Theoretical modelling of the three-dimensional wake of vertical axis turbines

Arrays of Vertical Axis Wind Turbines (VAWTs) can achieve larger power generation per land area than horizontal axis turbines farms, due to the positive synergy between VATs in close proximity. Theoretical wake models enable the reliable design of the array layout that maximises the energy output, which need to depict the driving wake dynamics. VAWTs generate a highly complex wake that evolves according to two governing length-scales, namely the turbine rotor's diameter and height which define a rectangular shape of the wake cross-section, and feature distinct wake expansion rates. This paper presents analytical VAWT wake models that account for an asymmetric distribution of such wake expansion adopting a top-hat and Gaussian velocity deficit distribution. Our proposed analytical Gaussian model leads to an enhanced initial wake expansion prediction with the wake width ($\varepsilon$) behind the rotor equal to $(\beta/4 \pi)^{1/2}$ with $\beta$ being the ratio of initial wake area to the VAWT's frontal area, which addresses the limitations of previous models that under-predicted the wake onset area. Velocity deficit predictions are calculated in a series of numerical benchmarks consisting of a single and an array of four in-line vertical axis wind turbines. In comparisons with field data and large-eddy simulations, our models provide a good accuracy to represent the mean wake distribution, maximum velocity deficit, and momentum thickness, with the Gaussian model attaining the best predictions.These models will aid to drive the design of VAT arrays and accelerate this technology.Comment: 22 pages, 8 figure

Cervical spine surgery in ankylosing spondylitis : review and current concept

Ankylosing spondylitis of the cervical spine is associated with stiff kyphosis and increased risk of transversal unstable fracture. A spine surgeon may be involved mainly in the management of trauma cases, but in some situations, corrective surgery of a kyphotic cervical deformity is needed. Both types of cases carry specific aspects and rely on principles that differ from those associated with more common cervical surgery. This paper is a review of the literature regarding cervical surgery in cases of ankylosing spondylitis. It addresses practical technical questions

Identification of a second GTP-bound magnesium ion in archaeal initiation factor 2

International audienceEukaryotic and archaeal translation initiation processes involve a heterotrimeric GTPase e/aIF2 crucial for accuracy of start codon selection. In eu-karyotes, the GTPase activity of eIF2 is assisted by a GTPase-activating protein (GAP), eIF5. In ar-chaea, orthologs of eIF5 are not found and aIF2 GT-Pase activity is thought to be non-assisted. However , no in vitro GTPase activity of the archaeal factor has been reported to date. Here, we show that aIF2 significantly hydrolyses GTP in vitro. Within aIF2␥, H97, corresponding to the catalytic histidine found in other translational GTPases, and D19, from the GKT loop, both participate in this activity. Several high-resolution crystal structures were determined to get insight into GTP hydrolysis by aIF2␥. In particular, a crystal structure of the H97A mutant was obtained in the presence of non-hydrolyzed GTP. This structure reveals the presence of a second magnesium ion bound to GTP and D19. Quantum chemical/molecular mechanical simulations support the idea that the second magnesium ion may assist GTP hydrolysis by helping to neutralize the developing negative charge in the transition state. These results are discussed in light of the absence of an identified GAP in archaea to assist GTP hydrolysis on aIF2

Roles of yeast eIF2α and eIF2β subunits in the binding of the initiator methionyl-tRNA

International audienceHeterotrimeric eukaryotic/archaeal translation initiation factor 2 (e/aIF2) binds initiator methionyl-tRNA and plays a key role in the selection of the start codon on messenger RNA. tRNA binding was extensively studied in the archaeal system. The γ subunit is able to bind tRNA, but the α subunit is required to reach high affinity whereas the β subunit has only a minor role. In Saccharomyces cerevisiae however, the available data suggest an opposite scenario with β having the most important contribution to tRNA-binding affinity. In order to overcome difficulties with purification of the yeast eIF2γ subunit, we designed chimeric eIF2 by assembling yeast α and β subunits to archaeal γ subunit. We show that the β subunit of yeast has indeed an important role, with the eukaryote-specific N- and C-terminal domains being necessary to obtain full tRNA-binding affinity. The α subunit apparently has a modest contribution. However, the positive effect of α on tRNA binding can be progressively increased upon shortening the acidic C-terminal extension. These results, together with small angle X-ray scattering experiments, support the idea that in yeast eIF2, the tRNA molecule is bound by the α subunit in a manner similar to that observed in the archaeal aIF2-GDPNP-tRNA complex. © The Author(s) 2012. Published by Oxford University Press

Mathematical evaluation of jumping distance in total hip arthroplasty: Influence of abduction angle, femoral head offset, and head diameter

Background and purpose The jumping distance (JD) is the degree of lateral translation of the femoral head center required before dislocation occurs. The smaller the distance, the higher the theoretical risk of dislocation. The aim of our study was to evaluate this jumping distance and its variation according to the characteristics of the implant, and also the theoretical gain in using large head diameters of above 38 mm