Inviscid instability of a stably stratified boundary layer

International audienceThe three-dimensional stability of an inflection-free boundary layer flow of length scale L and maximum velocity U0 in a stably stratified fluid of constant Brunt-Väisälä frequency N is examined in an inviscid Boussinesq framework. The plane of the boundary layer is assumed to be inclined with an angle θ with respect to the vertical direction of stratification. The stability analysis is performed using both numerical and theoretical methods for all the values of θ and Froude number F = U0 /(LN ). The boundary layer flow is found to be unstable whatever F as soon as θ ̸= 0. The growth rate of the most unstable mode is shown to increase with the inclination angle to reach its maximum for a vertical boundary layer θ = π/2. The unstable modes are 3D in nature. The mechanism of instability is shown to be associated with internal gravity wave emission. In the weakly stratified limit, both the oscillation frequency and the growth rate are found to scale with the Brunt-Väisälä frequency N. In the strongly stratified limit, frequency and growth rate become independent of the Froude number and proportional to the sine of the inclination angle (as long as F / sin θ ≪ 1)

The future of global sugar markets: Policies, reforms, and impact

"Sugar is one of the most highly protected agricultural commodities worldwide. This protection depresses trade opportunities and the prices received by exporters without preferential market access. For this reason, dialogues about sugar policy are often polarized and short sound bites caustic. Yet today's sugar markets are being driven by a complex array of dynamic and emerging supply, demand, and policy forces that need to be understood. A number of these forces have the potential to reshape the global market scene. Recent sugar policy reforms in the European Union (EU) have received little attention in North America but may turn the EU into a net importer, with substantial compensation paid to its farmers and displaced processing facilities. High oil prices and the related ethanol boom place Brazil at the fulcrum of new market developments. In the United States, corn sweetener and sugar markets are being integrated with Mexican markets under the North American Free Trade Agreement (NAFTA), raising the question of whether the EU reforms provide a template for new policies. And among developing countries in Africa and elsewhere there are low-cost producers that would benefit from more open trade but others who would be disadvantaged by the loss of preferential markets. This discussion paper presents the proceedings of a one-day conference that served as a forum for the discussion of these and other critical issues affecting global sugar markets, policies, and reform options. The conference was attended by 60 representatives of governments, research institutions, producers and processors from the sugar sector, and other groups interested in sugar markets and policies. The four papers were presented by internationally recognized experts from the EU, Brazil, the United States, and South Africa. Discussion openers and general discussion at the conference added further policy insights, and the papers were edited and revised after the conference to reflect the dialogue that had occurred." from authors' abstractsugar, Ethanol, NAFTA, WTO, Trade policy,

Instabilités rayonnantes tri-dimensionnelles d'un jet plan stratifié

La stabilité tri-dimensionnelle d'un jet plan de Bickley est étudiée sous l'hypothèse de Boussinesq en faisant varier l'angle formé par le plan de cisaillement et la stratification. Nous analysons dans quelle mesure le mode de Kelvin-Helmholtz peut avoir une structure rayonnante et plus généralement comment des ondes internes (ou ondes de gravité) associées à des modes rayonnants instables, peuvent être générées spontanément

Les enjeux du développement des biocarburants dans l’Union européenne

En mars 2007, le Conseil européen a annoncé qu’à l’horizon 2020, une proportion minimale de 10 % de biocarburants devrait être incorporée dans les carburants utilisés pour le transport routier. Les pouvoirs publics communautaires mettent en avant trois facteurs principaux pour justifier une telle ambition : la réduction des émissions de gaz à effet de serre (GES), la diversification des approvisionnements énergétiques et le soutien des revenus agricoles. Alors qu’on s’interroge sur les conditions dans lesquelles les 5,75 % d’incorporation fixés par le cadre législatif actuel pourraient être atteints en 2010, ce nouvel élan donné à la politique communautaire des biocarburants suscite plusieurs interrogations.

Modélisation de l'étirage à froid de tubes par analyse éléments-finis

Le procédé d'étirage permet de fabriquer des tubes minces, en réduisant progressivement leur épaisseur ainsi que les diamètres intérieur et extérieur. Dans ce cadre, deux procédés d'étirage à froid sont étudiés avec deux matériaux, un acier austénitique (316LVM) et un acier cobalt (L605). Cette étude aborde différentes problématiques telles que le comportement élastoplastique d'un matériau, les contacts, les frottements et la convergence numérique. Des essais sur banc d'étirage sont réalisés pour enregistrer les efforts et les dimensions. Dans une première approche, des essais de traction quasistatique conduisent à appliquer une loi de comportement élastoplastique avec un écrouissage isotrope. Un modèle statique axisymétrique est utilisé dans les simulations. Finalement, après comparaison des résultats expérimentaux et numériques, cette étude souligne la nécessité d'une meilleure compréhension et modélisation du comportement du matériau, dans des conditions de sollicitations représentatives de celles rencontrées lors de l'étirage

Safety and efficacy of a 6-month home-based exercise program in patients with facioscapulohumeral muscular dystrophy

