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Keqing Xia - One of the best experts on this subject based on the ideXlab platform.

  • temperature fluctuation profiles in turbulent thermal Convection a logarithmic dependence versus a power law dependence
    Physical Review Letters, 2019
    Co-Authors: Keqing Xia
    Abstract:

    We report an experimental measurement of the rms temperature (σ_{T}) profiles in two regions inside a large aspect ratio (Γ=4.2) rectangular Convection Cell. It is found that, in the region where the boundary layer is sheared by a large-scale wind, σ_{T} has a power-law dependence on the vertical distance (z) from the plate, whereas in the region where plumes are abundant, σ_{T} has a logarithmic dependence on z. The power-law profile may be understood by balancing the inertia force and the viscous force in the equations of motion, and the logarithmic profile may be understood in terms of the balance between the buoyancy and the inertia forces. When normalized by a convective temperature scale, θ_{*}, the profiles of σ_{T} collapse onto a single curve for different values of the Rayleigh number. This shows that the convective temperature first proposed by Deardorff is the suitable temperature scale outside the thermal boundary layer for both logarithmic and power-law profiles. Our finding suggests a strong connection between plumes and the logarithmic rms temperature profile. The present Letter reveals that multiple force balance mechanisms can coexist in the bulk of highly turbulent flows.

  • confinement induced heat transport enhancement in turbulent thermal Convection
    Physical Review Letters, 2013
    Co-Authors: Shidi Huang, Matthias Kaczorowski, Keqing Xia
    Abstract:

    We report an experimental and numerical study of the effect of spatial confinement in turbulent thermal Convection. It is found that when the width of the Convection Cell is narrowed, the heat-transfer efficiency increases significantly despite the fact that the overall flow is slowed down by the increased drag force from the sidewalls. Detailed experimental and numerical studies show that this enhancement is brought about by the changes in the dynamics and morphology of the thermal plumes in the boundary layers and in the large-scale flow structures in the bulk. It is found that the confined geometry produces more coherent and energetic hot and cold plume clusters that go up and down in random locations, resulting in more uniform and thinner thermal boundary layers. The study demonstrates how changes in turbulent bulk flow can influence the boundary layer dynamics and shows that the prevalent mode of heat transfer existing in larger aspect ratio Convection Cells, in which hot and cold thermal plumes are carried by the large-scale circulation along opposite sides of the sidewall, is not the most efficient way for heat transport.

  • local dissipation scales and integral scale reynolds number scalings in thermally driven turbulence
    Journal of Physics: Conference Series, 2011
    Co-Authors: Keqing Xia, Quan Zhou
    Abstract:

    We present direct multi-point velocity measurements of the two-dimensional velocity fields in a cylindrical Rayleigh-Benard Convection Cell using the particle image velocimetry (PIV) technique over the Rayleigh number range 5.9 × 109 Ra 1.1 × 1011. The longitudinal integral length scale of the horizontal and vertical velocity fields are obtained at the Cell center, near the Cell sidewall, and near the bottom plate of the Cell, respectively. In addition, the Reynolds number based on these scales, ReLx and ReLz, are obtained. It is found that all measured ReL scales as ReL ~ Raβ, with the exponent β 0.5, except ReLx for the horizontal velocity at the Cell center, which has a β 0.75. The local dissipation scale field η at the three different places are also studied. Our results reveal two types of universality of η. The first one is that, for the same flow, the probability density functions (PDF) of η are insensitive to turbulent intensity and large-scale inhomogeneity and anisotropy of the system. The second is that the small-scale dissipation dynamics in buoyancy-driven turbulence can be described by the same models developed for homogeneous and isotropic turbulence. However, the exact functional form of the PDF of the local dissipation scale is not universal with respect to different types of flows, but depends on the integral-scale velocity boundary condition, which is found to have an exponential, not Gaussian, distribution in turbulent Rayleigh-Benard Convection.

