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排序方式: 共有484条查询结果,搜索用时 15 毫秒
71.
Benjamin?KrumbeinEmail author Pourya?Forooghi Suad?Jakirli? Franco?Magagnato Bettina?Frohnapfel 《Flow, Turbulence and Combustion》2017,99(3-4):685-703
Turbulent flow over variably-shaped rough walls, characterized by either a regular or a random arrangement of axisymmetric roughness elements in an open channel flow configuration, is investigated computationally within a VLES (Very Large Eddy Simulation) framework by utilizing a volumetric forcing-based roughness model. The prime objective of the present work is to assess the roughness model’s capability to predict mean velocities and turbulent intensities in conjunction with this recently formulated hybrid LES/RANS (Reynolds-Averaged Navier-Stokes) model. The friction velocity-based Reynolds number is in the range Reτ =?460 ? 500. A non-dimensional drag function accounting for the shape of the roughness elements is introduced and evaluated based on the results of complementary direct numerical simulations (DNS). The dynamics of the residual motion of the presently adopted VLES methodology is described by an appropriately modified elliptic-relaxation-based ζ ? f (\(\zeta =\overline {v^{2}}/k\)) RANS model. 相似文献
72.
Ruthenium Lewis Acid‐Catalyzed Asymmetric Diels–Alder Reactions: Reverse‐Face Selectivity for α,β‐Unsaturated Aldehydes and Ketones
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Sirinporn Thamapipol Bettina Ludwig Céline Besnard E. Peter Kündig 《Helvetica chimica acta》2016,99(10):774-789
Acrolein, methacrolein, methyl vinyl ketone, ethyl vinyl ketone, 3‐methyl‐3‐en‐2‐one, and divinyl ketone were coordinated to a cationic cyclopentadienyl ruthenium(II) Lewis acid incorporating the electron‐poor bidentate BIPHOP–F ligand. Analysis by NOESY and ROESY NMR techniques allowed the determination of conformations of enals and enones present in solution in CD2Cl2. The results were compared to solid‐state structures and to the facial selectivities of catalytic asymmetric Diels–Alder reactions with cyclopentadiene. X‐Ray structures of four Ru‐enal and Ru‐enone complexes show the α,β‐unsaturated C=O compounds to adopt an anti‐s‐trans conformation. In solution, enals assume both anti‐s‐trans and anti‐s‐cis conformations. An additional conformation, syn‐s‐trans, is present in enone complexes. Enantioface selectivity in the cycloaddition reactions differs for enals and enones. Reaction products indicate enals to react exclusively in the anti‐s‐trans conformation, whereas with enones, the major product results from the syn‐s‐trans conformation. The alkene in s‐cis conformations, while present in solution, is shielded and cannot undergo cycloaddition. A syn‐s‐trans conformation is found in the solid state of the bulky 6,6‐dimethyl cyclohexanone‐Ru(II) complex. The X‐ray structure of divinyl ketone is unique in that the Ru(II) center binds the enone via a η2 bond to one of the alkene moieties. In solution, coordination to Ru–C=O oxygen is adopted. A comparison of facial preference is also made to the corresponding indenyl Lewis acids. 相似文献
73.
Matthias Mendt Bettina Jee Dieter Himsl Lutz Moschkowitz Tim Ahnfeldt Norbert Stock Martin Hartmann Andreas Pöppl 《Applied magnetic resonance》2014,45(3):269-285
Continuous-wave electron paramagnetic resonance spectroscopy is applied to explore the adsorption of carbon dioxide (CO2) over the metal organic framework (MOF) MIL-53. Therefore, paramagnetic Cr3+ ions, which replace a small amount of the bulk Al3+ ions in MIL-53(Al/Cr), are used as magnetically active probes. CO2 was adsorbed on samples of MIL-53(Al/Cr) at equilibrium pressures between 0 and 2.5 bar. The transformation from the large pore phase to the narrow pore phase of MIL-53 was observed by electron paramagnetic resonance spectroscopy at small CO2 pressures between 0.2 and 0.4 bar, which is in accordance with adsorption results reported in literature. By analyzing the electron paramagnetic resonance signal intensities of the corresponding Cr3+ probes, the ratio between the amount of the narrow pore phase and the large pore phase before and after this phase transformation was quantified. A small fraction of the large pore phase remains even after this phase transition. CO2 adsorption at 77 K indicates the occurrence of the transformation of this MOF from a narrow pore phase to a large pore phase triggered by the adsorbed CO2. Similar observations were already made using powder X-ray diffraction or infrared spectroscopy. But in contrast to these methods electron paramagnetic resonance spectroscopy on Cr3+ seems to be very sensitive not only to large differences between crystallographic conformations like large pores and narrow pores but also to different amounts and configurations of CO2 molecules trapped in the same structural phase of MIL-53, taking advantage of the high sensitivity of the fine structure interaction of Cr3+. 相似文献
74.
