Guenter Ahlers

Guenter Ahlers
University of California, Santa Barbara | UCSB · Department of Physics

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Publications

Publications (335)
Article
Full-text available
While the heat transfer and the flow dynamics in a cylindrical Rayleigh-Bénard (RB) cell are rather independent of the aspect ratio Γ (diameter/height) for large Γ, a small-Γ cell considerably stabilizes the flow and thus affects the heat transfer. Here, we first theoretically and numerically show that the critical Rayleigh number for the onset of...
Article
We report measurements of the temperature frequency spectra $P(\,f, z, r)$ , the variance $\sigma ^2(z,r)$ and the Nusselt number $Nu$ in turbulent Rayleigh–Bénard convection (RBC) over the Rayleigh number range $4\times 10^{11} \underset{\smash{\scriptscriptstyle\thicksim}} { and for a Prandtl number $Pr \simeq ~0.8$ ( $z$ is the vertical distance...
Article
We propose a theoretical model for spatial variations of the temperature variance σ2(z,r) (z is the distance from the sample bottom and r the radial coordinate) in turbulent Rayleigh-Bénard convection (RBC). Adapting the “attached-eddy” model of shear flow to the plumes of RBC, we derived an equation for σ2 which is based on the universal scaling o...
Article
DOI:https://doi.org/10.1103/PhysRevLett.124.229402
Article
Full-text available
For rapidly rotating turbulent Rayleigh--B\'enard convection in a slender cylindrical cell, experiments and direct numerical simulations reveal a boundary zonal flow (BZF) that replaces the classical large-scale circulation. The BZF is located near the vertical side wall and enables enhanced heat transport there. Although the azimuthal velocity of...
Article
Full-text available
In Rayleigh–Bénard convection experiments, the thermal coupling between the sidewall and fluid is unavoidable. As a result, the thermal properties of the sidewall can influence the flow structure that develops. To get a better understanding of the influence of the sidewall, we performed a one-to-one comparison between experiments and direct numeric...
Preprint
Full-text available
For rapidly rotating turbulent Rayleigh--B\'enard convection in a slender cylindrical cell, experiments and direct numerical simulations reveal a boundary zonal flow (BZF) that replaces the classical large-scale circulation. The BZF is located near the vertical side wall and enables enhanced heat transport there. Although the azimuthal velocity of...
Article
We critically analyse the different ways to evaluate the dependence of the Nusselt number ( $\mathit{Nu}$ ) on the Rayleigh number ( $\mathit{Ra}$ ) in measurements of the heat transport in turbulent Rayleigh–Bénard convection under general non-Oberbeck–Boussinesq conditions and show the sensitivity of this dependence to the choice of the reference...
Article
Recently Schumacher et al. [Phys. Rev. Fluids 1, 084402 (2016)] used direct numerical simulation to calculate the shear stress exerted on the top and bottom viscous boundary layers (BLs) of Rayleigh-Bénard convection with a Prandtl number Pr=0.021 and aspect ration Γ=1 for Rayleigh numbers Ra up to 4×108. By extrapolating their results to larger Ra...
Article
We present new Nusselt-number (Nu) measurements for slowly rotating turbulent thermal convection in cylindrical samples with aspect ratio Γ=1.00 and provide a comprehensive correlation of all available data for that Γ. In the experiment compressed gasses (nitrogen and sulfur hexafluride) as well as the fluorocarbon C6F14 (3M Fluorinert FC72) and is...
Article
We present measurements of the orientation $\theta_0$ and temperature amplitude $\delta$ of the large-scale circulation in a cylindrical sample of turbulent Rayleigh-Benard convection (RBC) with aspect ratio $\Gamma \equiv D/L = 1.00$ ($D$ and $L$ are the diameter and height respectively) and for the Prandtl number $Pr \simeq 0.8$. Results for $\th...
Article
Full-text available
We report results of Reynolds-number measurements, based on multi-point temperature measurements and the elliptic approximation (EA) of He and Zhang (2006 Phys. Rev. E 73 055303), Zhao and He (2009 Phys. Rev. E 79 046316) for turbulent Rayleigh–Bénard convection (RBC) over the Rayleigh-number range 1011 ≲ Ra ≲ 2 × 1014 and for a Prandtl number Pr ≃...
