High-Rayleigh-Number Convection in a Vertical Channel

See Also * Phys. Rev. Focus 17, story 9International audienceWe measure the relation between convective heat flux and temperature gradient in a vertical channel filled with water, the average vertical mass flux being zero. Compared to the classical Rayleigh-Bénard case, this situation has the advantage of avoiding plates and, thus, their neighborhood, in which is usually concentrated most of the temperature gradient. Consequently, inertial processes should control the convection, with poor influence of the viscosity. This idea gives a good account of our observations, if we consider that a natural vertical length, different from the channel width, appears. Our results also suggest that heat fluxes can be deduced from velocity measurements in free convective flows. This confers to our results a wide range of applications

Natural cross-ventilation of buildings, an experimental study

International audienceNatural cross ventilation can be a promising passive solution for summer thermal comfort in buildings. It takes advantage of the night temperature of the air to cool the walls of the building. Although this technique is well-known in hot climate, its use in new buildings requires being able to predict the quantity of heat that can be dissipated. There is indeed a lack of experimental data either to build design rules for engineers or to validate numerical code dedicated to the design. In this study, experimental research is carried out on a full-scale dwelling and a small-scale model, and evidence is given on the relevance of such a comparison. The flow has been estimated with spatially discrete local sensors in both systems, and flow visualizations have been additionally performed in the small-scale controlled model. For windy periods, the indoor airflow is found to be driven by the wind, as expected. For calm periods which are critical for summer comfort, the flows look much more complex with the observation of unsteady flow reversals

Être femme de rois karanga à la fin du XVIe et au début du XVIIe siècle

Certains Portugais influents du Sud-Est africain, territoire compris entre la baie Delagoa et le cap Delgado – dans les actuels Mozambique et Zimbabwe – sont considérés, à la fin du xvie et au début du xviie siècle, comme les femmes du Mwene Mutapa et du Quiteve, souverains de royaumes shona karanga au sud du Zambèze. Les Portugais d’alors n’y voient qu’un titre honorifique, thèse que reprennent des historiens du xxe siècle et tout particulièrement W.G.L. Randles. La documentation portugaise fait part de l’existence de « grandes femmes » gouvernant auprès des souverains et ayant des prérogatives éminemment politiques. Elles sont des seigneurs dominant des territoires proches de la cour et interviennent dans la nomination et l’intronisation des nouveaux rois. Leur existence laisse entrevoir l’incompréhension des contemporains, et plus tard des historiens. Informés par leur vision masculine de la royauté où les reines sont avant tout des épouses, ils ne comprennent pas le rôle des femmes, et par extension, celui de ces Portugais, « femmes des rois ».Cet article montre la capacité d’adaptation des royautés karanga par l’intégration d’étrangers dans leur système politique, qui les transforment en seigneurs et en relais du pouvoir. Ces Portugais permettent aux rois karanga de réaffirmer leur autorité, dans des territoires éloignés de la cour, toujours plus difficiles à défendre face aux désirs d’indépendance de certains de leurs vassaux. Il met également en évidence la lecture biaisée qu’en font les Portugais contemporains, mais également des historiens du xxe siècle.No final do século xvi e no início do século xvii, na África do Sudeste, entre a baía Delagoa e o cabo Delgado – no actual Moçambique e Zimbabué – alguns Portugueses influentes eram considerados como mulheres do Mwene Mutapa e do Quiteve, soberanos dos reinos shona karanga ao sul do Zambeze. Os Portugueses daquela época viam naquilo apenas um título honorífico e muitos historiadores do século xx, como W.G.L. Randles, recuperam esta visão.A documentação portuguesa informa-nos sobre a existência de « grandes mulheres » que governavam ao lado destes reis. Elas tinham um grande papel na vida política daqueles reinos. Podemos considerá-las como Suzeranas dos territórios situados perto da corte e intervinham na nomeação e na entronização dos novos reis. Estas mulheres não foram sempre bem compreendidas pelos seus contemporâneos e, mais tarde pelos historiadores do século xx. Envolvidos num mundo dominado pelo sistema patriarcal onde as mulheres eram, antes de tudo, esposas, estes não podiam compreender o papel destas mulheres ; e por extensão, o papel desses Portugueses, que eram « mulheres dos reis ».Este artigo mostra a capacidade de adaptação das realezas karanga, integrando estrangeiros no seu sistema político, estabelecendo-os como senhores e intermediários do poder. Graças a estes Portugueses, os reis karanga reafirmavam a sua autoridade, nos territórios afastados da corte, custosos a defender frente ao desejo de independência dos seus vassalos. Mostra também a leitura torcida feita pelos Portugueses dos séculos xvi-xvii mas, igualmente, pelos historiadores do século xx.Certain influential Portuguese men from South Eastern Africa, the territory between the Delagoa Bay and Cape Delgado – now Mozambique and Zimbabwe – were considered in the late 16th and early 17th centuries, as the wives of Mwene Mutapa and Quiteve, the sovereigns of the Shona Karanga kingdoms south of the Zambesi. The Portuguese of the time saw it merely as an honorary title, an opinion shared by 20th century historians, and in particular W.G.L. Randles.Portuguese documentation informs us, however, of the existence of important female figures governing alongside these sovereigns, with eminently political prerogatives. These women dominated the territories close to the royal courts and took part in the naming and enthronement of new kings. These women were not always well understood by their contemporaries, or later, by 20th century historians. Informed by their masculine vision of royalty where women were above all spouses, they could not understand the role of these women and, by extension, that of the Portuguese so-called «kings’ wives».This article shows the capacity of the Karanga royals to adapt by integrating foreigners into their political system, setting them up as lords, as relays of power. These Portuguese figures enabled the Karanga kings to reaffirm their authority over territories further from the court, areas ever more difficult to defend against the aspirations for independence of certain vassals. It also clearly shows the biased vision not only of the contemporary Portuguese, but also of 20th century historians

