Articles de revista
http://hdl.handle.net/2117/1087
Wed, 26 Oct 2016 17:26:27 GMT2016-10-26T17:26:27ZA WRF simulation of an episode of contrails covering the entire sky
http://hdl.handle.net/2117/91007
A WRF simulation of an episode of contrails covering the entire sky
Mazón Bueso, Jordi; Pino González, David
On 21 September 2012 the entire sky was covered by contrails over the Gulf of Lyon (NW of the Mediterranean basin). These clouds were well recorded by ground observers as well as by Meteosat imagery. The atmospheric characteristics at the levels where these anthropic clouds formed are analyzed by performing a WRF simulation in the area where Meteosat recorded contrail clouds. According to the vertical profiles of temperature and the relative humidity respect to the ice (RHI), the environmental condition favors that the water vapor exhaust emitted by the aircraft engines reaches the deposition point and form crystal clouds, which spread out because the temperature remained below 230 K and the RHI was higher than 70% during the whole episode.
Mon, 24 Oct 2016 15:03:44 GMThttp://hdl.handle.net/2117/910072016-10-24T15:03:44ZMazón Bueso, JordiPino González, DavidOn 21 September 2012 the entire sky was covered by contrails over the Gulf of Lyon (NW of the Mediterranean basin). These clouds were well recorded by ground observers as well as by Meteosat imagery. The atmospheric characteristics at the levels where these anthropic clouds formed are analyzed by performing a WRF simulation in the area where Meteosat recorded contrail clouds. According to the vertical profiles of temperature and the relative humidity respect to the ice (RHI), the environmental condition favors that the water vapor exhaust emitted by the aircraft engines reaches the deposition point and form crystal clouds, which spread out because the temperature remained below 230 K and the RHI was higher than 70% during the whole episode.Meteorological and hydrological analysis of major floods in NE Iberian Peninsula
http://hdl.handle.net/2117/91005
Meteorological and hydrological analysis of major floods in NE Iberian Peninsula
Pino González, David; Ruiz Bellet, Josep Lluís; Balasch Solanes, Josep Carles; Tuset Mestre, Jordi; Barriendos Valve, Mariano; Mazón Bueso, Jordi; Castelltort, Xavier
This paper analyses the meteorological conditions and the specific peak flows of 24 catastrophic floods that affected NE Iberian Peninsula in the period 1842–2000. We classify these floods according to the affected area, peak flow magnitude, and damages. Additionally, the NOAA 6 Hourly 20th Century V2 Reanalysis Data Composites have been used to analyze the synoptic conditions during each flood and to evaluate several stability indices, such as the convective available potential energy (CAPE), or the lift index.
We found a good correlation between stability indices and the season when the flood occurred. For instance, if maximum CAPE is considered, larger values are found for summer floods, moderate for autumn, and low values during winter floods. We select 5 representative episodes occurred in different seasons and areas to describe in detail the synoptic conditions and to show the temporal evolution of the stability indices. In one the summer floods analyzed in detail, the largest instability, according to all the convective indices, is found. On the contrary, the winter case shows very low values of the convective indices, and autumn cases lay in between. During the other analyzed summer flood instability was low but snow thaw played an important role in producing the flood.
Regarding hydrological variables, clear differences between floods occurred at the coast or at the Pyrenees are found. Coastal specific peak flows are larger than Pyrenean ones, especially for small catchment areas.
We also combine meteorological (rainfall duration, CAPE), hydrological (specific peak flow) and geomorphological (catchment area) variables to show that for many of the analyzed floods these variables are related: the specific peak flow generally shows larger values when CAPE is also large. However some differences appear depending on the season and area. In those summer floods, where snow thaw played doesn’t played any role, either Pyrenean or coastal, specific peak flow seems to be correlated with CAPE. For autumn floods, depending on the area different correlations were found: Pyrenean floods seem to be a correlation between CAPE and specific peak flows, but not for coastal ones. For winter coastal floods we couldn’t find any correlation between CAPE and specific peak flows.
