Fluctuations of the wall shear stress vector in a large-scale natural convection cell

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

Barrel of Ilmenau: a large-scale convection experiment to study dust devil-like flow structures

We present an experimental facility for the validation of numerical simulations on atmospheric dust devils in a controlled laboratory experiment. Dust devils are atmospheric air vortices with a vertical axis, and are formed by intense solar radiation and the resulting vertical temperature gradient. The structure of a typical dust devil is dominated by a radial inflow near the surface and a vertical upward flow within the vortex. These vortices have been studied in recent years using field observations, in situ measurements, and large-eddy simulation (LES). Field tests suffer from the limited area and their unpredictable behavior, while the LES approach cannot resolve the dust devils well enough. Dust devil-like structures may also occur in direct numerical simulation (DNS) with a Rayleigh number of at least Ra = 10^7 in Rayleigh-Bénard convection, with the advantage that the structures can be resolved more precisely. In order to validate the DNS approach and provide measurement data, the airflow is measured inside of a large-scale Rayleigh-Bénard cell of similar geometry (i.e. inside the Barrel of Ilmenau) to the DNS set-up for Rayleigh numbers from Ra = 10^6 to Ra = 10^12. For the measurement of the flow in a large volume, an optical measurement method is used to obtain the trajectories of single particles. Since there are no commercial systems that are suitable for such a large measurement volume, we developed our own system

Viscous boundary layers in turbulent Rayleigh-Bénard convection

Highly resolved local velocity profiles inside the boundary layers in turbulent Rayleigh-Bénard convection in air are presented and discussed. The present work makes progress to our work in the past (see du Puits & Resagk, 2007) that our actual set-up permits the measurement of the wall-normal velocity component w up to a distance of 200 mm away from the wall. All component profiles were performed in a cylindrical box with an aspect ratio Γ = 1, a Prandtl number Pr = 0.7 and Rayleigh numbers Ra = 3 × 10 9 , Ra = 3 × 10 10 . We compare the experimental results with numerics at Ra = 3 × 10 10 directly. We found that the profiles of mean velocity from both experiments and numerics collapse very well with each other and both of the mean horizontal velocity profiles differ from the laminar Blasius prediction at the boundary layer. The wall-normal mean velocity at the central window tends to zero in both experiment and numerics

Wärmetransport in turbulenten Konvektionsströmungen

Abschlussarbeit zur experimentellen Untersuchung des Wärmetransportes in turbulenten Konvektionsströmungen, studiert an einem großskaligem Rayleigh-Bénard-Experiment mit dem besonderem Schwerpunkt auf der Charakterisierung des Geschwindigkeits- und Temperaturfeldes in der Nähe der Kühlplatte, der Analyse der kohärenten Oszillationen in den Zeitreihen beider Größen und der Bestimmung von globalen Ähnlichkeitsparametern in ihrer Abhängigkeit von den thermischen und den geometrischen Randbedingungen.Auch im Buchhandel erschienen: Wärmetransport in turbulenten Konvektionsströmungen / Ronald du Puits. Aachen: Shaker, 2008 DOI: 10.2370/436_282 Preis: 15,00

The cause of oscillations of the large-scale circulation of turbulent Rayleigh-B{\'e}nard convection

In agreement with a recent experimental discovery by Xia et. al. (2009), we also find a sloshing mode in experiments on the large-scale circulation (LSC) of turbulent Rayleigh-Benard convection in a cylindrical sample of aspect ratio one. The sloshing mode has the same frequency as the torsional oscillation discovered by Funfschilling and Ahlers (2004). We show that both modes can be described by an extension of a model developed previously [Brown and Ahlers (2008)] which consists of permitting a lateral displacement of the LSC circulation plane away from the vertical center line of the sample as well as a variation in displacements with height (such displacements had been excluded in the original model). Pressure gradients produced by the side wall of the container on average center the plane of the LSC so that it prefers to reach its longest diameter. If the LSC is displaced away from this diameter, the walls provide a restoring force. Turbulent fluctuations drive the LSC away from the central alignment, and combined with the restoring force they lead to oscillations. These oscillations are advected along with the LSC. This model predicts the correct wavenumber and phase of the oscillations, as well as estimates of the frequency, amplitude, and probability distributions of the displacements.Comment: 16 pages, 6 figures, submitted to Journal of Fluid Mechanic

Fine-scale statistics of temperature and its derivatives in convective turbulence

We study the fine-scale statistics of temperature and its derivatives in turbulent Rayleigh-Benard convection. Direct numerical simulations are carried out in a cylindrical cell with unit aspect ratio filled with a fluid with Prandtl number equal to 0.7 for Rayleigh numbers between 10^7 and 10^9. The probability density function of the temperature or its fluctuations is found to be always non-Gaussian. The asymmetry and strength of deviations from the Gaussian distribution are quantified as a function of the cell height. The deviations of the temperature fluctuations from the local isotropy, as measured by the skewness of the vertical derivative of the temperature fluctuations, decrease in the bulk, but increase in the thermal boundary layer for growing Rayleigh number, respectively. Similar to the passive scalar mixing, the probability density function of the thermal dissipation rate deviates significantly from a log-normal distribution. The distribution is fitted well by a stretched exponential form. The tails become more extended with increasing Rayleigh number which displays an increasing degree of small-scale intermittency of the thermal dissipation field for both the bulk and the thermal boundary layer. We find that the thermal dissipation rate due to the temperature fluctuations is not only dominant in the bulk of the convection cell, but also yields a significant contribution to the total thermal dissipation in the thermal boundary layer. This is in contrast to the ansatz used in scaling theories and can explain the differences in the scaling of the total thermal dissipation rate with respect to the Rayleigh number.Comment: 22 pages and 15 figure