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Claus Wagner - One of the best experts on this subject based on the ideXlab platform.
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analysis of the Thermal plumes in turbulent rayleigh benard convection based on well resolved numerical simulations
Journal of Fluid Mechanics, 2009Co-Authors: Matthias Kaczorowski, Claus WagnerAbstract:For the study presented DNS and high-resolved LES of turbulent Rayleigh-Benard convection were conducted with fluid of Pr=0.7 in a long rectangular cell of aspect ratio unity in the cross-section and periodic boundaries in a horizontal longitudinal direction. The analysis of the Thermal and kinetic energy spectra suggests that temperature and velocity fields are correlated within the Thermal boundary layers and tend to be uncorrelated in the core region of the flow. A tendency of decorrelation of the temperature and velocity field is also observed for increasing Ra when the flow has become fully turbulent, which is thought to characterise this regime. This argument is also supported by the analysis of the correlation of the turbulent fluctuations |u|' and θ'. The plume and mixing layer dominated region is found to be separated from the Thermal Dissipation rates of the bulk and conductive sublayer by the inflection points of the PDF. In order to analyse the contributions of bulk, boundary layers and plumes to the mean heat transfer, the Thermal Dissipation rate PDFs of four different Ra are integrated over these three regions. Hence it is shown that the core region is dominated by the turbulent fluctuations of the Thermal Dissipation rate throughout the range of simulated Ra, whereas the contributions from the conductive sublayer due to turbulent fluctuations increase rapidly with Ra. The latter contradicts results by He et al. (2007). The results also show that the plumes and mixing layers are increasingly dominated by the mean gradient contributions. The PDFs of the core region are compared to an analytical scaling law for passive scalar turbulence which is found to be in good agreement with the results of the present study. It is noted that the core region scaling seems to approach the behaviour of a passive scalar as Ra increases, i.e. it changes from pure exponential to a stretched exponential scaling.
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analysis of sheet like Thermal plumes in turbulent rayleigh benard convection
Journal of Fluid Mechanics, 2008Co-Authors: Olga Shishkina, Claus WagnerAbstract:Sheet-like Thermal plumes are investigated using time-dependent and three-dimensional flow fields obtained from direct numerical simulations and well-resolved large-eddy simulations of turbulent Rayleigh-Benard convection in water (Prandtl number Pr = 5.4) in a cylindrical container with the aspect ratio Γ = 1 and for the Rayleigh numbers Ra = 2 x 10 9 and 2 x 10 10 . To analyse quantitatively the physical properties of the sheet-like Thermal plumes and the turbulent background and to obtain the temperature threshold which separates these two different flow regions, the temperature dependences of the conditionally averaged local heat flux, Thermal Dissipation rate and selected components of the velocity and vorticity fields are studied. It is shown that the sheet-like plumes are characterized by high values of the local heat flux and relatively large absolute values of the vertical components of the vorticity and velocity fields. The borders of these plumes are indicated by large values of the Thermal Dissipation rate and large absolute values of the horizontal vorticity components. In contrast to the sheet-like Thermal plumes, the turbulent background is characterized by low values of the Thermal Dissipation rate, local heat flux and vertical vorticity component. The highest values of the local heat flux and the highest absolute values of the vertical vorticity component are found in the regions where the sheet-like plumes strike against each other. Fluid swirling at these places forms the stems of the mushroom-like Thermal plumes which develop in the bulk of the Rayleigh-Benard cell. Further, formulae to calculate the curvature, thickness and length of the plumes are introduced. Geometrical properties such as plume area, diameter, curvature, thickness and aspect ratio together with the physical properties of the sheet-like plumes such as temperature, heat flux, Thermal Dissipation rate, velocity and vorticity are investigated.
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analysis of Thermal Dissipation rates in turbulent rayleigh benard convection
Journal of Fluid Mechanics, 2005Co-Authors: Olga Shishkina, Claus WagnerAbstract:Direct numerical simulations (DNS) of turbulent Rayleigh–Benard convection in a wide cylindrical container (aspect ratio are defined to determine quantitatively the role of the turbulent background. Evaluating these functions from the DNS data, it is shown that the turbulent background pushes the Thermal plumes back and that its contribution to the volume-averaged Thermal Dissipation rate increases with the Rayleigh number. Further, it is proven analytically that the ratio of the area-averaged (over the top or the bottom plates) to the volume-averaged Thermal Dissipation rate is greater than or equal to the Nusselt number for all aspect ratios, and Prandtl and Rayleigh numbers.
Alain Rocheteau - One of the best experts on this subject based on the ideXlab platform.