Mon, 24 Oct 2016 14:38:14 GMThttp://hdl.handle.net/2117/910052016-10-24T14:38:14ZPino González, DavidRuiz Bellet, Josep LluísBalasch Solanes, Josep CarlesTuset Mestre, JordiBarriendos Valve, MarianoMazón Bueso, JordiCastelltort, XavierThis paper analyses the meteorological conditions and the specific peak flows of 24 catastrophic floods that affected NE Iberian Peninsula in the period 1842–2000. We classify these floods according to the affected area, peak flow magnitude, and damages. Additionally, the NOAA 6 Hourly 20th Century V2 Reanalysis Data Composites have been used to analyze the synoptic conditions during each flood and to evaluate several stability indices, such as the convective available potential energy (CAPE), or the lift index.
We found a good correlation between stability indices and the season when the flood occurred. For instance, if maximum CAPE is considered, larger values are found for summer floods, moderate for autumn, and low values during winter floods. We select 5 representative episodes occurred in different seasons and areas to describe in detail the synoptic conditions and to show the temporal evolution of the stability indices. In one the summer floods analyzed in detail, the largest instability, according to all the convective indices, is found. On the contrary, the winter case shows very low values of the convective indices, and autumn cases lay in between. During the other analyzed summer flood instability was low but snow thaw played an important role in producing the flood.
Regarding hydrological variables, clear differences between floods occurred at the coast or at the Pyrenees are found. Coastal specific peak flows are larger than Pyrenean ones, especially for small catchment areas.
We also combine meteorological (rainfall duration, CAPE), hydrological (specific peak flow) and geomorphological (catchment area) variables to show that for many of the analyzed floods these variables are related: the specific peak flow generally shows larger values when CAPE is also large. However some differences appear depending on the season and area. In those summer floods, where snow thaw played doesn’t played any role, either Pyrenean or coastal, specific peak flow seems to be correlated with CAPE. For autumn floods, depending on the area different correlations were found: Pyrenean floods seem to be a correlation between CAPE and specific peak flows, but not for coastal ones. For winter coastal floods we couldn’t find any correlation between CAPE and specific peak flows.On the morphodynamic stability of intertidal environments and the role of vegetation
http://hdl.handle.net/2117/90168
On the morphodynamic stability of intertidal environments and the role of vegetation
Kakeh Burgada, Nabil; Giovanni, Coco; Marani, Marco
We describe the coupled biotic and abiotic dynamics in intertidal environments using a point model that includes suspended sediment deposition, wave-and current-driven erosion, biofilm sediment stabilization, and sediment production and stabilization by vegetation. We explore the effects of two widely different types of vegetation: salt-marsh vegetation and mangroves. These two types of vegetation, which colonize distinct geographical areas, are characterized by different biomass productivities and stabilization mechanisms. We show that changing vegetation and biofilm properties result in differing stable states, both in their type and number. The presence of the biofilm exerts a dominant control on the tidal flat (lower intertidal) equilibrium elevation and stability. Vegetation controls the elevation of the marsh platform (i.e., the upper intertidal equilibrium). The two types of vegetation considered lead to similar effects on the stability of the system despite their distinct biophysical interactions, they ultimately lead to similar e¿ects on the stability of the system.
Fri, 23 Sep 2016 12:54:55 GMThttp://hdl.handle.net/2117/901682016-09-23T12:54:55ZKakeh Burgada, NabilGiovanni, CocoMarani, MarcoWe describe the coupled biotic and abiotic dynamics in intertidal environments using a point model that includes suspended sediment deposition, wave-and current-driven erosion, biofilm sediment stabilization, and sediment production and stabilization by vegetation. We explore the effects of two widely different types of vegetation: salt-marsh vegetation and mangroves. These two types of vegetation, which colonize distinct geographical areas, are characterized by different biomass productivities and stabilization mechanisms. We show that changing vegetation and biofilm properties result in differing stable states, both in their type and number. The presence of the biofilm exerts a dominant control on the tidal flat (lower intertidal) equilibrium elevation and stability. Vegetation controls the elevation of the marsh platform (i.e., the upper intertidal equilibrium). The two types of vegetation considered lead to similar effects on the stability of the system despite their distinct biophysical interactions, they ultimately lead to similar e¿ects on the stability of the system.Natural convection in a horizontal cylinder with axial rotation
http://hdl.handle.net/2117/89338
Natural convection in a horizontal cylinder with axial rotation
Sánchez Casals, Odalys de la Caridad; Mercader Calvo, María Isabel; Batiste Boleda, Oriol; Alonso Maleta, María Aránzazu
We study the problem of thermal convection in a laterally heated horizontal cylinder rotating about its axis. A cylinder of aspect ratio Gamma = H/2R = 2 containing a small Prandtl number fluid (sigma = 0.01) representative of molten metals and molten semiconductors at high temperature is considered. We focus on a slow rotation regime (Omega < 8), where the effects of rotation and buoyancy forces are comparable. The Navier-Stokes and energy equations with the Boussinesq approximation are solved numerically to calculate the basic states, analyze their linear stability, and compute several secondary flows originated from the instabilities. Due to the confined cylindrical geometry-the presence of lateral walls and lids-all the flows are completely three dimensional, even the basic steady states. Results characterizing the basic states as the rotation rate increases are presented. As it occurred in the nonrotating case for higher values of the Prandtl number, two curves of steady states with the same symmetric character coexist for moderate values of the Rayleigh number. In the range of Omega considered, rotation has a stabilizing effect only for very small values. As the value of the rotation rate approaches Omega = 3.5 and Omega = 4.5, the scenario of bifurcations becomes more complex due to the existence in both cases of very close bifurcations of codimension 2, which in the latter case involve both curves of symmetric solutions.
