10:30
Thermally driven turbulence 1
Chair: Enrico Stalio
10:30
15 mins
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Buoyancy-driven turbulent convection in a bundle of vertical heated cylinders
Diego Angeli, Enrico Stalio
Abstract: Buoyant, turbulent convective heat transfer around cylindrical rods arranged in bundles is a technically relevant heat transfer configuration which finds application in steam generators, cooling of reactor core fuel assemblies and heat exchangers in general. Most of the research performed so far considered forced convection conditions on vertical rod bundles, corresponding for example to the configuration of a nuclear reactor primary loop. Fewer works have focused on the effect of buoyancy, with or without an external source of momentum.
This paper reports a Direct Numerical Simulation study of the fully developed, mixed convection in a bundle of vertical, heated cylinders, where each cylinder releases an equal, uniform heat flux distribution, and a Boussinesq approximation is introduced to represent buoyancy effects. Simulations are performed for Gr = 10^7 , Reτ = 180 and Pr = 0.1. As experimental velocity and temperature profiles are not available in the literature, a preliminary validation of the method employed is performed by the comparison of the global quantities like f and Nu. The time averaged velocity and temperature fields are reported in the paper, together with first order and higher-order statistics. In addition the main turbulent flow features are described through coherent structure eduction methods.
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10:45
15 mins
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Logarithmic variance profiles and the corresponding {\lowercase {$ f^{-1}$}} spectra of temperature fluctuations in turbulent Rayleigh-B\'enard convection
Xiaozhou He, Dennis van Gils, Eberhard Bodenschatz, Guenter Ahlers
Abstract: We report experimental results for the temperature variance $\sigma^2(z)$ and the corresponding frequency spectra $P(f)$ in turbulent Rayleigh-B\'enard convection (RBC) in a cylindrical sample of aspect ratio $\Gamma \equiv D/L = 1.00$ ($D = 1.12$ m is the diameter and $L = 1.12$ m the height). The measurements were conducted in the Rayleigh-number range $10^{11} \alt$ Ra $\alt 1.35\times10^{14}$ and Pr $\simeq 0.8$. For Ra $= 1.35\times 10^{14}$, $\sigma^2(z)$ could be described well by a logarithmic dependence on the vertical position $z$ in a range of $z^*_1 \alt z \alt z^*_2$ with $z^*_1 \simeq 70 \lambda_{\theta}$ and $z^*_2 = 0.1L$. Here $\lambda_\theta \equiv L/(2Nu)$ is the thickness of a thin thermal sublayer adjacent to the horizontal plate where the heat flux (denoted by the Nusselt number $Nu$) is carried mostly by thermal diffusion. In the log layer, we found that the temperature spectra had a significant frequency range over which $P(f) \sim f^{-\alpha}$ with $\alpha$ close to $1$. As Ra decreased, $\lambda_\theta$ increased so that the log layer became thinner. At Ra $= 2.05 \times 10^{11}$, $z^*_2 \alt z^*_1$ and therefore there was no range for a log layer. Correspondingly, the temperature spectrum near the horizontal plate did not have the $f^{-1}$ scaling form either.
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11:00
15 mins
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LARGE SCALE CIRCULATION IN TURBULENT RAYLEIGH-BÉNARD CONVECTION
Tomáš Králík, Marco La Mantia, Ladislav Skrbek, Pavel Urban, Věra Musilová
Abstract: Statistical properties of turbulent Rayleigh-Bénard convection (RBC) are investigated experimentally in a cylindrical cell of aspect ratio one. We specifically analyze the large scale circulation of RBC based on measurements of temperature fluctuations by small Ge sensors placed inside the cell. The resulting dependencies of Reynolds numbers on Rayleigh number up to 2x10^13 are compared to available theoretical and experimental results for similar geometries.