  • horizontal structures of velocity and temperature boundary layers in two dimensional numerical turbulent rayleigh benard Convection
    Physics of Fluids, 2011
    Co-Authors: Quan Zhou, Richard J A M Stevens, Detlef Lohse, Kazuyasu Sugiyama, Siegfried Grossmann, Keqing Xia
    Abstract:

    We investigate the structures of the near-plate velocity and temperature profiles at different horizontal positions along the conducting bottom (and top) plate of a Rayleigh-Benard Convection Cell, using two-dimensional (2D) numerical data obtained at the Rayleigh number Ra = 108 and the Prandtl number Pr = 4.4 of an Oberbeck-Boussinesq flow with constant material parameters. The results show that most of the time, and for both velocity and temperature, the instantaneous profiles scaled by the dynamical frame method [Q. Zhou and K.-Q. Xia, “Measured instantaneous viscous boundary layer in turbulent Rayleigh-Benard Convection,” Phys. Rev. Lett. 104, 104301 (2010)] agree well with the classical Prandtl-Blasius laminar boundary layer (BL) profiles. Therefore, when averaging in the dynamical reference frames, which fluctuate with the respective instantaneous kinematic and thermal BL thicknesses, the obtained mean velocity and temperature profiles are also of Prandtl-Blasius type for nearly all horizontal positions. We further show that in certain situations the traditional definitions based on the time-averaged profiles can lead to unphysical BL thicknesses, while the dynamical method also in such cases can provide a well-defined BL thickness for both the kinematic and the thermal BLs

  • local energy dissipation rate balances local heat flux in the center of turbulent thermal Convection
    Physical Review Letters, 2011
    Co-Authors: Shidi Huang, Keqing Xia
    Abstract:

    The local kinetic energy dissipation rateu;c in Rayleigh-Benard Convection Cell was measured experimentally using the particle tracking velocimetry method, with varying Rayleigh number Ra, Prandtl number Pr, and Cell height H. It is found thatu;c=ð� 3 H � 4 Þ¼ 1:05 � 10 � 4 Ra 1:55� 0:02 Pr 1:15� 0:38 . TheRaand H dependenciesofthemeasuredresultsarefoundtobeconsistentwiththeassumptionmadefor the bulk energy dissipation rateu;bulk in the Grossmann-Lohse model. A remarkable finding of the study is thatu;c balancesthedirectlymeasuredlocalNusseltnumber Nuc intheCellcenter,notonlyscalingwisebut also in magnitude.

Penger Tong - One of the best experts on this subject based on the ideXlab platform.

  • turbulent temperature fluctuations in a closed rayleigh benard Convection Cell
    Journal of Fluid Mechanics, 2019
    Co-Authors: Yin Wang, Penger Tong
    Abstract:

    We report a systematic study of spatial variations of the probability density function (PDF) .

  • statistics of the locally averaged thermal dissipation rate in turbulent rayleigh benard Convection
    Journal of Turbulence, 2010
    Co-Authors: Penger Tong, Emily S C Ching
    Abstract:

    From the measured thermal dissipation rate in turbulent Rayleigh–Benard Convection in a cylindrical Cell, we construct a locally averaged thermal dissipation rate χ fτ by averaging over a time interval τ. We study how the statistical moments ⟨(χ fτ) p ⟩ depend on τ at various locations along the vertical axis of the Convection Cell. We find that ⟨(χ fτ) p ⟩ exhibits good scaling in τ, of about a decade long, with scaling exponents μ(p) for p = 1–6. For Rayleigh number (Ra) around 8×109, the scaling range is 1.4–21 s at the Cell center and 4–21 s at the bottom plate. The dissipative and turnover times are about 0.8 s and 35 s respectively, while the timescale corresponding to the local Bolgiano scale is estimated to be about 31 s at the Cell center and 3.5 s at the bottom plate. On the basis of several assumptions, we derive theoretical predictions for μ(p) at the different locations. The measured values of μ(p) are presented and shown to be in good agreement with our theoretical predictions.