Article
Full-text available
Sometimes it is thought that sharp transitions between potentially different turbulent states should be washed out by the prevailing intense fluctuations and short coherence lengths and times. Contrary to this expectation, we found a sequence of such transitions in turbulent rotating Rayleigh-Bénard convection as the rotation rate was increased. Th...
Article
Full-text available
We report measurements of logarithmic temperature profiles Θ(z, r) = A(r) × ln(z/L) + B(r) in the bulk of turbulent Rayleigh–Bénard convection (here Θ is a scaled and time-averaged local temperature in the fluid, z is the vertical and r the radial position, and L is the sample height). Two samples had aspect ratios Γ ≡ D/L = 1.00 and 0.50 (where D...
Article
Full-text available
We report on experimental determinations of the temperature field in the interior (bulk) of turbulent Rayleigh-Benard convection for a cylindrical sample with aspect ratio (diameter over height) of 0.50, both in the classical and in the ultimate state. The Prandtl number was close to 0.8. We find a "logarithmic layer" in which the temperature varie...
Article
Full-text available
We report measurements of the temperature variance $\sigma^2(z,r)$ and frequency power spectrum $P(f,z,r)$ ($z$ is the distance from the sample bottom and $r$ the radial coordinate) in turbulent Rayleigh-B\'enard convection (RBC) for Rayleigh numbers $\textrm{Ra} = 1.6\times10^{13}$ and $1.1\times10^{15}$ and for a Prandtl number $\textrm{Pr} \sime...
Article
We report on turbulent Rayleigh-Bénard convection of a nematic liquid crystal while it undergoes a transition from the nematic to the isotropic phase in a cylindrical convection cell with a height equal to twice the diameter (aspect ratio Γ = 0.50). The difference between the top and bottom plate temperature ΔT=T b -T t was held constant, while the...
Article
We present experimental results for the Reynolds number Re U based on the horizontal mean-flow velocity U and for Re V based on the root-mean-square horizontal fluctuation velocity V for turbulent Rayleigh-Bénard convection in a cylindrical sample of aspect ratio Γ=10·9 over the Prandtl number range 0·18≤Pr≤0·88. The results were derived from space...
Article
Full-text available
A Comment on the Letter by P. Urban, P. Hanzelka, T. Kralik, V. Musilova, A. Srnka, and L. Skrbek, Phys. Rev. Lett. 109, 154301 (2012). The authors of the Letter offer a Reply.
Article
Full-text available
We report on near-turbulent thermal convection of a nematic liquid crystal heated from below in a cylindrical cell with an aspect ratio (diameter/height) equal to 0.50 for Rayleigh numbers 2 × 107 ≤ Ra ≤ 3 × 108 and a Prandtl number of about 355. The Nusselt number Nu as a function of Ra did not differ significantly from that of an isotropic fluid....
Article
Full-text available
We report on the experimental results for heat-transport measurements, in the form of the Nusselt number Nu, by turbulent Rayleigh–Bénard convection (RBC) in a cylindrical sample of aspect ratio Γ ≡ D/L = 0.50 (D = 1.12 m is the diameter and L = 2.24 m the height). The measurements were made using sulfur hexafluoride at pressures up to 19 bar as th...
Article
Full-text available
We report experimental results for heat-transport measurements, in the form of the Nusselt number Nu, by turbulent Rayleigh–Bénard convection (RBC) in a cylindrical sample of aspect ratio Γ ≡ D/L = 1.00 (D = 1.12 m is the diameter and L = 1.12 m the height) and compare them with previously reported results for Γ = 0.50. The measurements were made u...
Article
Full-text available
We report measurements of properties of turbulent thermal convection of a fluid with a Prandtl number $\Pra=4.38$ in a cylindrical cell with an aspect ratio $\Gamma=0.50$. The rotational symmetry was broken by a small tilt of the sample axis relative to gravity. Measurements of the heat transport (as expressed by the Nusselt number \Nu), as well as...
