Autoxidation of Hydrocarbons: From Chemistry to Catalysis

This contribution summarizes our recent efforts to unravel the radical chemistry of autoxidations. Abstraction of the weakly bonded αH-atom of the primary hydroperoxide product by chain carrying peroxyl radicals is identified as the source of major end products such as alcohol and ketone/aldehyde. In the case of cyclohexane autoxidation, this reaction is even responsible for the majority of ring-opened by-products. In a second part, different approaches to autoxidation catalysis are discussed, ranging from transition metal ion catalysis to organocatalysis with immobilized N-hydroxyphthalimid

Interplay between toroidal rotation and flow shear in turbulence stabilisation

International audienceThe interplay between toroidal rotation u, parallel flow shear u ′ and perpendicular flow shear γE in the stabilisation of tokamak turbulence is investigated in non-linear flux-tube gyrokinetic simulations. The simulations are performed for a reference L-mode DIII-D plasma (the so-called shortfall case) at r/a = 0.8, varying the flow parameters around their nominal values. Depending on the respective signs of u, u ′ and γE, turbulence is found to be enhanced, reduced or unchanged. When the coupling is favorable, the overall effect on the non-linear heat fluxes can be very large, even at moderate flow values. The ion heat flux is for instance decreased by a factor of three when the direction of the parallel flow shear is reversed with respect to its nominal value. Even more surprising, keeping u ′ and γE at their nominal values, the ion heat flux decreases by more than 50% when the toroidal flow is reversed. The relevance of this mechanism in the experiments which depends on the ability to decouple u, u ′ and γE is discussed. The interplay between u and u ′ observed in the non-linear simulations qualitatively follows the linear stability results and is interpreted in the frame of a simple fluid model

Non-renormalization conditions for four-gluon scattering in supersymmetric string and field theory

The constraints imposed by maximal supersymmetry on multi-loop contributions to the scattering of four open superstrings in the U(N) theory are examined by use of the pure spinor formalism. The double-trace term k^2 t_8(tr F^2)^2 (where k represents an external momentum and F the Yang--Mills field strength) only receives contributions from L<=2 (where L is the loop number) while the single-trace term k^2 t_8(tr F^4) receives contributions from all L. We verified these statements up to L=5, but arguments based on supersymmetry suggest they extend to all L. This explains why the single-trace contributions to low energy maximally supersymmetric Yang--Mills field theory are more divergent in the ultraviolet than the double-trace contributions. We also comment further on the constraints on closed string amplitudes and their implications for ultraviolet divergences in N=8 supergravity.Comment: 25 pages. 2 eps figures. Harvmac format. v2 qualifications regarding comments on closed strings. References adde

String loop corrections to the universal hypermultiplet

We study loop corrections to the universal dilaton supermultiplet for type IIA strings compactified on Calabi-Yau threefolds. We show that the corresponding quaternionic kinetic terms receive non-trivial one-loop contributions proportional to the Euler number of the Calabi-Yau manifold, while the higher-loop corrections can be absorbed by field redefinitions. The corrected metric is no longer Kahler. Our analysis implies in particular that the Calabi-Yau volume is renormalized by loop effects which are present even in higher orders, while there are also one-loop corrections to the Bianchi identities for the NS and RR field strengths.Comment: 30 pages, harvmac, 1 figure. v2: minor typos corrected. Version to appear in Classical and Quantum Gravit

Flash floods and muddy floods in Wallonia: recent temporal trends, spatial distribution and reconstruction of the hydrosedimentological fluxes using flood marks and sediment deposits

Flash floods and muddy floods may cause severe human and material damage despite their small spatial extent and low occurrence. In late May 2008, a major event has affected the area of Liège. This paper describes the methodology used to reconstruct the hydrosedimentological parameters of the flood from the geomorphological evidences observed in the field. Bed load and suspended load transport rates estimated during this extreme event were compared to the average values observed in other Walloon rivers and more specifically in the Ardennes Massif. The spatial distribution and the temporal evolution of the flash flood and muddy flood events are then analysed across Wallonia based on several works compiling press archives since the early twentieth century. The biases associated with this type of historical sources and the consequences of flash floods and muddy floods on the vulnerability and the risk of flooding in Wallonia are finally addressed.Les crues éclairs et les coulées boueuses, malgré leur extension spatiale réduite et leur occurrence peu fréquente, peuvent être à l’origine de dégâts importants tant sur le plan matériel que sur le plan humain. Fin mai 2008, un événement majeur a affecté la région liégeoise. Ce papier décrit la méthodologie employée pour reconstituer, en milieu forestier, ses paramètres hydrosédimentologiques à partir des éléments géomorphologiques observés sur le terrain. Les taux de transport de la charge de fond et de la charge en suspension au cours de cet événement extrême sont comparés aux valeurs moyennes observées dans d’autres rivières wallonnes et plus spécifiquement dans le massif ardennais. La répartition spatiale et l’évolution temporelle des crues éclairs sont ensuite analysées à l’échelle de la Wallonie sur base de plusieurs travaux académiques compilant l’ensemble des événements recensés dans la presse depuis le début du XXème siècle. Les biais liés à ce type de sources historiques sont abordés ainsi que les conséquences des crues éclairs sur la vulnérabilité et le risque d’inondation soudaine en Wallonie