  • extraction of plumes in turbulent thermal Convection
    Physical Review Letters, 2004
    Co-Authors: Emily S C Ching, Penger Tong, Xiaodong Shang, Hong Guo, Keqing Xia
    Abstract:

    We present a scheme to extract the velocity of buoyant structures in turbulent thermal Convection from simultaneous local velocity and temperature measurements. Applying this scheme to measurements taken at positions within the Convection Cell where the buoyant structures are dominated by plumes, we obtain the temperature dependence of the plume velocity and understand our results using the equations of motion. We further obtain the scaling behavior of the average local heat flux in the vertical direction at the Cell center with the Rayleigh number and find that the scaling exponent is different from that measured for the Nusselt number. This difference leads to the conclusion that heat cannot be mainly transported through the central region of the Convection Cell.

  • velocity oscillations in turbulent rayleigh benard Convection
    Physics of Fluids, 2004
    Co-Authors: Xinliang Qiu, Penger Tong, Xiaodong Shang, Keqing Xia
    Abstract:

    A systematic study of velocity oscillations in turbulent thermal Convection is carried out in small aspect-ratio Cells filled with water. Local velocity fluctuations and temperature-velocity cross-correlation functions are measured over varying Rayleigh numbers and spatial positions across the entire Convection Cell. These structural measurements reveal how the thermal plumes interact with the bulk fluid in a closed Cell and provide an interesting physical picture for the dynamics of the temperature and velocity oscillations in turbulent Convection.

  • large scale velocity structures in turbulent thermal Convection
    Physical Review E, 2001
    Co-Authors: Xinliang Qiu, Penger Tong
    Abstract:

    A systematic study of large-scale velocity structures in turbulent thermal Convection is carried out in three different aspect-ratio Cells filled with water. Laser Doppler velocimetry is used to measure the velocity profiles and statistics over varying Rayleigh numbers Ra and at various spatial positions across the whole Convection Cell. Large velocity fluctuations are found both in the central region and near the Cell boundary. Despite the large velocity fluctuations, the flow field still maintains a large-scale quasi-two-dimensional structure, which rotates in a coherent manner. This coherent single-roll structure scales with Ra and can be divided into three regions in the rotation plane: (1) a thin viscous boundary layer, (2) a fully mixed central core region with a constant mean velocity gradient, and (3) an intermediate plume-dominated buffer region. The experiment reveals a unique driving mechanism for the large-scale coherent rotation in turbulent Convection.

Jorg Schumacher - One of the best experts on this subject based on the ideXlab platform.

  • on the challenges for reliable measurements of Convection in large aspect ratio rayleigh benard Cells in air and sulfur hexafluoride
    Experimental Thermal and Fluid Science, 2019
    Co-Authors: Christian Cierpka, Christian Kastner, Christian Resagk, Jorg Schumacher
    Abstract:

    Abstract Heat transfer through natural turbulent thermal Convection is an important mechanism in geophysics and many engineering problems. A canonical well-known experiment is the Rayleigh-Benard (RB) Cell, which consists of an enclosure with adiabatic side walls uniformly heated from below and cooled from above. The direct correlation between the local momentum and heat transfer is currently under investigation, especially for large aspect ratios. In this situation the dominant large-scale circulation roll, as typically found for low aspect ratios, is substituted by a whole pattern of long-living rolls. Nowadays, commonly advanced optical methods are applied to investigate velocity fields, which makes a transparent heating or cooling plate necessary. For the current study glass plates were coated with a thin metal oxide layer enabling heating via the Joule effect. However, since the temperature homogeneity is crucial for the setup, two heating plates had to be combined to enable a homogeneous temperature distribution at the bottom of the Convection Cell. Additional difficulties arise from the large aspect ratio and thus the large observation angle. This may cause optical aberrations and systematic errors due to the perspective bias. Furthermore, the observation duration has to be in the order of minutes to hours to resolve the dynamics of the slowly evolving large-scale flow. This results in additional challenges for the seeding generation. To prove the reliability of the experimental approach, a Convection Cell with an aspect ratio Γ = l / h = 10 was placed in the SCALEX (Scaled Convective Airflow Laboratory Experiment) facility. This facility enables experiments with sulfur-hexafluoride (SF6) and air under pressures of up to 10 bar to achieve very high Rayleigh numbers. Here, results of measurements for a Rayleigh number of 5 × 10 5 in SF6 at ambient pressure and in air at a pressure of 5.7 bar will be discussed and show the applicability of this approach. Stereoscopic particle image velocimetry (PIV) in horizontal planes covering the whole cross section of the Cell ( 300 × 300 mm 2 ) was used for the estimation of all three components of the velocity vector. The optical access for the laser light sheet was provided by transparent sidewalls. The aim of the current study was to prove the possibility of reliable PIV measurements with reproducible homogenous temperature boundary conditions in the SCALEX facility. Issues of the temperature distribution at the heating plate, tracer particles, illumination and data evaluation will therefore be addressed in greater detail.