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Transient Thermal Dissipation method of xylem sap flow measurement: Multi-species calibration and field evaluation
Tree Physiology, 2010Co-Authors: S. Isarangkool Na Ayutthaya, F. C. Do, K. Pannengpetch, J. Junjittakarn, Jean-luc Maeght, Alain Rocheteau, HervÉ CochardAbstract:The transient Thermal Dissipation (TTD) method developed by Do and Rocheteau (2002b) is a close evolution of the original constant Thermal Dissipation (CTD) method of Granier (1985). The TTD method has the advantage of limiting the influence of passive natural temperature gradients and of yielding more stable zero-flux references at night. By analogy with the CTD method, the transient method was first calibrated on synthetic porous material (sawdust) on the assumption that the relationship was independent of the woody species. Here, our concern was to test the latter hypothesis with a 10-min heating time in three tropical species: Hevea brasiliensis Müll. Arg., Mangifera indica L. and Citrus maxima Merr. A complementary objective was to compare the field estimates of daily transpiration for mature rubber trees with estimates based on a simplified soil water balance in the dry season. The calibration experiments were carried out in the laboratory on cut stems using an HPFM device and gravimetric control of water flow up to 5 L dm(-2) h(-1). Nineteen response curves were assessed on fully conductive xylem, combining 11 cut stems and two probes. The field evaluation comprised five periods from November 2007 to February 2008. Estimates of daily transpiration from the measurement of sap flow were based on the 41 sensors set up on 11 trees. Soil water depletion was monitored by neutron probe and 12 access tubes to a depth of 1.8 m. The calibrations confirmed that the response of the transient Thermal index to flow density was independent of the woody species that were tested. The best fit was a simple linear response (R(2) = 0.88, n = 276 and P < 0.0001). The previous calibration performed by Do and Rocheteau (2002b) on sawdust fell within the variability of the multi-species calibration; however, there were substantial differences with the average curve at extreme flow rates. Field comparison with soil water depletion in the dry season validated to a reasonable extent the absolute estimates of transpiration acquired with the 10-min TTD method. In conclusion, evidence for the independence of calibration from woody species and the simple linear response of the Thermal index strengthen the interest of the TTD method with 10-min heating.
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influence of natural temperature gradients on measurements of xylem sap flow with Thermal Dissipation probes 2 advantages and calibration of a noncontinuous heating system
Tree Physiology, 2002Co-Authors: F Do, Alain RocheteauAbstract:: Natural temperature gradients in stems of trees growing in open stands give rise to errors when measuring sap flow by the continuous Thermal Dissipation method. Previously, we obtained evidence from field measurements that a noncontinuous Thermal Dissipation system can prevent these errors (Do and Rocheteau 2002). Cyclic heating (i.e., 45 minutes on and 15 minutes off; 45/15) allowed the derivation of an alternate signal, defined as the difference between the temperature signal at the end of the heating period and the temperature signal at the end of the cooling period. By analogy with the continuous system, we calculated an alternate flow index. Analysis in an artificial flow system confirmed that external temperature gradients have an additive effect on the continuous signal, whereas the alternate signal is unaffected by such gradients. The response of the alternate flow index to flow densities was similar for five combinations of heating and cooling times (45/15, 40/20, 30/30, 15/15 and 10/10 min). The relationship was markedly different from Granier's (1985) calibration because measurements in cyclic systems are made under non-steady-state temperature conditions. We recommend the 15/15 min cycle, which allows two sap flow measurements per hour. We compared flow density estimates obtained from field measurements with the continuous and cyclic systems over 192 days with 1-day lags between systems. Comparisons based on daily maximum values (between 0.5 and 2.5 l dm(-2) h(-1) for the cyclic system) confirmed that differences between the continuous and cyclic systems, which could be greater than 100%, were linked to the effect of temperature gradients on the continuous system. The results demonstrate that, in situations where significant natural temperature gradients (i.e., > 0.2 degrees C) are likely, the cyclic system improves the accuracy of sap flow measurements made with Thermal Dissipation probes.
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influence of natural temperature gradients on measurements of xylem sap flow with Thermal Dissipation probes 1 field observations and possible remedies
Tree Physiology, 2002Co-Authors: F Do, Alain RocheteauAbstract:: The Thermal Dissipation method is simple and widely used for measuring sap flow in large stems. As with several other Thermal methods, natural temperature gradients are assumed to be negligible in the sapwood being measured. We studied the magnitude and variability of natural temperature gradients in sapwood of Acacia trees growing in the Sahelian zone of Senegal, analyzed their effects on sap flow measurements, and investigated possible solutions. A new measurement approach employing cyclic heating (45 minutes of heating and 15 minutes of cooling; 45/15) was also tested. Three-day measurement sequences that included 1 day without heating, a second day with continuous heating and a third day with cyclic heating were recorded during a 6.5-month period using probes installed at three azimuths in a tree trunk. Natural temperature gradients between the two probes of the sensor unit, spaced 8 to 10 cm vertically, were rarely negligible (i.e., < 0.2 degrees C): they were positive during the night and negative during the day, with an amplitude ranging from 0.3 to 3.5 degrees C depending on trunk azimuth, day and season. These temperature gradients had a direct influence on the signal from the continuously heated sensors, inducing fluctuations in the nighttime reference signal. The resulting errors in sap flow estimates can be greater than 100%. Correction protocols have been proposed in previous studies, but they were unsuitable because of the high spatial and temporal variability of the natural temperature gradients. We found that a measurement signal derived from a noncontinuous heating system could be an attractive solution because it appears to be independent of natural temperature gradients. The magnitude and variability of temperature gradients that we observed were likely exacerbated by the combination of open stand, high solar radiation and low sap flow rate. However, for all applications of the Thermal Dissipation method, it is wise to check regularly for natural temperature gradients by switching off the heater.
