation. We found the amplitude of short-term (turbulent) fluctuations to be following a highly skewed Weibull distribution, while the long-term fluctuations are dominated by the modulation effect of a quasi-regular angular precession of the outer flow around a constant mean, the time-scale of which is coupled to the characteristic eddy turn-over time of the global recirculation roll. Events of instantaneous negative streamwise wall shear occur when rapid twisting of the local flow happens. A mechanical model is used to explain the precession by tilting the spin moment of the large circulation roll and conservation of angular momentum. A slow angular drift of the mean orientation is observed in a phase of considerable weakening of mean wind magnitude.

Publication Type:	Article
Additional Information:	All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).
Subjects:	T Technology > TA Engineering (General). Civil engineering (General) T Technology > TJ Mechanical engineering and machinery T Technology > TL Motor vehicles. Aeronautics. Astronautics
Departments:	School of Science & Technology > Engineering
SWORD Depositor:	Symplectic Administrator

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