Thu, 28 Jul 2016 14:41:31 GMThttp://hdl.handle.net/2117/893382016-07-28T14:41:31ZSánchez Casals, Odalys de la CaridadMercader Calvo, María IsabelBatiste Boleda, OriolAlonso Maleta, María AránzazuWe study the problem of thermal convection in a laterally heated horizontal cylinder rotating about its axis. A cylinder of aspect ratio Gamma = H/2R = 2 containing a small Prandtl number fluid (sigma = 0.01) representative of molten metals and molten semiconductors at high temperature is considered. We focus on a slow rotation regime (Omega < 8), where the effects of rotation and buoyancy forces are comparable. The Navier-Stokes and energy equations with the Boussinesq approximation are solved numerically to calculate the basic states, analyze their linear stability, and compute several secondary flows originated from the instabilities. Due to the confined cylindrical geometry-the presence of lateral walls and lids-all the flows are completely three dimensional, even the basic steady states. Results characterizing the basic states as the rotation rate increases are presented. As it occurred in the nonrotating case for higher values of the Prandtl number, two curves of steady states with the same symmetric character coexist for moderate values of the Rayleigh number. In the range of Omega considered, rotation has a stabilizing effect only for very small values. As the value of the rotation rate approaches Omega = 3.5 and Omega = 4.5, the scenario of bifurcations becomes more complex due to the existence in both cases of very close bifurcations of codimension 2, which in the latter case involve both curves of symmetric solutions.Prediction of image partitions using Fourier descriptors: application to segmentation-based coding schemes
http://hdl.handle.net/2117/89013
Prediction of image partitions using Fourier descriptors: application to segmentation-based coding schemes
Marqués Truyol, Francisco; Gasull Llampallas, Antoni
This paper presents a prediction technique for partition sequences. It uses a region-by-region approach that consists of four steps: region parameterization, region prediction, region ordering, and partition creation. The time evolution of each region is divided into two types: regular motion and shape deformation. Both types of evolution are parameterized by means of the Fourier descriptors and they are separately predicted in the Fourier domain. The final predicted partition is built from the ordered combination of the predicted regions, using morphological tools. With this prediction technique, two different applications are addressed in the context of segmentation-based coding approaches. Noncausal partition prediction is applied to partition interpolation, and examples using complete partitions are presented. In turn, causal partition prediction is applied to partition extrapolation for coding purposes, and examples using complete partitions as well as sequences of binary images-shape information in video object planes (VOPs)-are presented.
