Effect of deflector aerator on stepped spillway flow

Systematic physical model tests are performed on a stepped spillway equipped with a bottom aerator at the beginning of the stepped part. A deflector is used to issue a jet in order to initiate air entrainment into the flow. A horizontal slot located in the vertical face of the first step allows for air supply underneath the flow. The cavity subpressure was measured to ensure optimal aerator performance, namely atmospheric pressure conditions. The air discharge entrained below the jet is measured to derive the aerator air entrainment coefficient. The local air concentrations are spatially measured downstream of the aerator at regularly spaced profiles, allowing the investigation of air transport and detrainment as well as the average and bottom air concentrations. The present paper focuses on the resulting spatial distribution of air concentration for five deflector geometries. The chute angle, step height, approach flow Froude number and approach flow depth were kept constant, so that the differences occur mostly on the jet length and air entrainment coefficient. The flow depth and the air concentration rapidly converge towards quasi-uniform flow values downstream of the aerator

One-pot synthesis of an ionic half-sandwich complex of Neodymium. Application to stereospecific isoprene polymerization catalysis.

International audienceThe reaction of one equivalent of Nd(BH4)3(THF)3 with an half equivalent of dialkylmagnesium in the presence of a stoechiometric amount of pentamethylcyclopentadiene cleanly affords a new kind of half-sandwich of neodymium that is stable toward comproportionation. This strategy can be advantageously applied to generate in situ catalysts allowing the controlled polymerisation of isoprene

Controlled Trans-stereospecific Polymerisation of Isoprene with Neodymium (III) Borohydride-dialkylmagnesium system

International audienceNeodymium trisborohydride, associated with MgR2 alkylating agents, affords an efficient initiator for the stereospecific polymerization of isoprene. This system presents good activity (150 kg of polyisoprene/mol of Nd/h; 95% yield in 2 h) and is trans-stereospecific (up to 97.7%), and its polymerization shows a quasi-living character. The molecular weights are controlled by the [Isoprene]/[Nd] ratio, with a polydispersity index lower than 1.6. A full study is presented here, including the influence of the nature of the alkylating reagent (MgR2), the solvent, the reaction temperature, the [MgR2]/[Nd] ratio, and the replacement of neodymium by other lanthanides. This work also includes a kinetic study of the system and a detailed NMR study of the microstructure of the trans-polyisoprene obtained, providing mechanistic insights about the polymerization reaction

“Half-lanthanidocenes catalysts via the "borohydride/alkyl" route: A simple approach of ligand screening for the controlled polymerization of styrene.”

International audienceThe ‘‘borohydride/alkyl'' (B/A) route initially reported for isoprene has been applied successfully to the polymerization of styrene. This method provides via an in situ approach an interesting tool for the assessment of the influence of a ligand on the performance of half-lanthanidocene catalysts. All systems lead to well controlled oligomerization/polymerization processes. This method is thus a convenient tool for the controlled polymerization of styrene starting from a common trisborohydride precursor and commercial ligands. The influence of the nature of several ligands on the activity could be established, with trends corresponding to those obtained starting from the isolated precursors: HCp = HCpPh3 > HCp* (Cp = C5H5; CpPh3 = 1; 2; 4-Ph3C5H2; Cp* = C5Me5). These results suggest an influence of the electron donating ability of the ligand rather than steric requirements

New ionic half-metallocenes of early lanthanides

International audienceWe present in this study a new and one-step method allowing the preparation of an unprecedented family of stable half-lanthanidocenes. X-ray analysis shows that the isolated compounds all display the same ionic Ln–Mg bimetallic structure consisting of two anionic (CpR)Ln(BH4)3 species and one cationic Mg(THF)6 (CpR =C5Me5, Ln = Nd, 1a, Ln = La, 1b; CpR =C5H5, Ln = Nd, 2a; CpR =C5H2Ph3, Ln = Nd, 3a). Such complexes display high stability with respect to disproportionation in solution. Combined with dialkylmagnesium, neodymium complexes provide powerful catalysts for stereospecific isoprene polymerisation