Background: Previous randomized controlled trials investigating exercise training programs in facioscapulohumeral muscular dystrophy (FSHD) patients are scarce and of short duration only. This study assessed the safety and efficacy of a 6-month home-based exercise training program on fitness, muscle, and motor function in FSHD patients. Methods: Sixteen FSHD patients were randomly assigned to training (TG) and control (CG) groups (both n = 8) in a home-based exercise intervention. Training consisted of cycling 3 times weekly for 35 minutes (combination of strength, high-intensity interval, and low-intensity aerobic) at home for 24 weeks. Patients in CG also performed an identical training program (CTG) after 24 weeks. The primary outcome was change in peak oxygen uptake (VO 2 peak) measured every 6 weeks. The principal secondary outcomes were maximal quadriceps strength (MVC) and local quadriceps endurance every 12 weeks. Other outcome measures included maximal aerobic power (MAP) and experienced fatigue every 6 weeks, 6-minute walking distance every 12 weeks, and muscle characteristics from vastus lateralis biopsies taken pre- and postintervention. Results: The compliance rate was 91% in TG. Significant improvements with training were observed in the VO 2 peak (+19%, P = 0.002) and MAP by week 6 and further to week 24. Muscle endurance, MVC, and 6-minute walking distance increased and experienced fatigue decreased. Muscle fiber cross-sectional area and citrate synthase activity increased by 34% (P = 0.008) and 46% (P = 0.003), respectively. Dystrophic pathophysiologic patterns were not exacerbated. Similar improvements were experienced by TG and CTG. Conclusions: A combined strength and interval cycling exercise-training program compatible with patients' daily professional and social activities leads to significant functional benefits without compromising muscle tissue

Inertia-gravity waves in inertially stable and unstable shear flows

An inertia-gravity wave (IGW) propagating in a vertically sheared, rotating stratified fluid interacts with the pair of inertial levels that surround the critical level. An exact expression for the form of the IGW is derived here in the case of a linear shear and used to examine this interaction in detail. This expression recovers the classical values of the transmission and reflection coefficients $|T|=\exp(-\pi \mu )$ and $|R|=0$, where $\mu^2=J(1+\nu^2)-1/4$, $J$ is the Richardson number and $\nu$ the ratio between the horizontal transverse and along-shear wavenumbers. For large $J$, a WKB analysis provides an interpretation of this result in term of tunnelling: an IGW incident to the lower inertial level becomes evanescent between the inertial levels, returning to an oscillatory behaviour above the upper inertial level. The amplitude of the transmitted wave is directly related to the decay of the evanescent solution between the inertial levels. In the immediate vicinity of the critical level, the evanescent IGW is well represented by the quasi-geostrophic approximation, so that the process can be interpreted as resulting from the coupling between balanced and unbalanced motion. The exact and WKB solutions describe the so-called valve effect, a dependence of the behaviour in the region between the inertial levels on the direction of wave propagation. For $J < 1$ this is shown to lead to an amplification of the wave between the inertial levels. Since the flow is inertially unstable for $J<1$, this establishes a correspondence between the inertial-level interaction and the condition for inertial instability

Changes in ecosystems, climate and societies in the Jura Mountains between 40 and 8 ka cal BP

International audienceWe present radiometric, palaeoclimatological, palaeoenvironmental and archaeological data for the period 40 000–8000 cal BP in the Jura Mountains (eastern France). These mountains culminate at ∼1700 m a.s.l. and are today characterised by a semi-continental climate. During the Last Glacial Maximum, the range supported a local ice cap. While recent data suggest a possible early ice-cap development during MIS 4, the chronology of the regional LGM and following deglaciation has still to be refined. The complete disappearance of the local ice cap at ca 17 000–16 600 cal BP marked the beginning of accumulation of sediment archives in the Jurassian lakes and mires, which favoured the reconstruction of past changes in climatic and environmental conditions, in addition to faunal remains found in caves and in archaeological sites. Three main successive stages may be distinguished regarding the history of societies. The first stage at ca 40 000–18 700 cal BP was characterized by very few archaeological sites with only discontinuous intermittent occupations, always located outside the Jura range. The second stage, around 18 700–11 700 cal BP, corresponded to an increase in the population density, as suggested by an increasing number of archaeological sites and a progressive colonisation of elevated areas of the Jura Mountains. The third stage at ca 11 700–8000 cal BP coincided with a reinforcement of settlement in the lowland areas as well as a development of long-term occupations in elevated areas. The millennial-scale GS-1 cold event had a more long-lasting and stronger impact on societies than did the 200 year-long 8.2 ka cold event

Quinine-Resistant Malaria in Traveler Returning from Senegal, 2007

We describe clinical and parasitologic features of in vivo and in vitro Plasmodium falciparum resistance to quinine in a nonimmune traveler who returned to France from Senegal in 2007 with severe imported malaria. Clinical quinine failure was associated with a 50% inhibitory concentration of 829 nmol/L. Increased vigilance is required during treatment follow-up