Heat convection in a vertical channel : Plumes versus turbulent diffusion

11 pagesInternational audienceFollowing a previous study [Gibert , Phys. Rev. Lett. 96, 084501 (2006)], convective heat transfer in a vertical channel of moderate dimensions follows purely inertial laws. It would be therefore a good model for convective flows of stars and ocean. Here we report new measurements on this system. We use an intrinsic length in the definition of the characteristic Rayleigh and Reynolds numbers. We explicit the relation between this intrinsic length and the thermal correlation length. Using particle imaging velocimetry, we show that the flow undergoes irregular reversals. We measure the average velocity profiles and the Reynolds stress tensor components. The momentum flux toward the vertical walls seems negligible compared to the shear turbulent stress. A mixing length theory seems adequate to describe the horizontal turbulent heat and momentum fluxes, but fails for the vertical ones. We propose a naive model for vertical heat transport inspired by the Knudsen regime in gases

Convection naturelle dans un canal vertical chauffé asymétriquement : stratification thermique externe et écoulements de retour (S23).

Le refroidissement passif d'une façade double-peau photovoltaïque est étudié expérimentalement par l'analyse de la convection naturelle dans un canal vertical chauffé ouvert dans une cellule en eau. Une paroi est chauffée à une densité de flux de chaleur uniforme, l'autre étant adiabatique. L'étude est réalisée pour un nombre de Rayleigh modifié élevé (Ra* = 2, 75.10^7 ). Les mesures de température de la paroi chauffée et de vitesse en sortie du canal par PIV (Particle Image Velocimetry) ont mis en évidence deux états dits "mélangé" "et "stratifié" qui sont caractérisés par la position du maximum de température de paroi situé, respectivement, à mi-hauteur et en sortie. Les mesures de vitesse montre que l'état stratifié est toujours associé à l'existence d'un écoulement de retour. Les trois expériences, réalisées pour différentes stratifications thermiques de la cellule en eau, montrent trois dynamiques différentes de changement d'état

Comparison between rough and smooth plates within the same Rayleigh-Bénard cell

International audienceIn a Rayleigh-Bénard cell at high Rayleigh number, the bulk temperature is nearly uniform. The mean temperature gradient differs from zero only in the thin boundary layers close to the plates. Measuring this bulk temperature allows to separately determine the thermal impedance of each plate. In this work, the bottom plate is rough and the top plate is smooth; both interact with the same bulk flow. We compare them and address in particular the question whether the influence of roughness goes through a modification of the bulk flow

Risk factors associated with lambing traits

peer-reviewedThis article was first published in Animal (2016), 10:1, pp 89–95, © The Animal Consortium 2015The objective of this study was to establish the risk factors associated with both lambing difficulty and lamb mortality in the Irish sheep multibreed population. A total of 135 470 lambing events from 42 675 ewes in 839 Irish crossbred and purebred flocks were available. Risk factors associated with producer-scored ewe lambing difficulty score (scale of one (no difficulty) to four (severe difficulty)) were determined using linear mixed models. Risk factors associated with the logit of the probability of lamb mortality at birth (i.e. binary trait) were determined using generalised estimating equations. For each dependent variable, a series of simple regression models were developed as well as a multiple regression model. In the simple regression models, greater lambing difficulty was associated with quadruplet bearing, younger ewes, of terminal breed origin, lambing in February; for example, first parity ewes experienced greater (P7.0 kg) birth weights, quadruplet born lambs and lambs that experienced a more difficult lambing (predicted probability of death for lambs that required severe and veterinary assistance of 0.15 and 0.32, respectively); lambs from dual-purpose breeds and born to younger ewes were also at greater risk of mortality. In the multiple regression model, the association between ewe parity, age at first lambing, year of lambing and lamb mortality no longer persisted. The trend in solutions of the levels of each fixed effect that remained associated with lamb mortality in the multiple regression model, did not differ from the trends observed in the simple regression models although the differential in relative risk between the different lambing difficulty scores was greater in the multiple regression model. Results from this study show that many common flock- and animal-level factors are associated with both lambing difficulty and lamb mortality and management of different risk category groups (e.g. scanned litter sizes, ewe age groups) can be used to appropriately manage the flock at lambing to reduce their incidence