Oberacher H Parson W Oefner PJ Mayr BM Huber CG 《Journal of the American Society for Mass Spectrometry》2004,15(4):510-522
An algorithm for the comparative sequencing (COMPAS) of oligonucleotides is shown to be suitable for the sequence verification of nucleic acids ranging in length from a few to 80 nucleotides. The algorithm is based on the matching of a fragment ion spectrum generated by collision-induced dissociation to m/z values predicted from a known reference sequence employing established fragmentation pathways. Prior to mass spectrometric investigation, the oligonucleotides were on-line purified by ion-pair reversed-phase high-performance liquid chromatography using monolithic separation columns. This study evaluated the potential and the limits of COMPAS regarding the length and the charge state of oligonucleotides, the selected collision energy, and the analyzed amount of sample using a quadrupole ion trap mass spectrometer. The results revealed that oligonucleotides could be very reliably re-sequenced up to 60-mers, although the algorithm was successfully used to even verify sequences up to 80-mers. The relative collision energy was typically in the range between 13 and 20%, which allowed in most cases a verification of the reference sequence in a window of at least three consecutive collision energies. To select a proper charge state for fragmentation, a compromise had to be found between high signal intensity and low charge state. Furthermore, by reducing the eluent flow rate during elution of the oligonucleotide, the sequence of a 50-mer was successfully verified from the analysis of 295 fmol of the raw product. COMPAS was proven to be reproducible and was applied to the genotyping of the polymorphic, Y-chromosomal locus M9 contained in a 62-base pair polymerase chain reaction product. 相似文献
75.
76.
Bettina Lotsch Franck Millange Richard I. Walton Dermot O'Hare 《Solid State Sciences》2001,3(8):883-886
We report the first example of selective intercalation of nucleoside monophosphates in a layered host material. The intercalated nucleoside monophosphates can then be quantitatively recovered from the inorganic host and so this opens up the possibility of using simple layered inorganic hosts as rapid, cost effective and recyclable materials for the purification and separation of complex biomolecules. 相似文献
77.
Bettina Immitzer Christoph Etzlstorfer Roland A. Obermüller Max Sonnleitner Gerhard J. Schütz Heinz Falk 《Monatshefte für Chemie / Chemical Monthly》2000,131(10):1039-1045
Summary. Using simultaneous two-photon excitation of fringelite D and a fluorescence indicator embedded in a vesicle system it was
demonstrated that after excitation a proton was transferred from the pigment to the indicator similarly as recently documented
for hypericin. Semiempirical AM1 calculations were used to show that the radical species formed by electron transfer from
the excited pigment state constitutes an acid which is therefore well suited for intermolecular proton transfer. Accordingly,
this process constitutes a suited candidate for the primary photoprocess in the signal transduction cascade of the photosensory
pigments of the stentorin and blepharismin type.
Received April 26, 2000. Accepted May 16, 2000 相似文献
78.
Experiments in a refractive index matched pipe flow facility were conducted using state-of-the-art laser-Doppler anemometry to study turbulent drag reduction by dilute addition of high polymers. The results were analyzed employing the invariant theory of turbulence. It was thus possible to confirm the major conclusion of preceding theoretical work, namely that the mechanism of drag reduction by long-chain polymers is associated with an increase in anisotropy of turbulence at the wall. Furthermore, theoretical considerations based on the elastic behavior of a polymer and spatial intermittency of turbulence at small scales enabled quantitative estimates to be made for the relaxation time of a polymer and its concentration that ensures maximum drag reduction. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
79.
Bettina S. J. Heller Matthias Lexow Francesco Greco Dr. Sunghwan Shin Dr. Gabriel Partl Dr. Florian Maier Prof. Dr. Hans-Peter Steinrück 《Chemistry (Weinheim an der Bergstrasse, Germany)》2020,26(5):1117-1126
Using angle-resolved X-ray photoelectron spectroscopy (ARXPS), we investigate the topmost nanometers of various binary ionic liquid (IL) mixtures at different temperatures in the liquid state. The mixtures consist of ILs with the same [PF6]− anion but two different cations, namely 3-methyl-1-(3,3,4,4,4-pentafluorobutyl)imidazolium hexafluorophosphate, [PFBMIm][PF6], and 1-butyl-3-methylimidazolium hexafluorophosphate, [C4C1Im][PF6], with 10, 25, 50 and 75 mol % content of [PFBMIm][PF6]. We observe a preferential enrichment of the fluorinated chain in the topmost layer, relative to the bulk composition, which is most pronounced for the lowest content of [PFBMIm][PF6]. Upon cooling the mixtures stepwise from 95 °C until surface charging effects in XPS indicate solidification, we observe a pronounced increase in surface enrichment of the fluorinated chain with decreasing temperature in the liquid state. In contrast to the mixtures with lower [PFBMIm][PF6] contents, cooling the 75 mol % mixture additionally shows an abrupt decrease of the fluorinated chain signal before complete solidification occurs, which is assigned to partial precipitation effects. 相似文献
80.