scholarly journals Numerical simulation of helical-vortex effects in Rayleigh-Bénard convection

2006 ◽  
Vol 13 (2) ◽  
pp. 205-222 ◽  
Author(s):  
G. V. Levina ◽  
I. A. Burylov
Keyword(s):  
Large Scale ◽  
Internal Heat ◽  
Main Idea ◽  
Numerical Approach ◽  
Small Scale ◽  
Forcing Function ◽  
Bénard Convection ◽  
Benard Convection ◽  
Rayleigh Benard Convection ◽  
Rayleigh Benard

Abstract. A numerical approach is substantiated for searching for the large-scale alpha-like instability in thermoconvective turbulence. The main idea of the search strategy is the application of a forcing function which can have a physical interpretation. The forcing simulates the influence of small-scale helical turbulence generated in a rotating fluid with internal heat sources and is applied to naturally induced fully developed convective flows. The strategy is tested using the Rayleigh-Bénard convection in an extended horizontal layer of incompressible fluid heated from below. The most important finding is an enlargement of the typical horizontal scale of the forming helical convective structures accompanied by a cells merging, an essential increase in the kinetic energy of flows and intensification of heat transfer. The results of modeling allow explaining how the helical feedback can work providing the non-zero mean helicity generation and the mutual intensification of horizontal and vertical circulation, and demonstrate how the energy of the additional helical source can be effectively converted into the energy of intensive large-scale vortex flow.

Torsional oscillations of the large-scale circulation in turbulent Rayleigh–Bénard convection

2008 ◽  
Vol 607 ◽  
pp. 119-139 ◽  
Author(s):  
DENIS FUNFSCHILLING ◽  
ERIC BROWN ◽  
GUENTER AHLERS
Keyword(s):  
Rayleigh Number ◽  
Aspect Ratio ◽  
Large Scale ◽  
Small Scale ◽  
Large Scale Circulation ◽  
Number Range ◽  
Bénard Convection ◽  
Benard Convection ◽  
Rayleigh Benard Convection ◽  
Rayleigh Benard

Measurements over the Rayleigh-number range 108 ≲ R ≲ 1011 and Prandtl-number range 4.4≲σ≲29 that determine the torsional nature and amplitude of the oscillatory mode of the large-scale circulation (LSC) of turbulent Rayleigh–Bénard convection are presented. For cylindrical samples of aspect ratio Γ=1 the mode consists of an azimuthal twist of the near-vertical LSC circulation plane, with the top and bottom halves of the plane oscillating out of phase by half a cycle. The data for Γ=1 and σ=4.4 showed that the oscillation amplitude varied irregularly in time, yielding a Gaussian probability distribution centred at zero for the displacement angle. This result can be described well by the equation of motion of a stochastically driven damped harmonic oscillator. It suggests that the existence of the oscillations is a consequence of the stochastic driving by the small-scale turbulent background fluctuations of the system, rather than a consequence of a Hopf bifurcation of the deterministic system. The power spectrum of the LSC orientation had a peak at finite frequency with a quality factor Q≃5, nearly independent of R. For samples with Γ≥2 we did not find this mode, but there remained a characteristic periodic signal that was detectable in the area density ρp of the plumes above the bottom-plate centre. Measurements of ρp revealed a strong dependence on the Rayleigh number R, and on the aspect ratio Γ that could be represented by ρp ~ Γ2.7±0.3. Movies are available with the online version of the paper.

The large-scale flow structure in turbulent rotating Rayleigh–Bénard convection

2011 ◽  
Vol 688 ◽  
pp. 461-492 ◽  
Author(s):  
Stephan Weiss ◽  
Guenter Ahlers
Keyword(s):  
Large Scale ◽  
Working Fluid ◽  
Flow Mode ◽  
Unexpected Result ◽  
Vertical Sidewall ◽  
Number Range ◽  
Bénard Convection ◽  
Benard Convection ◽  
Rayleigh Benard Convection ◽  
Rayleigh Benard