Article
Full-text available
We report results for the temperature profiles of turbulent Rayleigh-Bénard convection (RBC) in the interior of a cylindrical sample of aspect ratio Γ ≡ D/L = 0.50 (D and L are the diameter and height respectively). Results from experiment over the Rayleigh number range 4×10 12 < ∼ Ra < ∼ 10 15 for a Prandtl number Pr ≃ 0.8 and from direct numerica...
Article
We report experimental results for heat-transport measurements by turbulent Rayleigh-B\'enard convection in a cylindrical sample of aspect ratio $\Gamma \equiv D/L = 1.00$ ($D = 1.12$ m is the diameter and $L = 1.12$ m the height). They are for the Rayleigh-number range $4\times10^{11} \alt \Ra \alt 2\times10^{14}$ and for Prandtl numbers \Pra\ bet...
Article
We report experimental results for heat-transport measurements, in the form of the Nusselt number \Nu, by turbulent Rayleigh-B\'enard convection in a cylindrical sample of aspect ratio $\Gamma \equiv D/L = 0.50$ ($D = 1.12$ m is the diameter and $L = 2.24$ m the height). The measurements were made using sulfur hexafluoride at pressures up to 19 bar...
Article
Full-text available
Using compressed gases with Prandtl numbers near 0.7, we obtained flow visualizations of turbulent Rayleigh-Bénard convection in a cylindrical sample with an aspect ratio Γ≡D/L≅10 (D is the diameter and L the height) by the shadowgraph method. Focusing on the plumes under the top plate, we found that their length had a log-normal distribution, sugg...
Article
Full-text available
Measurements of the Nusselt number Nu and of a Reynolds number Re(eff) for Rayleigh-Bénard convection (RBC) over the Rayleigh-number range 10(12)≲Ra≲10(15) and for Prandtl numbers Pr near 0.8 are presented. The aspect ratio Γ≡D/L of a cylindrical sample was 0.50. For Ra≲10(13) the data yielded Nu∝Ra(γ(eff)) with γ(eff)≃0.31 and Re(eff)∝Ra(ζ(eff)) w...
Article
Full-text available
In turbulent rotating Rayleigh-Bénard convection Ekman vortices extract hot or cold fluid from thermal boundary layers near the bottom or top plate and enhance the Nusselt number. It is known from experiments and direct numerical simulation on cylindrical samples with aspect ratio Γ ≡ D/L (D is the diameter and L the height) that the enhancement oc...
Article
Full-text available
We report experimental results for the heat transport, as expressed by the Nusselt number Nu, by turbulent Rayleigh-Bénard convection in a cylindrical sample of aspect ratio Γ = D/L = 0.50 (D = 1.12 m is the diameter and L = 2.24 m the height). The measurements are for the Rayleigh-number range 1012 ≲ Ra ≲ 1015 and for a Prandtl number Pr ≃ 0.86. A...
Article
Full-text available
We 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 (where is the diameter and the height of the sample). The working fluid is water at an average temperature with a Prandtl number . For rotation rates , corresponding to i...
Article
We report spatial statistics of thermal plumes based on an analysis of shadowgraph images for a cylindrical Rayleigh-B'enard sample of height L = 9.5 mm and aspect ratio γ= 10.6. The fluids consisted of pure gases with Prandtl- numbers Pr 0.7 and gas mixtures with 0.17 <=Pr < 0.7. The Rayleigh-number range was 3x10^5 <=Ra <=5x10^6. We found the plu...
Article
While studying Rayleigh-B'enard convection (RBC) of Newtonian fluids is a challenging and interesting topic on its own, we want to expand this field by including anisotropic fluids such as liquid crystals. In these liquids certain fluid properties, such as for example the heat conductivity λ and the diamagnetic susceptibility χ, depend on the avera...
Article
We present experimental results for Rayleigh-Bénard convection with rotation about a vertical axis at a Prandtl number sigma=0.17. The working fluid is a 50-50 gas mixture of H_2-Xe at a pressure of 16.2 bar with a separation ratio of 0.21 and a Lewis number of 1.2. The dimensionless rotation rate Omega is varied from 0 to 120. Linear stability ana...