  • assessment of horizontal velocity fields in square thermal Convection Cells with large aspect ratio
    Experiments in Fluids, 2018
    Co-Authors: Christian Kastner, Christian Cierpka, Christian Resagk, Jasper Westphalen, Manuela Junghähnel, Jorg Schumacher
    Abstract:

    Transparent heating plates, consisting of glass coated with a transparent conductive metal oxide, are applied in large aspect ratio turbulent Rayleigh–Benard Convection (RBC) to investigate the large-scale patterns of velocity fields with optical flow measurement techniques across the whole horizontal cross section. The square Convection Cell with an aspect ratio Γ = L/h = 10 was tested inside the scaled convective airflow laboratory experiment (SCALEX) facility which enables experiments with gases as working fluids for pressures of up to 10 bar to achieve very high Rayleigh numbers Ra. For the current study, Ra = 2 × 104 was applied. The velocity fields are measured with 2D3C particle image velocimetry (PIV). The possibility of reliable PIV measurements with reproducible homogenous temperature boundary conditions was demonstrated in the SCALEX facility. The seeding of the tracer particles, their illumination and data evaluation are addressed in detail. The final comparison of experimental data and numerical simulations shows a good agreement for the probability density functions of the horizontal velocity components. Deviations for the vertical out-of-plane velocity component and their dependence on the thickness of the laser sheet are discussed in detail and quantified by measurements with light sheets of different thickness.

  • role of critical points of the skin friction field in formation of plumes in thermal Convection
    Physical Review E, 2015
    Co-Authors: Vinodh Bandaru, Anastasiya Kolchinskaya, Kathrin Padberggehle, Jorg Schumacher
    Abstract:

    The dynamics in the thin boundary layers of temperature and velocity is the key to a deeper understanding of turbulent transport of heat and momentum in thermal Convection. The velocity gradient at the hot and cold plates of a Rayleigh-Benard Convection Cell forms the two-dimensional skin friction field and is related to the formation of thermal plumes in the respective boundary layers. Our analysis is based on a direct numerical simulation of Rayleigh-Benard Convection in a closed cylindrical Cell of aspect ratio Γ=1 and focused on the critical points of the skin friction field. We identify triplets of critical points, which are composed of two unstable nodes and a saddle between them, as the characteristic building block of the skin friction field. Isolated triplets as well as networks of triplets are detected. The majority of the ridges of linelike thermal plumes coincide with the unstable manifolds of the saddles. From a dynamical Lagrangian perspective, thermal plumes are formed together with an attractive hyperbolic Lagrangian coherent structure of the skin friction field. We also discuss the differences from the skin friction field in turbulent channel flows from the perspective of the Poincare-Hopf index theorem for two-dimensional vector fields.