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11:15
15 mins
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Aspect-ratio dependence of the transition to the ultimate state of turbulent Rayleigh-B\'enard convection
Eberhard Bodenschatz, Xiaozhou He, Guenter Ahlers, Dennis Van-Gils
Abstract: We report on measurements of the ultimate-state transition in turbulent Rayleigh-B\'enard convection obtained in a large facility known as the ``Uboot of G\"ottingen" and using pressurized sulfur hexafluoride as the convecting fluid. We found that the transition occurs over a range of $Ra$ which becomes more narrow as $\Gamma$ increases, ranging from $Ra_1^*$ which is at most weakly dependent on $\Gamma$ and close to $10^{13}$ to $Ra_2^*$ which varies from about $2\times 10^{15}$ for $\Gamma = 0.33$ to about $7\times 10^{13}$ for $\Gamma = 1.00$.
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11:30
15 mins
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Direct numerical simulation of turbulent Taylor-Couette flow with grooved walls
Xiaojue Zhu, Rodolfo Ostilla-Monico, Roberto Verzicco, Detlef Lohse
Abstract: We present direct numerical simulations of Taylor-Couette flow with grooved walls up to inner cylinder Reynolds number of $Re_i=3.76\times10^4$, corresponding to Taylor number of $Ta=2.15\times10^9$. The simulations are performed at a fixed radius ratio $\eta=r_i/r_o=0.714$. The grooves are axisymmetric V-shaped obstacles attached to the wall with a tip angle of $90^\circ$. Results are compared with the smooth wall case in order to investigate the effects of the grooved walls. In particular, we focus on the effective scaling laws for torque, boundary layers and flow structures. With increasing $Ta$, the boundary layer thickness finally becomes smaller than the groove height. When this happens, the plumes are ejected from tips of the grooves and a secondary circulation between the grooves is formed. This is associated with a sharp increase of the torque and thus the effective scaling law for the torque becomes much steeper. Further increasing $Ta$ does not result in an additional slope increases. Instead, the effective scaling law saturates to the same ``ultimate'' regime effective exponents seen for smooth walls.
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11:45
15 mins
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Roughness-triggered turbulent boundary layers in Rayleigh-Bénard convection
Julien Salort, Olivier Liot, Robert Kaiser, Ronald du Puits, Francesca Chillà
Abstract: We present measurements carried out inside the Barrel of Ilmenau as part of the European EuHIT transnational infrastructure access program. The Barrel of Ilmenau is the worldwide largest experiment to study highly turbulent convection in air. A rectangular cell, with proportions strictly identical to the water cell in Lyon [Salort, et al. PoF 26:015112 (2014)], but six times larger, has been inserted inside the Barrel. The top plate is smooth, and the bottom plate is rough. The roughness are also similar to the one in Lyon, but six times larger. We have obtained velocity fields using PIV near the obstacles, as well as the local heat-flux on the bottom plate. This has allowed us to test and improve our previous interpretation of the roughness-induced heat transfer enhancement mechanisms.
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12:00
15 mins
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Reynolds numbers near the ultimate state of turbulent Rayleigh-B\'enard convection
Guenter Ahlers, Xiaozhou He, Dennis van Gils, Eberhard Bodenschatz
Abstract: We report on measurements of the mean-flow Reynolds number $Re_U$ and the rms fluctuation Reynolds number $Re_V$ in turbulent Rayleigh-B\'enard convection as a function of the Rayleigh number $Ra$ for $4\times 10^{11} \alt Ra \alt 2\times 10^{14}$ and $Pr \simeq 0.8$. Both can be described by the same power law with an effective exponent $\zeta = 0.44$, in agreement with predictions for $Re_U$ but in disagreement with predictions for $Re_V$.
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12:15
15 mins
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Multiple transitions in rotating turbulent Rayleigh-B\`enard convection
Stephan Weiss, Ping Wei, Guenter Ahlers
Abstract: Sharp transitions between potentially different turbulent states are unexpected because one might think that they should be washed out by the prevailing intense fluctuations and short coherence lengths and times. Contrary to this expectation, we found a {\it sequence} of such transitions in turbulent rotating Rayleigh-B\'enard convection as the rotation rate was increased. This phenomenon became most prominent at very large Rayleigh numbers up to $2\times 10^{12}$ where the fluctuations are extremely vigorous. It was found in the heat transport as well as in the temperature gradient near the sample center. We conjecture that the transitions are between different large-scale structures which involve changes of symmetry and thus can not be gradual.
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