15:00
Thermally driven turbulence 2
Chair: Xiaozhou He
15:00
15 mins
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Thermal boundary layer equation for turbulent Rayleigh-Benard convection
Olga Shishkina, Susanne Horn, Sebastian Wagner, Emily S. C. Ching
Abstract: We report a new thermal boundary layer equation for turbulent Rayleigh-Benard convection for Prandtl number P>1that takes into account the effect of turbulent fluctuations. These fluctuations are neglected in existing equations, which are based on steady-state and laminar assumptions. Using this new equation, we derive analytically the mean temperature profiles in two limits: Pr~1 and Pr>>1. These two theoretical predictions are in excellent agreement with the results of our direct numerical simulations for Pr=4.38 and Pr=2547.9, respectively.
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15:15
15 mins
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LOCAL HEAT FLUX MEASUREMENTS USING A MICRO-MACHINED INTEGRATED PROBE
Eléonore RUSAOUEN, Bernard CASTAING, Francesca CHILLA, Philippe-Emmanuel ROCHE, Julien SALORT
Abstract: Thermal flows are ubiquitous in natural and industrial systems, this is why they have been studied for decades. In these kinds
of flows, an important quantity is the local heat flux, but it remains experimentally nearly unstudied since it is difficult to measure it.
We introduce a new local convective heat flux probe, based on a joint measurement of temperature and velocity within the flow.
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15:30
15 mins
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Analysis of turbulent Rayleigh-Bénard convection in the compound physical/scale space domain
Riccardo Togni, Andrea Cimarelli, Elisabetta De Angelis
Abstract: We report the results from two distinct direct numerical simulations (DNS) of turbulent Rayleigh-Bénard convection (RBC) for Rayleigh number of 10^5 and Prandtl number of 0.7 in a laterally unbounded domain confined between two horizontal isothermal plates with no-slip and free-slip boundary conditions respectively. The central aim of the present work consists in a simultaneous description of both flows in a compound physical/scale space domain by using a generalized form of the classical Kolmogorov equation for the second-order velocity structure function. It has been found that the dynamics of the coherent structures in RBC, the so-called thermal plumes, are clearly reflected in the multi-scale energy budgets. In particular, the enlargement of thermal plumes following the impingement at the wall entails a transfer of scale-energy from small turbulent scales toward larger ones. This aspect shed light on the role of thermal plumes in turbulent RBC and could have a direct impact on future attempts to model the effects of small-scale motions in thermal convection.
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15:45
15 mins
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Turbulent convective heat transfer in an inclined cylinder with liquid sodium
Andrei Mamykin, Andrei Vasiliev, Ruslan Khalilov, Ilya Kolesnichenko, Peter Frick
Abstract: The natural turbulent convection of liquid sodium in a cell with end heat exchangers providing a fixed temperature drop is investigated experimentally. The cell is a straight thermally isolated tube with inner diameter D = 96 mm and length L ≈ 20D. Experiments are carried out for a fixed Rayleigh number Ra = 2.4 • 10^6 and for different tube orientations with respect to the gravity. A strong dependence of power transferred along the tube on the inclination angle is discovered: Nusselt number varies by an order in the investigated range of angles with a maximum approximately at 65 degrees to the vertical. Presented characteristics of the large-scale circulation (LSC) and turbulent temperature fluctuations demonstrate the fact that the convective heat transfer is mainly determined by the velocity of the LSC.
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16:00
15 mins
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Large-Scale Circulation Reversals in a 2D Rayleigh-Bénard cell
Berengere Podvin, Anne Sergent
Abstract: We consider the numerical simulation of a two-dimensional Rayleigh-B\'enard cell in the turbulent regime $Ra=5$ $10^{7}$ and $Pr=4.3$.
The flow is dominated by a large-scale inclined roll, the orientation of which switches intermittently in time. We use Proper Orthogonal Decomposition to identify the most energetic modes. We find that the first two modes respectively correspond to an antisymmetric and
a symmetric mode. The next most energetic mode breaks the symmetry of the flow. During
reversals, sharp variations in the temporal amplitudes of the modes are observed. We derive
a low-dimensional model based on the first three most energetic modes which is able to reproduce the
large-scale circulation reversals, in quantitative agreement with the simulation.
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16:15
15 mins
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ONSET OF REVERSAL AND CHAOS IN THERMALLY DRIVEN CAVITY FLOW
Masaki Shimizu, Kentaro Cho, Genta Kawahara
Abstract: We investigate the onset of chaotic reversals of thermal convection cell in a two–dimensional square cavity using direct numerical simulation. To our knowledge, the chaotic reversal motion at the lowest Rayleigh number is related to the unicellular motion in this system. As Rayleigh number increases, the two antisymmetric attractors, which arise from the supercritical Hopf bifurcation, approach each other. However, because the basin boundaries of these attractors have multiple unstable directions, the behavior of the global changes seems to be complex for the small range of Rayleigh number around the onset of the chaotic reversal. On the other hand, period–doubling cascade from periodic reversal solutions leads to chaotic reversal as Rayleigh number decreases. At the conference we will report observation about the beginning of reversal and the quantities of the chaotic attractor.
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