Genetic analyses of live weight and carcass composition traits in purebred Texel, Suffolk and Charollais lambs

peer-reviewedLamb live weight is one of the key drivers of profitability on sheep farms. Previous studies in Ireland have estimated genetic parameters for live weight and carcass composition traits using a multi-breed population rather than on an individual breed basis. The objective of the present study was to undertake genetic analyses of three lamb live weight and two carcass composition traits pertaining to purebred Texel, Suffolk and Charollais lambs born in the Republic of Ireland between 2010 and 2017, inclusive. Traits (with lamb age range in parenthesis) considered in the analyses were: pre-weaning weight (20 to 65 days), weaning weight (66 to 120 days), post-weaning weight (121 to 180 days), muscle depth (121 to 180 days) and fat depth (121 to 180 days). After data edits, 137 402 records from 50 372 lambs across 416 flocks were analysed. Variance components were derived using animal linear mixed models separately for each breed. Fixed effects included for all traits were contemporary group, age at first lambing of the dam, parity of the dam, a gender by age of the lamb interaction and a birth type by rearing type of the lamb interaction. Random effects investigated in the pre-weaning and weaning weight analyses included animal direct additive genetic, dam maternal genetic, litter common environment, dam permanent environment and residual variances. The model of analysis for post-weaning, muscle and fat depth included an animal direct additive genetic and litter common environment effect only. Significant direct additive genetic variation existed in all cases. Direct heritability for pre-weaning weight ranged from 0.14 to 0.30 across the three breeds. Weaning weight had a direct heritability ranging from 0.17 to 0.27 and post-weaning weight had a direct heritability ranging from 0.15 to 0.27. Muscle and fat depth heritability estimates ranged from 0.21 to 0.31 and 0.15 to 0.20, respectively. Positive direct correlations were evident for all traits. Results revealed ample genetic variation among animals for the studied traits and significant differences between breeds to suggest that genetic evaluations could be conducted on a per-breed basis

Genetic selection for hoof health traits and cow mobility scores can accelerate the rate of genetic gain in producer-scored lameness in dairy cows

peer-reviewedCattle breeding programs that strive to reduce the animal-level incidence of lameness are often hindered by the availability of informative phenotypes. As a result, indicator traits of lameness (i.e., hoof health and morphological conformation scores) can be used to improve the accuracy of selection and subsequent genetic gain. Therefore, the objectives of the present study were to estimate the variance components for hoof health traits using various phenotypes collected from a representative sample of Irish dairy cows. Also of interest to the present study was the genetic relationship between both hoof health traits and conformation traits with producer-scored lameness. Producer-recorded lameness events and linear conformation scores from 307,657 and 117,859 Irish dairy cows, respectively, were used. Data on hoof health (i.e., overgrown sole, white line disease, and sole hemorrhage), mobility scores, and body condition scores were also available from a research study on up to 11,282 Irish commercial dairy cows. Linear mixed models were used to quantify variance components for each trait and to estimate genetic correlations among traits. The estimated genetic parameters for hoof health traits in the present study were greater (i.e., heritability range: 0.005 to 0.27) than previously reported in dairy cows. With the exception of analyses that considered hoof health traits in repeatability models, little difference in estimated variance components existed among the various hoof-health phenotypes. Results also suggest that producer-recorded lameness is correlated with both hoof health (i.e., genetic correlation up to 0.48) and cow mobility (i.e., genetic correlation = 0.64). Moreover, cows that genetically tend to have rear feet that appear more parallel when viewed from the rear are also genetically more predisposed to lameness (genetic correlation = 0.39); genetic correlations between lameness and other feet and leg type traits, as well as between lameness and frame type traits, were not different from zero. Results suggest that if the population breeding goal was to reduce lameness incidence, improve hoof health, or improve cow mobility, genetic selection for either of these traits should indirectly benefit the other traits. Results were used to quantify the genetic gains achievable for lameness when alternative phenotypes are available