Article
Full-text available
We report on the influence of rotation about a vertical axis on heat transport by turbulent Rayleigh–Bénard convection in a cylindrical vessel with an aspect ratio ( is the diameter and the height of the sample) and compare the results with those for larger . The working fluid was water at where the Prandtl number is 4.38. For rotation rates , corr...
Article
Full-text available
Measurements of the Nusselt number and properties of the large-scale circulation (LSC) are presented for turbulent Rayleigh–Bénard convection in water-filled cylindrical containers (Prandtl number Pr = 4.38) with aspect ratio Γ = 0.50. They cover the range 2 × 108 ≲ Ra ≲ 1 × 1011 of the Rayleigh number Ra. We confirm the occurrence of a double-roll...
Article
Full-text available
In the original paper by Ahlers et al ( 2009 New J. Phys. 11 123001) a transition was reported at a Rayleigh number Ra = Ra* similar or equal to 4x10(13). For Ra > Ra*, the Nusselt number Nu rose less rapidly with increasing Ra than it did below Ra*. In a note added in proof the authors reported the subsequent discovery of a coexisting additional `...
Conference Paper
Measurements of a Reynolds number Re for Rayleigh-B'enard convection (RBC) of a cylindrical sample over the Rayleigh-number range 2x10^12 <=Ra <=2x10^15 and the Prandtl-number range 0.79 <=Pr <=0.86 are presented. The aspect ratio γ≡D/L was 0.50 (D= 1.12 m was the diameter and L = 2.24 m was the height). We used the elliptic approximation of He and...
Conference Paper
Measurements of the Nusselt number Nu for Rayleigh-B'enard convection (RBC) of a cylindrical sample over the Rayleigh-number range 2x10^12 <=Ra <=2x10^15 and the Prandtl-number range 0.79 <=Pr <=0.86 are presented. The aspect ratio γ≡D/L was 0.50 (D= 1.12 m was the diameter and L = 2.24 m was the height). For Ra <=2x10^13 the data yielded Nu = Nu0R...
Article
Full-text available
Measurements of the Nusselt number Nu and of properties of the large-scale circulation (LSC) for turbulent Rayleigh–Bénard convection are presented in the presence of rotation about a vertical axis at angular speeds 0 ≤ Ω ≲ 2 rad s−1. The sample chamber was cylindrical with a height equal to the diameter, and the fluid contained in it was water. Th...
Article
We report Nusselt-number measurements for a cylindrical Rayleigh-B'enard sample of height L = 49.6 cm and aspect ratio gamma= 0.497 that were made using three pure gases: helium (Prandtl number Pr=0.67), nitrogen (Pr=0.73), and argon (Pr=0.67-0.70) at pressures up to 47 bars. They cover the Rayleigh number range 9x10^6 < Ra < 2x10^11. The uncorrect...
Article
Full-text available
In turbulent thermal convection in cylindrical samples with an aspect ratio $\Gamma \equiv D/L$ ($D$ is the diameter and $L$ the height), the Nusselt number Nu is enhanced when the sample is rotated about its vertical axis because of the formation of Ekman vortices that extract additional fluid out of thermal boundary layers at the top and bottom....
Chapter
After a brief review in the introduction of the major breakthroughs in the study of Rayleigh-Bénard convection (RBC) since the experiments of Henri Bénard, a few selected topics are presented in more detail. The effect of on the bifurcation to convection is discussed because experimental work on this is quite recent and as yet incomplete. Examples...
Article
We determined the critical Rayleigh numbers Ra{c} for the onset of convection in cylindrical containers with aspect ratios 1 approximately <Gamma [ triple bond] D/L approximately <9 ( D is the diameter and L the height) and the patterns that form just above Ra{c}, both from experiment and by direct numerical simulation (DNS). Results for Ra{c} agre...
Article
Measurements of heat transport, as expressed by the Nusselt number Nu, are reported for turbulent Rayleigh-B\'enard convection of water containing up to 120 ppm by weight of poly-[ethylene oxide] with a molecular weight of $4\ifmmode\times\else\texttimes\fi{}{10}^{6}\text{ }\text{ }\mathrm{g}/\mathrm{mole}$. Over the Rayleigh number range $5\ifmmod...