  • lagrangian tracer dynamics in a closed cylindrical turbulent Convection Cell
    Physical Review E, 2010
    Co-Authors: Mohammad S Emran, Jorg Schumacher
    Abstract:

    Turbulent Rayleigh-Benard Convection in a closed cylindrical Cell is studied in the Lagrangian frame of reference with the help of three-dimensional direct numerical simulations. The aspect ratio of the Cell r is varied between 1 and 12, and the Rayleigh number Ra between 10 7 and 10 9 . The Prandtl number Pr is fixed at 0.7. It is found that both the pair dispersion of the Lagrangian tracer particles and the statistics of the acceleration components measured along the particle trajectories depend on the aspect ratio for a fixed Rayleigh number for the parameter range covered in our studies. This suggests that large-scale circulations present in the Convection Cell affect the Lagrangian dynamics. Our findings are in qualitative agreement with existing Lagrangian laboratory experiments on turbulent Convection.

Shengqi Zhou - One of the best experts on this subject based on the ideXlab platform.

  • measured oscillations of the velocity and temperature fields in turbulent rayleigh benard Convection in a rectangular Cell
    Physical Review E, 2007
    Co-Authors: Shengqi Zhou, Chao Sun, Keqing Xia
    Abstract:

    Temperature and velocity oscillations have been found in a rectangular Rayleigh-Benard Convection Cell, in which one large-scale Convection roll exists. At Rayleigh number Ra=8.9X 10 11 and Prandtl number Pr=4, temperature oscillations can be observed in most parts of the system and the oscillation period remains almost constant, t T =74±2 s. Velocity oscillation can only be found in its horizontal component ν y (perpendicular to the large-scale circulation plane) near the Cell sidewall, its oscillation period is also constant, t υ =65±2 s, at these positions. Temperature and velocity oscillations have different Ra dependences, which are, respectively, indicated by the Peclect number Pe T =0.55Ra 0.47 and Pe υ =0.28Ra 0.50 . In comparison to the case of a cylindrical Cell, we find that velocity oscillations are affected by the system geometry.

  • particle image velocimetry measurement of the velocity field in turbulent thermal Convection
    Physical Review E, 2003
    Co-Authors: Keqing Xia, Chao Sun, Shengqi Zhou
    Abstract:

    The spatial structure of the velocity field in turbulent Rayleigh-Benard Convection in water has been measured using the particle image velocimetry technique, with the Rayleigh number Ra varying from 9 x 10(8) to 9 x 10(11) and the Prandtl number remaining approximately constant (Pr approximately 4). The study provides a direct confirmation that a rotatory mean wind indeed persists for the highest value of Ra reached in the experiment. The measurement reveals that the mean flow in the central region of the Convection Cell is of the shape of a coherent elliptical rotating core for Ra below 1 x 10(10). Above this Ra, the orientation of the elliptical core changes by a 90 degrees angle and an inner core rotating at a lower rate inside the original bulk core emerges. It is further found that the rotation frequencies of the inner core and the outer shell have distinct scalings with Ra; the scaling exponent for the outer-shell is 0.5 and it is 0.4 for the inner core. From the measured rms and skewness distributions of the velocity field, we find that velocity fluctuations at the Cell center are neither homogeneous nor isotropic. The turbulent energy production fields further reveal that the mean wind is not driven by turbulent fluctuations associated with Reynolds stress.

  • prandtl number dependence of the viscous boundary layer and the reynolds numbers in rayleigh benard Convection
    Physical Review E, 2002
    Co-Authors: Siu Lam, Shengqi Zhou, Xiaodong Shang, Keqing Xia
    Abstract:

