Mechanism of large-scale flow reversals in turbulent thermal convection

It is commonly believed that heat flux passing through a closed thermal convection system is balanced so that the convection system can remain at a steady state. Here, we report a new kind of convective instability for turbulent thermal convection, in which the convective flow stays over a long steady “quiet period” having a minute amount of heat accumulation in the convection cell, followed by a short and intermittent “active period” with a massive eruption of thermal plumes to release the accumulated heat. The rare massive eruption of thermal plumes disrupts the existing large-scale circulation across the cell and resets its rotational direction. A careful analysis reveals that the distribution of the plume eruption amplitude follows the generalized extreme value statistics with an upper bound, which changes with the fluid properties of the convecting medium. The experimental findings have important implications to many closed convection systems of geophysical scale, in which massive eruptions and sudden changes in large-scale flow pattern are often observed.

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Combined measurement of velocity and temperature in liquid metal convection

Journal of Fluid Mechanics ◽

10.1017/jfm.2019.556 ◽

2019 ◽

Vol 876 ◽

pp. 1108-1128 ◽

Cited By ~ 11

Author(s):

Till Zürner ◽

Felix Schindler ◽

Tobias Vogt ◽

Sven Eckert ◽

Jörg Schumacher

Keyword(s):

Reynolds Number ◽

Liquid Metal ◽

Large Scale ◽

Scale Up ◽

Convection Cell ◽

Small Scale ◽

Convection Flow ◽

Large Scale Circulation ◽

Large Scale Flow ◽

The Impact

Combined measurements of velocity components and temperature in a turbulent Rayleigh–Bénard convection flow at a low Prandtl number of $Pr=0.029$ and Rayleigh numbers of $10^{6}\leqslant Ra\leqslant 6\times 10^{7}$ are conducted in a series of experiments with durations of more than a thousand free-fall time units. Multiple crossing ultrasound beam lines and an array of thermocouples at mid-height allow for a detailed analysis and characterization of the complex three-dimensional dynamics of the single large-scale circulation roll in a cylindrical convection cell of unit aspect ratio which is filled with the liquid metal alloy GaInSn. We measure the internal temporal correlations of the complex large-scale flow and distinguish between short-term oscillations associated with a sloshing motion in the mid-plane as well as varying orientation angles of the velocity close to the top/bottom plates and the slow azimuthal drift of the mean orientation of the roll as a whole that proceeds on a time scale up to a hundred times slower. The coherent large-scale circulation drives a vigorous turbulence in the whole cell that is quantified by direct Reynolds number measurements at different locations in the cell. The velocity increment statistics in the bulk of the cell displays characteristic properties of intermittent small-scale fluid turbulence. We also show that the impact of the symmetry-breaking large-scale flow persists to small-scale velocity fluctuations thus preventing the establishment of fully isotropic turbulence in the cell centre. Reynolds number amplitudes depend sensitively on beam-line position in the cell such that different definitions have to be compared. The global momentum and heat transfer scalings with Rayleigh number are found to agree with those of direct numerical simulations and other laboratory experiments.

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Large-scale flow properties of turbulent thermal convection

Physical Review E ◽

10.1103/physreve.54.r5901 ◽

1996 ◽

Vol 54 (6) ◽

pp. R5901-R5904 ◽

Cited By ~ 108

Author(s):

S. Ciliberto ◽

S. Cioni ◽

C. Laroche

Keyword(s):

Thermal Convection ◽

Large Scale ◽

Flow Properties ◽

Large Scale Flow

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How heat transfer efficiencies in turbulent thermal convection depend on internal flow modes

Journal of Fluid Mechanics ◽

10.1017/jfm.2011.50 ◽

2011 ◽

Vol 676 ◽

pp. 1-4 ◽

Cited By ~ 11

Author(s):

KE-QING XIA

Keyword(s):

Heat Transfer ◽

Nusselt Number ◽

Thermal Convection ◽

Large Scale ◽

Internal Flow ◽

Global Transport ◽

Large Scale Flow ◽

Rayleigh Benard Convection ◽

Rayleigh Benard ◽

Flow States

How internal flow states can influence the global transport properties in a turbulent system has always been an intriguing question. Weiss & Ahlers (J. Fluid Mech., this issue, vol. 676, 2011, pp. 5–40) have provided an example by measuring the instantaneous Nusselt number in turbulent Rayleigh-Bénard convection and correlating it to the different modes of large-scale flow.

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Effects of polymer additives in the bulk of turbulent thermal convection

Journal of Fluid Mechanics ◽

10.1017/jfm.2015.618 ◽

2015 ◽

Vol 784 ◽

Cited By ~ 14

Author(s):

Yi-Chao Xie ◽

Shi-Di Huang ◽

Denis Funfschilling ◽

Xiao-Ming Li ◽

Rui Ni ◽

...

Keyword(s):

Heat Flux ◽

Heat Transport ◽

Thermal Convection ◽

Polymer Concentration ◽

Local Heat ◽

Heat Fluxes ◽

Small Scale ◽

Polymer Additives ◽

Thermal Plumes ◽

Minute Amount

We present experimental evidence that a minute amount of polymer additives can significantly enhance heat transport in the bulk region of turbulent thermal convection. The effects of polymer additives are found to be the enhancement of coherent heat fluxes and suppression of incoherent heat fluxes. The enhanced heat transport is associated with the increased coherency of thermal plumes, as a result of the suppression of small-scale turbulent fluctuations by polymers. The incoherent heat flux, arising from turbulent background fluctuations, makes no net contribution to heat transport. The fact that polymer additives can increase the coherency of thermal plumes is supported by the measurements of a number of local quantities, such as the extracted plume amplitude and width, the velocity autocorrelation functions and the velocity–temperature cross-correlation coefficient. The results from local measurements also suggest the existence of a threshold value for the polymer concentration, only above which significant modification of the plume coherent properties and enhancement of the local heat flux can be observed. Estimation of the plume emission rate suggests a stabilization of the thermal boundary layer by polymer additives.