Thu, 21 Jul 2016 09:12:40 GMThttp://hdl.handle.net/2117/890132016-07-21T09:12:40ZMarqués Truyol, FranciscoGasull Llampallas, AntoniThis paper presents a prediction technique for partition sequences. It uses a region-by-region approach that consists of four steps: region parameterization, region prediction, region ordering, and partition creation. The time evolution of each region is divided into two types: regular motion and shape deformation. Both types of evolution are parameterized by means of the Fourier descriptors and they are separately predicted in the Fourier domain. The final predicted partition is built from the ordered combination of the predicted regions, using morphological tools. With this prediction technique, two different applications are addressed in the context of segmentation-based coding approaches. Noncausal partition prediction is applied to partition interpolation, and examples using complete partitions are presented. In turn, causal partition prediction is applied to partition extrapolation for coding purposes, and examples using complete partitions as well as sequences of binary images-shape information in video object planes (VOPs)-are presented.Continuation and stability of convective modulated rotating waves in spherical shells
http://hdl.handle.net/2117/86900
Continuation and stability of convective modulated rotating waves in spherical shells
García González, Fernando; Net Marcé, Marta; Sánchez Umbría, Juan
Modulated rotating waves (MRW), bifurcated from the thermal-Rossby waves that arise at the onset of convection of a fluid contained in a rotating spherical shell, and their stability, are studied. For this purpose, Newton-Krylov continuation techniques are applied. Nonslip boundary conditions, an Ekman number E=10-4, and a low Prandtl number fluid Pr=0.1 in a moderately thick shell of radius ratio ¿=0.35, differentially heated, are considered. The MRW are obtained as periodic orbits by rewriting the equations of motion in the rotating frame of reference where the rotating waves become steady states. Newton-Krylov continuation allows us to obtain unstable MRW that cannot be found by using only time integrations, and identify regions of multistability. For instance, unstable MRW without any azimuthal symmetry have been computed. It is shown how they become stable in a small Rayleigh-number interval, in which two branches of traveling waves are also stable. The study of the stability of the MRW helps to locate and classify the large sequence of bifurcations, which takes place in the range analyzed. In particular, tertiary Hopf bifurcations giving rise to three-frequency stable solutions are accurately determined.
Tue, 10 May 2016 17:55:53 GMThttp://hdl.handle.net/2117/869002016-05-10T17:55:53ZGarcía González, FernandoNet Marcé, MartaSánchez Umbría, JuanModulated rotating waves (MRW), bifurcated from the thermal-Rossby waves that arise at the onset of convection of a fluid contained in a rotating spherical shell, and their stability, are studied. For this purpose, Newton-Krylov continuation techniques are applied. Nonslip boundary conditions, an Ekman number E=10-4, and a low Prandtl number fluid Pr=0.1 in a moderately thick shell of radius ratio ¿=0.35, differentially heated, are considered. The MRW are obtained as periodic orbits by rewriting the equations of motion in the rotating frame of reference where the rotating waves become steady states. Newton-Krylov continuation allows us to obtain unstable MRW that cannot be found by using only time integrations, and identify regions of multistability. For instance, unstable MRW without any azimuthal symmetry have been computed. It is shown how they become stable in a small Rayleigh-number interval, in which two branches of traveling waves are also stable. The study of the stability of the MRW helps to locate and classify the large sequence of bifurcations, which takes place in the range analyzed. In particular, tertiary Hopf bifurcations giving rise to three-frequency stable solutions are accurately determined.Critical torsional modes of convection in rotating fluid spheres at high Taylor numbers
http://hdl.handle.net/2117/86793
Critical torsional modes of convection in rotating fluid spheres at high Taylor numbers
Sánchez Umbría, Juan; García González, Fernando; Net Marcé, Marta
A numerical study of the onset of convection in rotating internally heated self-gravitating fluid spheres is presented. The exploration of the stability of the conduction state versus the Taylor and Prandtl numbers supplies a detailed idea of the laws that fulfil the four types of solutions obtained at low Prandtl numbers. The main result found is that axisymmetric (torsional) modes of convection are preferred at high Taylor numbers in the zero-Prandtl-number limit. This instability appears at low Rayleigh numbers and gives rise to an oscillating single vortex of very high frequency.