Article
Full-text available
Measurements of fluctuations and convection patterns in horizontal layers of fluid heated from below and near the onset of Rayleigh–Bénard convection (RBC) are reported under conditions where the fluid properties vary strongly over the temperature range ΔT = Tb − Tt (Tb and Tt are the temperatures at the bottom and top of the sample, respectively)....
Article
Full-text available
For the Rayleigh-number range 10(7) less than or similar to Ra less than or similar to 10(11) we report measurements of the Nusselt number Nu and of properties of the large-scale circulation (LSC) for cylindrical samples of helium gas (Prandtl number Pr = 0.674) that have aspect ratio Gamma equivalent to D/L = 0.50 (D and L are the diameter and the...
Article
Full-text available
We describe a pressure vessel for conducting experiments in helium (He), air, nitrogen (N2) or sulfur hexafluoride (SF6) under pressures of up to 19 bars, and facilities for the study of Rayleigh-Bénard convection inside this pressure vessel. The convection cells, known as the high pressure convection facilities (HPCFs), can have interior heights u...
Article
In agreement with a recent experimental discovery by Xi et al. (Phys. Rev. Lett., vol. 102, 2009, paper no. 044503), we also find a sloshing mode in experiments on the large-scale circulation (LSC) of turbulent Rayleigh–Bénard convection in a cylindrical sample of aspect ratio one. The sloshing mode has the same frequency as the torsional oscillati...
Article
Measurements of the Nusselt number Nu will be reported for turbulent Rayleigh-B'enard convection of a cylindrical sample. They cover the Rayleigh-number range 10^11 Ra 2x10^15 using N2 (Pr = 0.72) and SF6 (Pr = 0.79 to 0.84) at pressures up to 19 bars and near-ambient temperatures. The sample had a height L=2.24m and diameter D = 1.12m and utilized...
Article
Full-text available
Measurements of the Nusselt number Nu and of temperature variations DeltaTb in the bulk fluid are reported for turbulent Rayleigh-Bénard convection of a cylindrical sample. They cover the Rayleigh-number range 10(9) less than or similar to Ra less than or similar to 3x10(14) using He (Prandtl number Pr=0.67), N2 (Pr=0.72) and SF6 (Pr=0.79 to 0.84)...
Article
Full-text available
Weakly rotating turbulent Rayleigh-Bénard convection was studied experimentally and numerically. With increasing rotation and large enough Rayleigh number a supercritical bifurcation from a turbulent state with nearly rotation-independent heat transport to another with enhanced heat transfer is observed at a critical inverse Rossby number 1/Roc app...
Article
Full-text available
Measurements of the Nusselt number $Nu$ and of temperature variations $\Delta T_b$ in the bulk fluid are reported for turbulent Rayleigh-B\'enard convection of a cylindrical sample. They cover the Rayleigh-number range $10^{9} \alt Ra \alt 3\times 10^{14}$ using He (Prandtl number $Pr = 0.67$), N$_2$ ($Pr = 0.72$) and SF$_6$ ($Pr = 0.79$ to 0.84) a...
Article
Full-text available
We report measurements of turbulent heat transport in samples of ethane (C2H6) heated from below while the applied temperature difference DeltaT straddled the liquid-vapor coexistence curve T(phi)(P). When the sample top temperature T(t) decreased below T(phi), droplet condensation occurred and the latent heat of vaporization H provided an addition...
Article
Full-text available
The progress in our understanding of several aspects of turbulent Rayleigh-Benard convection is reviewed. The focus is on the question of how the Nusselt number and the Reynolds number depend on the Rayleigh number Ra and the Prandtl number Pr, and on how the thicknesses of the thermal and the kinetic boundary layers scale with Ra and Pr. Non-Oberb...
Article
In agreement with a recent experimental discovery by Xia et. al. (2009), we also find a sloshing mode in experiments on the large-scale circulation (LSC) of turbulent Rayleigh-Benard convection in a cylindrical sample of aspect ratio one. The sloshing mode has the same frequency as the torsional oscillation discovered by Funfschilling and Ahlers (2...
