Velocity oscillations in turbulent Rayleigh–Bénard convection

Qiu, Xin; Shang, Xiaodong; Tong, Penger; Xia, Ke‐Qing

doi:10.1063/1.1637350

Cited by 78 publications

(98 citation statements)

References 33 publications

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“…[9] Thus the scalar and our vector measurements capture the same physical phenomena. In some cases this frequency was attributed to periodic plume emission [11,26,27,28]. Our results indicate that its origin is the horizontal oscillation of the large-scale flow.…”

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confidence: 63%

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Plume Motion and Large-Scale Circulation in a Cylindrical Rayleigh-Bénard Cell

2004

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We used the time correlation of shadowgraph images to determine the angle Θ of the horizontal component of the plume velocity above (below) the center of the bottom (top) plate of a cylindrical Rayleigh-Bénard cell of aspect ratio Γ ≡ D/L = 1 (D is the diameter and L ≃ 87 mm the height) in the Rayleigh-number range 7 × 10 7 ≤ R ≤ 3 × 10 9 for a Prandtl number σ = 6. We expect that Θ gives the direction of the large-scale circulation. It oscillates time-periodically. Near the top and bottom plates Θ(t) has the same frequency but is anti-correlated.PACS numbers: 44.25.+f,47.27.Te Turbulent Rayleigh-Bénard convection (RBC) is an important process that occurs in the oceans, the atmosphere, the outer layer of the sun, the Earth's mantle, and in many industrial processes.[1] Despite the seeming simplicity of the idealized laboratory experiment, i.e. a fluid between horizontal parallel plates heated from below and cooled from above, our understanding of the physical mechanism of turbulent RBC remains incomplete. [2,3,4] Much of the heat transport is mediated through the emission of hot (cold) plumes of fluid from a thin boundary layer adjacent to the bottom (top) plate, and these plumes are carried by a large-scale circulation [5,6,7,8,9,10,11] known as the "wind of turbulence". For systems with aspect ratio Γ ≡ D/L = O(1) (D is the diameter and L the height of a cylindrical cell) this wind, when time averaged, takes the form of a single convection roll filling the entire cell. The plume interaction with this circulation is a central component of the turbulent RBC problem, and yet only little is known quantitatively about plume motion and the wind. To a large extent we expect that at least the horizontal motion of the plumes is slaved to the wind velocity, since the only independent force acting on the plumes is the buoyancy force in the vertical direction. Here we present a quantitative study of plume motion and thus, by inference, of the wind direction above (below) the center of the bottom (top) plate. The measured horizontal direction oscillates time periodically, and the oscillations persist with a unique frequency over hundreds of cycles. The oscillations at the top and bottom have the same frequency but a phase which is displaced by half a cycle. We conclude that the wind is a significantly more complicated dynamical system than a simple Rayleigh-Bénard convection cell.Numerous measurements of the speed of the fluid and/or of the temperature at distinct points (i.e. of scalar quantities) were made before and revealed a timeperiodic component. [6,7,8,9,10,11] In some instances these results were interpreted as indicative of a timeperiodic plume emission from the plates. Where direct comparison is possible, we find that the measured frequencies agree quantitatively with our determination of the oscillation frequency of the horizontal wind direction. It has been shown [9] that the corresponding oscillation period is commensurate with the period of circulation of the wind. It had been suggested that periodic plu...

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confidence: 63%

“…Alhough this picture was appealing, it was called into question already by measurements of the characteristic frequency and of the amplitude of the oscillations at various places in a cell. [26] Our work focused on the plumes viewed from the top. Our first result is that near the bottom and the top plate, plumes when viewed from above look like sheets.…”

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confidence: 99%

Plume Motion and Large-Scale Circulation in a Cylindrical Rayleigh-Bénard Cell

2004

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“…Here δ decreased essentially to zero and then increased back up close to its average value. We interpret an amplitude drop to also indicate a velocity drop, since the temperature distribution -represented by δ -drives the LSC by buoyancy, and experiments have found a correlation between temperature and velocity [Niemela et al (2001), Qiu et al (2004)]. This means that the LSC gradually slowed to a stop, reversed direction, and gradually sped up again in another direction without significant rotation of the plane of circulation.…”

Section: The Nature Of Rotations and Cessationsmentioning

confidence: 99%

Rotations and cessations of the large-scale circulation in turbulent Rayleigh–Bénard convection

2006

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We present a broad range of measurements of the angular orientation θ 0 (t) of the largescale circulation (LSC) of turbulent Rayleigh-Bénard convection as a function of time. We used two cylindrical samples of different overall sizes, but each with its diameter nearly equal to its height. The fluid was water with a Prandtl number of 4.38. The time series θ 0 (t) consisted of meanderings similar to a diffusive process, but in addition contained large and irregular spontaneous reorientation events through angles ∆θ. We found that reorientations can occur by two distinct mechanisms. One consists of a rotation of the circulation plane without any major reduction of the circulation strength. The other involves a cessation of the circulation, followed by a restart in a randomly chosen new direction. Rotations occurred an order of magnitude more frequently than cessations. Rotations occurred with a monotonically decreasing probability distribution p(∆θ), i.e. there was no dominant value of ∆θ and small ∆θ were more common than large ones. For cessations p(∆θ) was uniform, suggesting that information of θ 0 (t) before the cessation is lost. Both rotations and cessations have Poissonian statistics in time, and can occur at any θ 0 . The average azimuthal rotation rate |θ| increased as the circulation strength of the LSC decreased. Tilting the sample relative to gravity significantly reduced the frequency of occurrence of both rotations and cessations.

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“…The corresponding aspect ratio ͑A = diameter D / height H͒ of the two cells is, respectively, A Ӎ 1 and A Ӎ 0.5. Details about the apparatus have been described elsewhere [16,19], and here we mention only some key points. The sidewall of the cells is made of a transparent Plexiglas ring with wall thickness 0.6 cm.…”

Section: Methodsmentioning

confidence: 99%

Measurements of the local convective heat flux in turbulent Rayleigh-Bénard convection

et al. 2004

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A systematic study of the local convective heat transport in turbulent thermal convection is carried out in small-aspect-ratio cells filled with water. The local convective heat flux is obtained from the simultaneous velocity and temperature measurements over varying Rayleigh numbers and spatial positions across the entire convection cell. Large fluctuations of the local convective heat flux are found mainly in the vertical direction and they are determined primarily by the thermal plumes in the system. The experiment reveals the spatial distribution of the local convective heat flux in a closed cell and thus settles a long-debated issue on how heat is transported in small-aspect-ratio cells.

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Velocity oscillations in turbulent Rayleigh–Bénard convection

Cited by 78 publications

References 33 publications

Plume Motion and Large-Scale Circulation in a Cylindrical Rayleigh-Bénard Cell

Plume Motion and Large-Scale Circulation in a Cylindrical Rayleigh-Bénard Cell

Rotations and cessations of the large-scale circulation in turbulent Rayleigh–Bénard convection

Measurements of the local convective heat flux in turbulent Rayleigh-Bénard convection

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