Olga Shishkina - One of the best experts on this subject based on the ideXlab platform.
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analysis of sheet like Thermal plumes in turbulent rayleigh benard convection
Journal of Fluid Mechanics, 2008Co-Authors: Olga Shishkina, Claus WagnerAbstract:Sheet-like Thermal plumes are investigated using time-dependent and three-dimensional flow fields obtained from direct numerical simulations and well-resolved large-eddy simulations of turbulent Rayleigh-Benard convection in water (Prandtl number Pr = 5.4) in a cylindrical container with the aspect ratio Γ = 1 and for the Rayleigh numbers Ra = 2 x 10 9 and 2 x 10 10 . To analyse quantitatively the physical properties of the sheet-like Thermal plumes and the turbulent background and to obtain the temperature threshold which separates these two different flow regions, the temperature dependences of the conditionally averaged local heat flux, Thermal Dissipation rate and selected components of the velocity and vorticity fields are studied. It is shown that the sheet-like plumes are characterized by high values of the local heat flux and relatively large absolute values of the vertical components of the vorticity and velocity fields. The borders of these plumes are indicated by large values of the Thermal Dissipation rate and large absolute values of the horizontal vorticity components. In contrast to the sheet-like Thermal plumes, the turbulent background is characterized by low values of the Thermal Dissipation rate, local heat flux and vertical vorticity component. The highest values of the local heat flux and the highest absolute values of the vertical vorticity component are found in the regions where the sheet-like plumes strike against each other. Fluid swirling at these places forms the stems of the mushroom-like Thermal plumes which develop in the bulk of the Rayleigh-Benard cell. Further, formulae to calculate the curvature, thickness and length of the plumes are introduced. Geometrical properties such as plume area, diameter, curvature, thickness and aspect ratio together with the physical properties of the sheet-like plumes such as temperature, heat flux, Thermal Dissipation rate, velocity and vorticity are investigated.
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analysis of Thermal Dissipation rates in turbulent rayleigh benard convection
Journal of Fluid Mechanics, 2005Co-Authors: Olga Shishkina, Claus WagnerAbstract:Direct numerical simulations (DNS) of turbulent Rayleigh–Benard convection in a wide cylindrical container (aspect ratio are defined to determine quantitatively the role of the turbulent background. Evaluating these functions from the DNS data, it is shown that the turbulent background pushes the Thermal plumes back and that its contribution to the volume-averaged Thermal Dissipation rate increases with the Rayleigh number. Further, it is proven analytically that the ratio of the area-averaged (over the top or the bottom plates) to the volume-averaged Thermal Dissipation rate is greater than or equal to the Nusselt number for all aspect ratios, and Prandtl and Rayleigh numbers.
F Do - One of the best experts on this subject based on the ideXlab platform.
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influence of natural temperature gradients on measurements of xylem sap flow with Thermal Dissipation probes 2 advantages and calibration of a noncontinuous heating system
Tree Physiology, 2002Co-Authors: F Do, Alain RocheteauAbstract:: Natural temperature gradients in stems of trees growing in open stands give rise to errors when measuring sap flow by the continuous Thermal Dissipation method. Previously, we obtained evidence from field measurements that a noncontinuous Thermal Dissipation system can prevent these errors (Do and Rocheteau 2002). Cyclic heating (i.e., 45 minutes on and 15 minutes off; 45/15) allowed the derivation of an alternate signal, defined as the difference between the temperature signal at the end of the heating period and the temperature signal at the end of the cooling period. By analogy with the continuous system, we calculated an alternate flow index. Analysis in an artificial flow system confirmed that external temperature gradients have an additive effect on the continuous signal, whereas the alternate signal is unaffected by such gradients. The response of the alternate flow index to flow densities was similar for five combinations of heating and cooling times (45/15, 40/20, 30/30, 15/15 and 10/10 min). The relationship was markedly different from Granier's (1985) calibration because measurements in cyclic systems are made under non-steady-state temperature conditions. We recommend the 15/15 min cycle, which allows two sap flow measurements per hour. We compared flow density estimates obtained from field measurements with the continuous and cyclic systems over 192 days with 1-day lags between systems. Comparisons based on daily maximum values (between 0.5 and 2.5 l dm(-2) h(-1) for the cyclic system) confirmed that differences between the continuous and cyclic systems, which could be greater than 100%, were linked to the effect of temperature gradients on the continuous system. The results demonstrate that, in situations where significant natural temperature gradients (i.e., > 0.2 degrees C) are likely, the cyclic system improves the accuracy of sap flow measurements made with Thermal Dissipation probes.
