Large-scale spiral structures in turbulent thermal convection between two vertical plates

We present the results of large-eddy simulations (LES) of turbulent thermal convection generated by surface cooling in a finite-depth stably stratified horizontal layer with an isothermal bottom surface. The flow is a simplified model of turbulent convection occurring in the warm shallow ocean during adverse weather events. Simulations are performed in a 6 × 6 × 1 aspect ratio computational domain using the pseudo-spectral Fourier method in the horizontal plane and finite-difference discretization on a high-resolution clustered grid in the vertical direction. A moderate value of the Reynolds number and two different values of the Richardson number corresponding to a weak initial stratification are considered. A version of the dynamic model is applied as a subgrid-scale (SGS) closure. Its performance is evaluated based on comparison with the results of direct numerical simulations (DNS) and simulations using the Smagorinsky model. Comprehensive study of the spatial structure and statistical properties of the developed turbulent state shows some similarity to Rayleigh–Bénard convection and other types of turbulent thermal convection in horizontal layers, but also reveals distinctive features such as the dominance of a large-scale pattern of descending plumes and strong turbulent fluctuations near the surface.

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Mechanism of large-scale flow reversals in turbulent thermal convection

Science Advances ◽

10.1126/sciadv.aat7480 ◽

2018 ◽

Vol 4 (11) ◽

pp. eaat7480 ◽

Cited By ~ 6

Author(s):

Yin Wang ◽

Pik-Yin Lai ◽

Hao Song ◽

Penger Tong

Keyword(s):

Thermal Convection ◽

Large Scale ◽

Careful Analysis ◽

Extreme Value Statistics ◽

Convection Cell ◽

Heat Accumulation ◽

Thermal Plumes ◽

Minute Amount ◽

Large Scale Flow ◽

Experimental Findings

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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Heat flux enhancement by regular surface roughness in turbulent thermal convection

Journal of Fluid Mechanics ◽

10.1017/jfm.2014.665 ◽

2014 ◽

Vol 763 ◽

pp. 109-135 ◽

Cited By ~ 34

Author(s):

Sebastian Wagner ◽

Olga Shishkina

Keyword(s):

Heat Flux ◽

Surface Roughness ◽

Thermal Convection ◽

Large Scale ◽

Secondary Flows ◽

Scaling Exponent ◽

Wall Roughness ◽

Regular Surface ◽

Large Scale Circulation ◽

Flux Enhancement

AbstractDirect numerical simulations (DNS) of turbulent thermal convection in a box-shaped domain with regular surface roughness at the heated bottom and cooled top surfaces are conducted for Prandtl number $\mathit{Pr}=0.786$ and Rayleigh numbers $\mathit{Ra}$ between $10^{6}$ and $10^{8}$. The surface roughness is introduced by four parallelepiped equidistantly distributed obstacles attached to the bottom plate, and four obstacles located symmetrically at the top plate. By varying $\mathit{Ra}$ and the height and width of the obstacles, we investigate the influence of the regular wall roughness on the turbulent heat transport, measured by the Nusselt number $\mathit{Nu}$. For fixed $\mathit{Ra}$, the change in the value of $\mathit{Nu}$ is determined not only by the covering area of the surface, i.e. the obstacle height, but also by the distance between the obstacles. The heat flux enhancement is found to be largest for wide cavities between the obstacles which can be ‘washed out’ by the flow. This is also manifested in an empirical relation, which is based on the DNS data. We further discuss theoretical limiting cases for very wide and very narrow obstacles and combine them into a simple model for the heat flux enhancement due to the wall roughness, without introducing any free parameters. This model predicts well the general trends and the order of magnitude of the heat flux enhancement obtained in the DNS. In the $\mathit{Nu}$ versus $\mathit{Ra}$ scaling, the obstacles work in two ways: for smaller $\mathit{Ra}$ an increase of the scaling exponent compared to the smooth case is found, which is connected to the heat flux entering the cavities from below. For larger $\mathit{Ra}$ the scaling exponent saturates to the one for smooth plates, which can be understood as a full washing-out of the cavities. The latter is also investigated by considering the strength of the mean secondary flow in the cavities and its relation to the wind (i.e. the large-scale circulation), that develops in the core part of the domain. Generally, an increase in the roughness height leads to stronger flows both in the cavities and in the bulk region, while an increase in the width of the obstacles strengthens only the large-scale circulation of the fluid and weakens the secondary flows. An increase of the Rayleigh number always leads to stronger flows, both in the cavities and in the bulk.

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Large-scale coherent rotation and oscillation in turbulent thermal convection

Physical Review E ◽

10.1103/physreve.61.r6075 ◽

2000 ◽

Vol 61 (6) ◽

pp. R6075-R6078 ◽

Cited By ~ 60

Author(s):

X.-L. Qiu ◽

S. H. Yao ◽

P. Tong

Keyword(s):

Thermal Convection ◽

Large Scale

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Large-scale thermal convection in a horizontal porous layer

Physical Review E ◽

10.1103/physreve.78.027301 ◽

2008 ◽

Vol 78 (2) ◽

Cited By ~ 8

Author(s):

Denis S. Goldobin ◽

Elizaveta V. Shklyaeva

Keyword(s):

Porous Layer ◽

Thermal Convection ◽

Large Scale

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Observations of Galactic Magnetic Fields