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Multi-point local temperature measurements inside the conducting plates in turbulent thermal convection

Journal of Fluid Mechanics ◽

10.1017/s0022112006003181 ◽

2007 ◽

Vol 570 ◽

pp. 479-489 ◽

Cited By ~ 10

Author(s):

CHAO SUN ◽

KE-QING XIA

Keyword(s):

Thermal Convection ◽

Large Scale ◽

Time Derivative ◽

Temperature Fluctuations ◽

Local Temperature ◽

Maximum Temperature ◽

Convection Cell ◽

Scaling Properties ◽

Plume Emission ◽

Plume Clusters

An experimental study of local temperature statistics in turbulent thermal convection is presented. The emissions of plumes and plume clusters are detected by an array of thermistors embedded in the top and bottom plates of a 1 m diameter convection cell. We found that the product STST′ of the temperature skewness ST and the skewness of the temperature time derivative ST′ from the embedded thermistors may be used as a measure of the intensity of plume emissions and that STST′ exhibits a pattern that corresponds well to the orientation of the large-scale circulation in the convecting flow. This is despite the fact that the temperature distribution across the plates is highly uniform, as indicated by the mean temperature of the embedded thermistors. By comparing the spatial distributions of STST′ and of the RMS temperature σ, we further find that the maximum temperature fluctuations take place in regions dominated by plume mixing instead of regions of plume emission. It is also found that temperature fluctuations inside the conducting plates have the same statistical and scaling properties as those in the cell centre.

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Two-layer thermal convection in miscible viscous fluids

Journal of Fluid Mechanics ◽

10.1017/s0022112098003322 ◽

1999 ◽

Vol 379 ◽

pp. 223-253 ◽

Cited By ~ 76

Author(s):

ANNE DAVAILLE

Keyword(s):

Thermal Convection ◽

Large Scale ◽

Scaling Laws ◽

Three Dimensional ◽

Interfacial Instability ◽

Small Scale ◽

Entrainment Rate ◽

Viscous Layer ◽

Stable Chemical ◽

Large Scale Flow

The influence of a viscosity stratification on the interaction between thermal convection and a stable density discontinuity is studied, using laboratory experiments. Initially, two superposed isothermal layers of high-Prandtl-number miscible fluids are suddenly cooled from above and heated from below. By adjusting the concentrations of salt and cellulose, Rayleigh numbers between 300 and 3×107 were achieved for density contrasts between 0.45 % and 5 % and viscosity ratios between 1 and 6.4×104. Heat and mass transfer through the interface were monitored.Two-layer convection is observed but a steady state is never obtained since penetrative convection occurs. A new interfacial instability is reported, owing to the nonlinear interaction of the unstable thermal and stable chemical density gradients. As a result, the temperature condition at the interface is highly inhomogeneous, driving, on top of the classical small-scale thermal convection, a large-scale flow in each layer which produces cusps at the interface. Entrainment, driven by viscous coupling between the two layers, proceeds through those cusps. The pattern of entrainment is asymmetric: two-dimensional sheets are dragged into the more viscous layer, while three-dimensional conduits are produced in the less viscous layer. A simple entrainment model is proposed and scaling laws for the entrainment rate are derived; they explain the experimental data well.

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Symmetry breaking, anomalous scaling, and large-scale flow generation in a convection cell

Physical Review E ◽

10.1103/physreve.63.035304 ◽

2001 ◽

Vol 63 (3) ◽

Cited By ~ 3

Author(s):

Ananias G. Tomboulides ◽

Victor Yakhot

Keyword(s):

Symmetry Breaking ◽

Large Scale ◽

Convection Cell ◽

Anomalous Scaling ◽

Flow Generation ◽

Large Scale Flow

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Prandtl number effects in convective turbulence

Journal of Fluid Mechanics ◽

10.1017/s0022112098003619 ◽

1999 ◽

Vol 383 ◽

pp. 55-73 ◽

Cited By ~ 98

Author(s):

R. VERZICCO ◽

R. CAMUSSI

Keyword(s):

Heat Transfer ◽

Rayleigh Number ◽

Prandtl Number ◽

Large Scale ◽

Quantitative Agreement ◽

Thermal Plumes ◽

Qualitative And Quantitative ◽

Series Of Experiments ◽

Large Scale Flow ◽

Scale Motion

The effect of Prandtl number on the dynamics of a convective turbulent flow is studied by numerical experiments. In particular, three series of experiments have been performed; in two of them the Rayleigh number spanned about two decades while the Prandtl number was set equal to 0.022 (mercury) and 0.7 (air). In the third series, in contrast, we fixed the Rayleigh number at 6×105 and the Prandtl number was varied from 0.0022 up to 15. The results have shown that, depending on the Prandtl number, there are two distinct flow regimes; in the first (Pr[lsim ]0.35) the flow is dominated by the large-scale recirculation cell that is the most important ‘engine’ for heat transfer. In the second regime, on the other hand, the large-scale flow plays a negligible role in the heat transfer which is mainly transported by the thermal plumes.For the low-Pr regime a model for the heat transfer is derived and the predictions are in qualitative and quantitative agreement with the results of the numerical simulations and of the experiments. All the hypotheses and the consequences of the model are directly checked and all the findings are consistent with the predictions and with experimental observations performed under similar conditions. Finally, in order to stress the effects of the large-scale flow some counter examples are shown in which the large-scale motion is artificially suppressed.

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