Article
Previously we published a dynamical model (E. Brown and G. Ahlers, Phys. Fluids, 20, 075101 (2008)) for the large-scale-circulation (LSC) dynamics of Rayleigh-Benard convection in cylindrical containers. The model consists of a pair of stochastic ordinary differential equations, motivated by the Navier-Stokes equations, one each for the strength de...
Article
Full-text available
Experimental and numerical data for the heat transfer as a function of the Rayleigh, Prandtl, and Rossby numbers in turbulent rotating Rayleigh-Bénard convection are presented. For relatively small Ra approximately 10(8) and large Pr modest rotation can enhance the heat transfer by up to 30%. At larger Ra there is less heat-transfer enhancement, an...
Chapter
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Stevens, Richard J. A. M. Zhong, Jin-Qiang Clercx, Herman J. H. Verzicco, Roberto Lohse, Detlef Ahlers, Guenter
Chapter
Turbulent convection in a fluid heated from below and cooled from above is an important process in many geo- and astro-physical processes with typical Rayleigh numbers RaH1020. It has been anticipated that the convective heat transport changes fundamentally for Ra = Ra*, where Ra* is a function of Prandtl number Pr. Below Ra* heat transport is expe...
Conference Paper
Full-text available
For given aspect ratio and given geometry, the nature of Rayleigh Benard convection (RBC) is determined by the Rayleigh number \(Ra = \beta g \Delta L^3 / (\kappa \nu)\) and by the Prandtl number \(Pr = \nu / \kappa\) is the thermal expansion coefficient, g the gravitational acceleration \(\Delta = T_b - T_t\) the difference between the imposed tem...
Article
Experimental measurements of properties of the large-scale circulation (LSC) in turbulent convection of a fluid heated from below in a cylindrical container of aspect ratio one are presented and used to test a model of diffusion in a potential well for the LSC. The model consists of a pair of stochastic ordinary differential equations motivated by...
Article
Measurements over the Rayleigh-number range 108 R 1011 and Prandtl-number range 4.4B=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 =4.4 showed that the oscillation amplitude varied irregularly in time, yielding...
Article
Full-text available
As shown in earlier work [Ahlers, J. Fluid Mech. 569, 409 (2006)], non-Oberbeck-Boussinesq (NOB) corrections to the center temperature in turbulent Rayleigh-Bénard convection in water and also in glycerol are governed by the temperature dependences of the kinematic viscosity and the thermal diffusion coefficient. If the working fluid is ethane clos...
Article
Measurements of the large-scale circulation (LSC) of turbulent Rayleigh-B'enard convection (RBC) in cylindrical samples of aspect ratio gamma= 1 (= diameter/height) are reported. They covered the Rayleigh-number range 9 x10^5 < R < 2 x10^9 and the Prandtl-number range 4.4 < sigma< 29. Using observations of the mid-plane temperature-profile around t...
Article
Measurements of oscillatory modes of the large-scale circulation (LSC) of turbulent Rayleigh-B'enard convection in water-filled cylindrical containers of equal height and diameter are presented. To observe and distinguish different modes, temperatures were measured simultaneously around the side wall at 8 azimuthal angles and 3 heights. A previousl...
Article
We present new measurements of the specific heat C P of 4He confined in cylindrical micro-channels of diameter L=1.89 μm at saturated vapor pressure and near the bulk superfluid-transition temperature T λ . The results, when combined with the specific-heat exponent α=−0.01264, and with the correlation-length exponent ν=0.6709 obtained from α by h...
Article
Full-text available
This paper reports measurements of Reynolds numbers Rep corresponding to the turnover time of thermal excitations ('plumes') and Re? corresponding to the twisting-oscillation period of the large-scale circulation (LSC) of turbulent Rayleigh?B?nard convection over the Rayleigh-number range and Prandtl-number range for cylindrical samples of aspect r...
Article
We report temperature measurements for a cylindrical sample of turbulent Rayleigh-Bénard convection (RBC) at points in the interior, well away from the thermal boundary layers near the top and bottom of the sample. The aspect ratio was equal to 1.00 and the Prandtl number σ was equal to 4.4 or 5.5. The data are in the range 5×107<R<1010, where R is...