Mon, 09 May 2016 16:44:28 GMThttp://hdl.handle.net/2117/867932016-05-09T16:44:28ZSánchez Umbría, JuanGarcía González, FernandoNet Marcé, MartaA numerical study of the onset of convection in rotating internally heated self-gravitating fluid spheres is presented. The exploration of the stability of the conduction state versus the Taylor and Prandtl numbers supplies a detailed idea of the laws that fulfil the four types of solutions obtained at low Prandtl numbers. The main result found is that axisymmetric (torsional) modes of convection are preferred at high Taylor numbers in the zero-Prandtl-number limit. This instability appears at low Rayleigh numbers and gives rise to an oscillating single vortex of very high frequency.Bone tissue properties measurement by reference point indentation in glucocorticoid-induced osteoporosis
http://hdl.handle.net/2117/86742
Bone tissue properties measurement by reference point indentation in glucocorticoid-induced osteoporosis
Mellibovsky, Leonardo; Prieto Alhambra, Daniel; Mellibovsky Elstein, Fernando; Gueerri Fernandez, Roberto; Nogués, Xavier; Randall, Connor; Hansma, Paul K.; Diez Perez, Adolfo
Glucocorticoids, widely used in inflammatory disorders, rapidly increase bone fragility and, therefore, fracture risk. However, common bone densitometry measurements are not sensitive enough to detect these changes. Moreover, densitometry only partially recognizes treatment-induced fracture reductions in osteoporosis. Here, we tested whether the reference point indentation technique could detect bone tissue property changes early after glucocorticoid treatment initiation. After initial laboratory and bone density measurements, patients were allocated into groups receiving calcium+vitamin D (Ca+D) supplements or anti-osteoporotic drugs (risedronate, denosumab, teriparatide). Reference point indentation was performed on the cortical bone layer of the tibia by a handheld device measuring bone material strength index (BMSi). Bone mineral density was measured by dual-energy X-ray absorptiometry (DXA). Although Ca+D-treated patients exhibited substantial and significant deterioration, risedronate-treated patients exhibited no significant change, and both denosumab- and teriparatide-treated participants exhibited significantly improved BMSi 7 weeks after initial treatment compared with baseline; these trends remained stable for 20 weeks. In contrast, no densitometry changes were observed during this study period. In conclusion, our study is the first to our knowledge to demonstrate that reference point indentation is sensitive enough to reflect changes in cortical bone indentation after treatment with osteoporosis therapies in patients newly exposed to glucocorticoids. (c) 2015 American Society for Bone and Mineral Research.
Mon, 09 May 2016 08:05:16 GMThttp://hdl.handle.net/2117/867422016-05-09T08:05:16ZMellibovsky, LeonardoPrieto Alhambra, DanielMellibovsky Elstein, FernandoGueerri Fernandez, RobertoNogués, XavierRandall, ConnorHansma, Paul K.Diez Perez, AdolfoGlucocorticoids, widely used in inflammatory disorders, rapidly increase bone fragility and, therefore, fracture risk. However, common bone densitometry measurements are not sensitive enough to detect these changes. Moreover, densitometry only partially recognizes treatment-induced fracture reductions in osteoporosis. Here, we tested whether the reference point indentation technique could detect bone tissue property changes early after glucocorticoid treatment initiation. After initial laboratory and bone density measurements, patients were allocated into groups receiving calcium+vitamin D (Ca+D) supplements or anti-osteoporotic drugs (risedronate, denosumab, teriparatide). Reference point indentation was performed on the cortical bone layer of the tibia by a handheld device measuring bone material strength index (BMSi). Bone mineral density was measured by dual-energy X-ray absorptiometry (DXA). Although Ca+D-treated patients exhibited substantial and significant deterioration, risedronate-treated patients exhibited no significant change, and both denosumab- and teriparatide-treated participants exhibited significantly improved BMSi 7 weeks after initial treatment compared with baseline; these trends remained stable for 20 weeks. In contrast, no densitometry changes were observed during this study period. In conclusion, our study is the first to our knowledge to demonstrate that reference point indentation is sensitive enough to reflect changes in cortical bone indentation after treatment with osteoporosis therapies in patients newly exposed to glucocorticoids. (c) 2015 American Society for Bone and Mineral Research.El canvi climàtic a través dels informes de l'IPCC (1990-2014)
http://hdl.handle.net/2117/86567
El canvi climàtic a través dels informes de l'IPCC (1990-2014)
Mazón Bueso, Jordi
Des de que l’any 1990 es va publicar el primer informe del Panell Intergovernamental
del Canvi Climàtic (en anglès IPCC) quatre
més
han aparegut (els anys 1995, 2001, 2007
i 2014), els quals ens donen una visió de l’evolució en l’estat del coneixement sobre el
canvi climàtic en les diferents etapes d’aquest període 1990-2014. A partir del contingut
científic i de les conclusions més destacables recollides en aquests cinc informes, s’ha
analitzat l’evolució del coneixement sobre el canvi climàtic, les polítiques i mesures
proposades per a combatre’l. En poc més de 20 anys s’ha passat d’una exhaustiva
descripció de les bases científiques de l’efecte hivernacle i del canvi climàtic en què
es basava l’informe del 1990, amb moltes incerteses i en el què era científicament
difícil atribuir a l’ésser humà com a unic responsable d’aquest canvi de clima, al darrer
informe del 2014, el qual qualifica d’extremament probable amb un 95% de certesa de
que l’escalfaent del planeta és d’origen antròpic, el qual pot esdevenir irreversible si no
s’actua de forma urgent en la reducció de les emissions.