Highlights of Astronomy ◽

10.1017/s153929960001474x ◽

2002 ◽

Vol 12 ◽

pp. 712-715 ◽

Cited By ~ 1

Author(s):

Rainer Beck

Keyword(s):

Magnetic Fields ◽

Large Scale ◽

Density Wave ◽

Upstream Region ◽

Galactic Magnetic Fields ◽

Barred Galaxies ◽

Angular Momentum Transport ◽

Central Regions ◽

Spiral Structures ◽

Field Lines

AbstractMagnetic fields are anchored in gas clouds. Field lines are tangled in spiral arms, but highly regularbetweenthe arms. The similarity of pitch angles between gaseous and magnetic arms suggests a coupling between the density wave and the magnetic wave. Observations of large-scale patterns in Faraday rotation favour a dynamo origin of the regular fields. Fields in barred galaxies do not reveal the strong shearing shocks observed in the cold gas, but swing smoothly from the upstream region into the bar. Magnetic fields are important for the dynamics of gas clouds, for the formation of spiral structures, bars and halos, and for mass and angular momentum transport in central regions.

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Numerical simulation of turbulent thermal convection based on LBM

Modern Physics Letters B ◽

10.1142/s0217984921500706 ◽

2020 ◽

pp. 2150070

Author(s):

Yuxian Xia ◽

Yuan Fu ◽

Jiahua Li ◽

Xiang Qiu ◽

Yuehong Qian ◽

...

Keyword(s):

Energy Spectrum ◽

Thermal Convection ◽

Large Scale ◽

Coarse Grained ◽

Field Energy ◽

Small Scale ◽

Temperature Spectrum ◽

Full Field ◽

Scale Range ◽

Boltzmann Method

The two-dimensional (2D) turbulent thermal convection is numerically investigated by using Lattice Boltzmann Method. The 2D turbulence is considered as 2D channel flow where the flow is forced by the arrays of adiabatic cylinders placed in the inlet and wall boundary of 2D channel, which is heated uniformly from the inlet as to inspire the paradigmatic motion of thermal convection. It is found that the spacing vortex number density distribution in the large-scale range [Formula: see text], based on the Liutex vortex definition criterion, which is in fair agreement with the Benzi prediction. The energy spectrum of the Liutex field [Formula: see text]. The scaling behavior of full-field energy spectrum in the large scale is [Formula: see text]. The temperature spectrum in the large-scale range is found to be approximate to [Formula: see text], which is according with the Bolgiano theory of 2D buoyancy driven turbulence. The energy flux cascades to the large scale, the enstrophy cascades to small scale. The moments of the energy dissipation field [Formula: see text] coarse grained at the scale [Formula: see text] have the power-law behaviors with the scale [Formula: see text]. The velocity intermittency measured by PDF exists in large-scale range of 2D turbulent thermal convection. The measured scaling exponents [Formula: see text] are determined by a lognormal formula. The measured intermittency parameter is [Formula: see text], which denotes the strong intermittency in the large-scale range of 2D turbulent thermal convection.

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Plume generation in natural thermal convection at high Rayleigh and Prandtl numbers

Journal of Fluid Mechanics ◽

10.1017/s0022112001003706 ◽

2001 ◽

Vol 434 ◽

pp. 1-21 ◽

Cited By ~ 22

Author(s):

C. LITHGOW-BERTELLONI ◽

M. A. RICHARDS ◽

C. P. CONRAD ◽

R. W. GRIFFITHS

Keyword(s):

Boundary Layer ◽

Thermal Convection ◽

Large Scale ◽

Lower Boundary ◽

Secular Change ◽

Scaling Analysis ◽

Temperature Dependent Viscosity ◽

Earth’S Mantle ◽

Dependent Viscosity ◽

Rayleigh Numbers

We study natural thermal convection of a fluid (corn syrup) with a large Prandtl number (103–107) and temperature-dependent viscosity. The experimental tank (1 × 1 × 0.3m) is heated from below with insulating top and side boundaries, so that the fluid experiences secular heating as experiments proceed. This setup allows a focused study of thermal plumes from the bottom boundary layer over a range of Rayleigh numbers relevant to convective plumes in the deep interior of the Earth's mantle. The effective value of Ra, based on the viscosity of the fluid at the interior temperature, varies from 105 at the beginning to almost 108 toward the end of the experiments. Thermals (plumes) from the lower boundary layer are trailed by continuous conduits with long residence times. Plumes dominate flow in the tank, although there is a weaker large-scale circulation induced by material cooling at the imperfectly insulating top and sidewalls. At large Ra convection is extremely time-dependent and exhibits episodic bursts of plumes, separated by periods of quiescence. This bursting behaviour probably results from the inability of the structure of the thermal boundary layer and its instabilities to keep pace with the rate of secular change in the value of Ra. The frequency of plumes increases and their size decreases with increasing Ra, and we characterize these changes via in situ thermocouple measurements, shadowgraph videos, and videos of liquid crystal films recorded during several experiments. A scaling analysis predicts observed changes in plume head and tail radii with increasing Ra. Since inertial effects are largely absent no transition to ‘hard’ thermal turbulence is observed, in contrast to a previous conclusion from numerical calculations at similar Rayleigh numbers. We suggest that bursting behaviour similar to that observed may occur in the Earth's mantle as it undergoes secular cooling on the billion-year time scale.

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