    We report results from high Prandtl number turbulent thermal Convection experiments. The viscous boundary layer and the Reynolds number are measured in four different fluids over wide ranges of the Prandtl number Pr and the Rayleigh number Ra, all in a single Convection Cell of unity aspect ratio. We find that the normalized viscous layer thickness may be represented as ${\ensuremath{\delta}}_{v}/L=0.65{\mathrm{Pr}}^{0.24}{\mathrm{Ra}}^{\ensuremath{-}0.16}.$ The Reynolds number based on the oscillation frequency of the large-scale flow is found as ${\mathrm{Re}}_{o}(\mathrm{R}\mathrm{a},\mathrm{P}\mathrm{r})=1.1{\mathrm{Ra}}^{0.43}{\mathrm{Pr}}^{\ensuremath{-}0.76}$ and that based on the rms velocity ${\mathrm{Re}}_{\mathrm{rms}}(\mathrm{R}\mathrm{a},\mathrm{P}\mathrm{r})=0.84{\mathrm{Ra}}^{0.40}{\mathrm{Pr}}^{\ensuremath{-}0.86}.$ Both the Ra and the Pr exponents of ${\mathrm{Re}}_{{V}_{m}}(\mathrm{R}\mathrm{a},\mathrm{P}\mathrm{r})$ based on the maximum velocity of the circulating wind appear to vary across the range of Pr covered, changing from 0.5 to 0.68 and $\ensuremath{-}0.88$ to $\ensuremath{-}0.95,$ respectively, as Pr is increased from 6 to 1027.

  • heat flux measurement in high prandtl number turbulent rayleigh benard Convection
    Physical Review Letters, 2002
    Co-Authors: Keqing Xia, Siu Lam, Shengqi Zhou
    Abstract:

    We report Nusselt number measurements from high Prandtl number turbulent thermal Convection experiments. The experiments are conducted in four fluids with the Prandtl number Pr varying from 4 to 1350 and the Rayleigh number Ra from 2x10(7) to 3x10(10), all in a single Convection Cell of unity aspect ratio. We find that the measured Nusselt number decreased about 20% over the range of Pr spanned in the experiment. The measure data are also found in good agreement with the prediction of a recent theory over the extended range of Pr covered in the experiment.

Xinliang Qiu - One of the best experts on this subject based on the ideXlab platform.

  • velocity oscillations in turbulent rayleigh benard Convection
    Physics of Fluids, 2004
    Co-Authors: Xinliang Qiu, Penger Tong, Xiaodong Shang, Keqing Xia
    Abstract:

    A systematic study of velocity oscillations in turbulent thermal Convection is carried out in small aspect-ratio Cells filled with water. Local velocity fluctuations and temperature-velocity cross-correlation functions are measured over varying Rayleigh numbers and spatial positions across the entire Convection Cell. These structural measurements reveal how the thermal plumes interact with the bulk fluid in a closed Cell and provide an interesting physical picture for the dynamics of the temperature and velocity oscillations in turbulent Convection.

  • large scale velocity structures in turbulent thermal Convection
    Physical Review E, 2001
    Co-Authors: Xinliang Qiu, Penger Tong
    Abstract:

    A systematic study of large-scale velocity structures in turbulent thermal Convection is carried out in three different aspect-ratio Cells filled with water. Laser Doppler velocimetry is used to measure the velocity profiles and statistics over varying Rayleigh numbers Ra and at various spatial positions across the whole Convection Cell. Large velocity fluctuations are found both in the central region and near the Cell boundary. Despite the large velocity fluctuations, the flow field still maintains a large-scale quasi-two-dimensional structure, which rotates in a coherent manner. This coherent single-roll structure scales with Ra and can be divided into three regions in the rotation plane: (1) a thin viscous boundary layer, (2) a fully mixed central core region with a constant mean velocity gradient, and (3) an intermediate plume-dominated buffer region. The experiment reveals a unique driving mechanism for the large-scale coherent rotation in turbulent Convection.

  • viscous boundary layers at the sidewall of a Convection Cell
    Physical Review E, 1998
    Co-Authors: Xinliang Qiu, Keqing Xia
    Abstract:

    large temperature gradients exist. This in turn signifies the interplay between the temperature and the velocity fields at the horizontal plates. The length scale associated with the root-mean-square velocity is found to be analogous to the thickness of the wall layer, while the length associated with the mean velocity is analogous to the thickness of the boundary layer, in conventional wall-bounded shear flows. Our measurements also reveal that the large-scale circulation is quite uniform over a large region on the sidewall plate. @S1063-651X~98!06807-X#