Wed, 04 May 2016 12:40:40 GMThttp://hdl.handle.net/2117/865672016-05-04T12:40:40ZMazón Bueso, JordiDes de que l’any 1990 es va publicar el primer informe del Panell Intergovernamental
del Canvi Climàtic (en anglès IPCC) quatre
més
han aparegut (els anys 1995, 2001, 2007
i 2014), els quals ens donen una visió de l’evolució en l’estat del coneixement sobre el
canvi climàtic en les diferents etapes d’aquest període 1990-2014. A partir del contingut
científic i de les conclusions més destacables recollides en aquests cinc informes, s’ha
analitzat l’evolució del coneixement sobre el canvi climàtic, les polítiques i mesures
proposades per a combatre’l. En poc més de 20 anys s’ha passat d’una exhaustiva
descripció de les bases científiques de l’efecte hivernacle i del canvi climàtic en què
es basava l’informe del 1990, amb moltes incerteses i en el què era científicament
difícil atribuir a l’ésser humà com a unic responsable d’aquest canvi de clima, al darrer
informe del 2014, el qual qualifica d’extremament probable amb un 95% de certesa de
que l’escalfaent del planeta és d’origen antròpic, el qual pot esdevenir irreversible si no
s’actua de forma urgent en la reducció de les emissions.Conductive and convective heat transfer in fluid flows between differentially heated and rotating cylinders
http://hdl.handle.net/2117/86416
Conductive and convective heat transfer in fluid flows between differentially heated and rotating cylinders
López, José M.; Marqués Truyol, Francisco; Àvila Cañellas, Marc
The flow of fluid confined between a heated rotating cylinder and a cooled stationary cylinder is a canonical experiment for the study of heat transfer in engineering. The theoretical treatment of this system is greatly simplified if the cylinders are assumed to be of infinite length or periodic in the axial direction. In these cases heat transfer in the laminar regime occurs only through conduction as in a solid. We here investigate numerically heat transfer and the onset of turbulence in such flows by using both periodic and no-slip boundary conditions in the axial direction. The influence of the geometric parameters is comprehensively studied by varying the radius ratio (0.1 <= eta <= 0.99) and the length-to-gap aspect ratio (5 <= Gamma <= 80). Similarly, a wide range of Prandtl, Rayleigh, and Reynolds numbers is explored (0.01 <= sigma <= 100, Ra <= 30,000, and Re <= 1000, respectively). We obtain a simple criterion, Ra which determines whether the infinite-cylinder assumption can be employed. The coefficient a is well approximated by a cubic fit over the whole n-range. Noteworthy the criterion is independent of the Prandtl number and appears robust with respect to Reynolds number even beyond the laminar regime. (C) 2015 Elsevier Ltd. All rights reserved.
Fri, 29 Apr 2016 10:13:31 GMThttp://hdl.handle.net/2117/864162016-04-29T10:13:31ZLópez, José M.Marqués Truyol, FranciscoÀvila Cañellas, MarcThe flow of fluid confined between a heated rotating cylinder and a cooled stationary cylinder is a canonical experiment for the study of heat transfer in engineering. The theoretical treatment of this system is greatly simplified if the cylinders are assumed to be of infinite length or periodic in the axial direction. In these cases heat transfer in the laminar regime occurs only through conduction as in a solid. We here investigate numerically heat transfer and the onset of turbulence in such flows by using both periodic and no-slip boundary conditions in the axial direction. The influence of the geometric parameters is comprehensively studied by varying the radius ratio (0.1 <= eta <= 0.99) and the length-to-gap aspect ratio (5 <= Gamma <= 80). Similarly, a wide range of Prandtl, Rayleigh, and Reynolds numbers is explored (0.01 <= sigma <= 100, Ra <= 30,000, and Re <= 1000, respectively). We obtain a simple criterion, Ra which determines whether the infinite-cylinder assumption can be employed. The coefficient a is well approximated by a cubic fit over the whole n-range. Noteworthy the criterion is independent of the Prandtl number and appears robust with respect to Reynolds number even beyond the laminar regime. (C) 2015 Elsevier Ltd. All rights reserved.