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influence of natural temperature gradients on measurements of xylem sap flow with Thermal Dissipation probes 1 field observations and possible remedies
Tree Physiology, 2002Co-Authors: F Do, Alain RocheteauAbstract:: The Thermal Dissipation method is simple and widely used for measuring sap flow in large stems. As with several other Thermal methods, natural temperature gradients are assumed to be negligible in the sapwood being measured. We studied the magnitude and variability of natural temperature gradients in sapwood of Acacia trees growing in the Sahelian zone of Senegal, analyzed their effects on sap flow measurements, and investigated possible solutions. A new measurement approach employing cyclic heating (45 minutes of heating and 15 minutes of cooling; 45/15) was also tested. Three-day measurement sequences that included 1 day without heating, a second day with continuous heating and a third day with cyclic heating were recorded during a 6.5-month period using probes installed at three azimuths in a tree trunk. Natural temperature gradients between the two probes of the sensor unit, spaced 8 to 10 cm vertically, were rarely negligible (i.e., < 0.2 degrees C): they were positive during the night and negative during the day, with an amplitude ranging from 0.3 to 3.5 degrees C depending on trunk azimuth, day and season. These temperature gradients had a direct influence on the signal from the continuously heated sensors, inducing fluctuations in the nighttime reference signal. The resulting errors in sap flow estimates can be greater than 100%. Correction protocols have been proposed in previous studies, but they were unsuitable because of the high spatial and temporal variability of the natural temperature gradients. We found that a measurement signal derived from a noncontinuous heating system could be an attractive solution because it appears to be independent of natural temperature gradients. The magnitude and variability of temperature gradients that we observed were likely exacerbated by the combination of open stand, high solar radiation and low sap flow rate. However, for all applications of the Thermal Dissipation method, it is wise to check regularly for natural temperature gradients by switching off the heater.
William W Adams - One of the best experts on this subject based on the ideXlab platform.
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photoprotection in an ecological context the remarkable complexity of Thermal energy Dissipation
New Phytologist, 2006Co-Authors: Barbara Demmigadams, William W AdamsAbstract:This review places photoprotection into the context of ecology and species diversity. The focus is on photoprotection via the safe removal - as Thermal energy - of excess solar energy absorbed by the light collecting system, which counteracts the formation of reactive oxygen species. An update on the surprisingly complex, multiple variations of Thermal energy Dissipation is presented, placing these different forms into ecological and genetic contexts. Zeaxanthin-facilitated, flexible Thermal Dissipation associated with the PsbS protein and controlled by the trans-thylakoid pH gradient apparently occurs ubiquitously in plants, and can become sustained (and thus less flexible) at low temperatures. Long-lived, slow-growing plants with low intrinsic capacities for photosynthesis have greater capacities for this flexible Dissipation than short-lived, fast-growing species. Furthermore, potent, but inflexible (zeaxanthin-facilitated) Thermal Dissipation, prominent in evergreen species under prolonged environmental stress, is characterized with respect to the involvement of photosystem II core rearrangement and/or degradation as well as the absence of control by trans-thylakoid pH and, possibly, PsbS. A role of PsbS-related proteins in photoprotection is discussed.
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seasonal changes in xanthophyll cycle dependent energy Dissipation in yucca glauca nuttall
Plant Cell and Environment, 1998Co-Authors: William W Adams, D H BarkerAbstract:The evergreen species Yucca glauca was characterized at the end of September and following exposure to low temperatures at the end of November. In November the diurnal pattern of xanthophyll cycle-dependent energy Dissipation was altered such that this Thermal Dissipation process was engaged at a high level throughout the day, whereas in September it only became engaged when leaves received direct sunlight. An analysis of the diurnal partitioning of the absorbed excitation energy into photochemistry versus Thermal Dissipation suggested that a smaller fraction of that energy was utilized in photochemistry and a greater fraction was dissipated Thermally at the end of November compared to September. Lower ratios of Chl a/b and β-carotene/xanthophylls both suggested a decrease in the ratio of reaction centre plus core antenna proteins compared to light-harvesting proteins, and a lower leaf chlorophyll content suggested a decrease in light-harvesting capacity in November versus September. Thus adjustments to the photosynthetic apparatus occurred on several levels in response to the increase in excess excitation energy that Y. glauca experienced during the onset of winter.