AbstractWe report on the influence of rotation about a vertical axis on the large-scale circulation (LSC) of turbulent Rayleigh–Bénard convection in a cylindrical vessel with aspect ratio $\Gamma \equiv D/ L= 0. 50$ (where $D$ is the diameter and $L$ the height of the sample). The working fluid is water at an average temperature ${T}_{av} = 40{~}^{\ensuremath{\circ} } \mathrm{C} $ with a Prandtl number $\mathit{Pr}= 4. 38$. For rotation rates $\Omega \lesssim 1~\mathrm{rad} ~{\mathrm{s} }^{\ensuremath{-} 1} $, corresponding to inverse Rossby numbers $1/ \mathit{Ro}$ between 0 and 20, we investigated the temperature distribution at the sidewall and from it deduced properties of the LSC. The work covered the Rayleigh-number range $2. 3\ensuremath{\times} 1{0}^{9} \lesssim \mathit{Ra}\lesssim 7. 2\ensuremath{\times} 1{0}^{10} $. We measured the vertical sidewall temperature gradient, the dynamics of the LSC and flow-mode transitions from single-roll states (SRSs) to double-roll states (DRSs). We found that modest rotation stabilizes the SRSs. For modest $1/ \mathit{Ro}\lesssim 1$ we found the unexpected result that the vertical LSC plane rotated in the prograde direction (i.e. faster than the sample chamber), with the rotation at the horizontal midplane faster than near the top and bottom. This differential rotation led to disruptive events called half-turns, where the plane of the top or bottom section of the LSC underwent a rotation through an angle of $2\lrm{\pi} $ relative to the main portion of the LSC. The signature of the LSC persisted even for large $1/ \mathit{Ro}$ where Ekman vortices are expected. We consider the possibility that this signature actually is generated by a two-vortex state rather than by a LSC. Whenever possible, we compare our results with those for a $\Gamma = 1$ sample by Zhong & Ahlers (J. Fluid Mech., vol. 665, 2010, pp. 300–333).

scholarly journals Temperature profiles measurements in turbulent Rayleigh-Bénard convection by optical fibre system at the Barrel of II-menau

2018 ◽  
Vol 180 ◽  
pp. 02020
Author(s):  
Jakub Drahotský ◽  
Pavel Hanzelka ◽  
Věra Musilová ◽  
Michal Macek ◽  
Ronald du Puits ◽  
...  
Keyword(s):  
Turbulent Flows ◽  
Optical Fibre ◽  
Large Scale ◽  
Temperature Profiles ◽  
Vertical Temperature ◽  
Thin Glass ◽  
Bénard Convection ◽  
Benard Convection ◽  
Rayleigh Benard Convection ◽  
Rayleigh Benard

Modelling of large-scale natural (thermally-generated) turbulent flows (such as the turbulent convection in Earth’s atmosphere, oceans, or Sun) is approached in laboratory experiments in the simplified model system called the Rayleigh-Bénard convection (RBC). We present preliminary measurements of vertical temperature profiles in the cell with the height of 4:7 m, 7:15m in diameter, obtained at the Barrel of Ilmenau (BOI), the worldwide largest experimental setup to study highly turbulent RBC, newly equipped with the Luna ODiSI-B optical fibre system. In our configuration, the system permits to measure the temperature with a high spatial resolution of 5mm along a very thin glass optical fibre with the length of 5m and seems to be perfectly suited for measurement of time series of instantaneous vertical temperature profiles. The system was supplemented with the two Pt100 vertically movable probes specially designed by us for reference temperature profiles measurements.

Flow reversals in Rayleigh–Bénard convection with non-Oberbeck–Boussinesq effects

2016 ◽  
Vol 798 ◽  
pp. 628-642 ◽  
Author(s):  
Shu-Ning Xia ◽  
Zhen-Hua Wan ◽  
Shuang Liu ◽  
Qi Wang ◽  
De-Jun Sun
Keyword(s):  
Large Scale ◽  
Mass Conservation ◽  
Working Fluid ◽  
Large Temperature ◽  
Cold Wall ◽  
Bénard Convection ◽  
Benard Convection ◽  
Rayleigh Benard Convection ◽  
Flow Reversals ◽  
Rayleigh Benard

Flow reversals in two-dimensional Rayleigh–Bénard convection led by non-Oberbeck–Boussinesq (NOB) effects due to large temperature differences are studied by direct numerical simulation. Perfect gas is chosen as the working fluid and the Prandtl number is 0.71 for the reference state. If NOB effects are included, the flow pattern $P_{11}$ with only one dominant roll often becomes unstable by the growth of the cold corner roll, which sometimes results in cession-led flow reversals. By exploiting the vorticity transport equation, it is found that the asymmetries of buoyancy and viscous forces are responsible for the growth of the cold corner roll because both such asymmetries cause an imbalance between the corner rolls and the large-scale circulation (LSC). The buoyancy force near the cold wall increases and decreases near the hot wall originating from the temperature-dependent isobaric thermal expansion coefficient ${\it\alpha}=1/T$ if NOB effects are included. Moreover, the decreased dissipation due to lower viscosity is favourable for the growth of the cold corner roll, while the increased viscosity further suppresses the growth of the hot corner roll. Finally, it is found that the boundary layer near the cold wall plays an important role in the mass transport from LSC to corner rolls subject to mass conservation.

scholarly journals Mixed thermal conditions in convection: how do continents affect the mantle’s circulation?