A New Family of Styrene/Diene Rubbers

International audienceThe insertion of single styrene units into polyisoprene is demonstrated using borohydrido rare earth/dialkylmagnesium systems. This yields a new family of styrene/diene copolymers (SBR rubbers). The resulting poly[(1,4-trans-isoprene)-co-styrene] exhibits quite narrow molecular weight distributions, up to 30% inserted styrene, and a 96–98% 1,4-trans-microstructure. The presence of a bulky and electron-rich ligand in the coordination sphere of the metal leads to an increase of the amount of styrene inserted and narrower chemical composition and molecular weight distributions. The presence of significant quantities of styrene in the medium does not alter the selectivity of the reaction, in contrast with cis-specific polymerizations

Comparison of chute aerator effect on stepped and smooth spillways

Bottom chute aerators are installed to prevent cavitation damages and they have been studied in detail on smooth spillways. In parallel, stepped spillways became widespread in the past decades. Research has shown that stepped spillways may be endangered even more by cavitation than smooth spillways, particularly for high unit discharges. As a consequence and besides issues of energy dissipation, the unit discharge of stepped spillways is usually limited to lower values than on smooth spillways. In order to overcome that limitation, flow aeration – mainly at the beginning of the chute – is necessary. Until now only fragmentary guidelines exists for the design of such aerators. Systematic tests with bottom chute aerators on stepped spillway are performed on a physical model. A deflector is used to separate the jet from the bottom in order to produce slight negative air pressures. A horizontal slot located in the vertical face of the first step allows for air supply underneath the flow. In addition of the global air entrainment by the aerator, the local air concentrations are spatially measured downstream of the deflector. The resulting air concentration distribution allows the investigation of air transport and detrainment as well as the streamwise average and bottom air concentration. The present paper discusses a test with a typical aerator and compares it with a reference test without aerator and a test on a smooth chute with a similar aerator

A flow sensing model for mesenchymal stromal cells using morphogen dynamics

The differentiation of mesenchymal stromal cells has been shown to be af- fected by many parameters such as morphogens, flow rate, medium viscosity, and shear stress when exposed to fluid flow. The mechanism by which these cells sense their environment is still under intense discussion. In particular, during flow chamber experiments, it is difficult to interpret the interplay of the above-mentioned parameters in the process of cell differentiation. In this work, we tested the hypothesis that the competition between diffusion and advection of paracrine morphogens could explain the dependency of the cell differentiation to the above-mentioned parameters. To evaluate this hy- pothesis, we developed a numerical model simulating a simplified version of the advection-diffusion-reaction of morphogens secreted by the cells within a flow chamber. The model predicted a sharp transition in the fraction of receptors bound to the morphogen. This transition was characterized by a new dimensionless number depending on flow rate, flow viscosity, flow chamber dimensions, and morphogen decay rate. We concluded that the competition between diffusion and advection of paracrine morphogens can act as a probe for the cells to sense their peri-cellular environment.LB

Simultaneous and multisite measure of micromotion, subsidence and gap to evaluate femoral stem stability

The initial stability of cementless femoral components is crucial for the long-term success of total hip arthroplasty. This has been reported in animal and clinical studies. Until now, the stability was evaluated by the measurement of relative micromotion on a few simultaneous locations around the stem in cadaveric experiments. This paper presents an extended experimental setup to measure simultaneously local micromotion, subsidence and gap on hundreds of points at the bone–stem interface. This technique we applied to anatomical and straight stems in three pairs of cadaveric femurs. Measurements were in agreement with typically reported values. Conversely to other methods, which measure micromotion between implant and bone anchoring points of the measuring device, our method provides local micromotion between stem surface and adjacent bone surface. The observed variation of micromotion at the peri-implant surface confirms the importance of this simultaneous measure on a lot of points around the implant