2017 ◽  
Vol 822 ◽  
pp. 1-4 ◽  
Author(s):  
R. Ostilla-Mónico
Keyword(s):  
Boundary Conditions ◽  
Plate Tectonics ◽  
Large Scale ◽  
Thermal Conditions ◽  
Experimental Proof ◽  
Bénard Convection ◽  
Benard Convection ◽  
Large Scale Flow ◽  
Rayleigh Benard Convection ◽  
Rayleigh Benard

Natural convection is omnipresent on Earth. A basic and well-studied model for it is Rayleigh–Bénard convection, the fluid flow in a layer heated from below and cooled from above. Most explorations of Rayleigh–Bénard convection focus on spatially uniform, perfectly conducting thermal boundary conditions, but many important geophysical phenomena are characterized by boundary conditions which are a mixture of conducting and adiabatic materials. For example, the differences in thermal conductivity between continental and oceanic lithospheres are believed to play an important role in plate tectonics. To study this, Wang et al. (J. Fluid Mech., vol. 817, 2017, R1), measure the effect of mixed adiabatic–conducting boundary conditions on turbulent Rayleigh–Bénard convection, finding experimental proof that even if the total heat transfer is primarily affected by the adiabatic fraction, the arrangement of adiabatic and conducting plates is crucial in determining the large-scale flow dynamics.

scholarly journals Convection due to an unstable density difference across a permeable membrane

2008 ◽  
Vol 609 ◽  
pp. 139-170 ◽  
Author(s):  
BABURAJ A. PUTHENVEETTIL ◽  
JAYWANT H. ARAKERI
Keyword(s):  
Large Scale ◽  
Dendritic Structure ◽  
Membrane Dynamics ◽  
Diffusion Regime ◽  
Permeable Membrane ◽  
Bénard Convection ◽  
Benard Convection ◽  
The Mean ◽  
Rayleigh Benard Convection ◽  
Rayleigh Benard

We study natural convection driven by unstable concentration differences of sodium chloride (NaCl) across a horizontal permeable membrane at Rayleigh numbers (Ra) of 1010 to 1011 and Schmidt number (Sc)=600. A layer of brine lies over a layer of distilled water, separated by the membrane, in square-cross-section tanks. The membrane is permeable enough to allow a small flow across it at higher driving potentials. Based on the predominant mode of transport across the membrane, three regimes of convection, namely an advection regime, a diffusion regime and a combined regime, are identified. The near-membrane flow in all the regimes consists of sheet plumes formed from the unstable layers of fluid near the membrane. In the advection regime observed at higher concentration differences (ΔC) across the membrane, there is a slow overturning through-flow across the membrane; the transport across the membrane occurs mostly by advection. This phenomenology explains the observed Nub~Ra2/Sc scaling of the Nusselt number. The planforms of sheet plumes near the membrane show a dendritic structure due to the combined influence of the mean shear due to the large-scale flow and the entrainment flow of the adjacent plumes. The near-membrane dynamics show initiation, elongation and merger of plumes; a movie is available with the online version of the paper. Increase in Ra results in a larger number of closely and regularly spaced sheet plumes. The mean plume spacing in the advection regime $\overline{\lambda}_b$, is larger than the mean plume spacing in Rayleigh–Bénard convection ($\overline{\lambda}$), and shows a different Ra-dependence. The plume spacings in the advection regime (λb) show a common log-normal probability density function at all Ra. We propose a phenomenology which predicts $\overline{\lambda}_b$ ~ $\sqrt{Z_w Z_{V_i}}$, where Zw and $Z_{V_i}$ are, respectively, the near-wall length scales in Rayleigh–Bénard convection (RBC) and due to the advection velocity. In the combined regime, which occurs at intermediate values of ΔC, the flux scales as (ΔC/2)4/3. At lower driving potentials, in the diffusion regime, the flux scaling is similar to that in turbulent RBC.

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