Unsteady axisymmetric stagnation flow on a circular cylinder

Unsteady, axisymmetric stagnation flow about a circular cylinderis examined when the far-field flow is a periodic function oftime with a fixed time average and an oscillatory part of prescribedamplitude and frequency. Solutions are computed for arbitraryvalues of the Reynolds number, quantifying the effects of surfacecurvature, and a frequency parameter based on the period ofthe far-field flow. It is found that solutions remain regularand periodic provided that the far-field amplitude lies belowa critical value. Above this value, solutions terminate in afinite-time singularity. The blow-up time is delayed by increasingthe curvature of the surface. These results are corroboratedby asymptotic predictions valid in the limits of small and largeamplitude and frequency. For large Reynolds number, the problemreduces to the two-dimensional stagnation-point flow againsta plane wall studied by previous authors.     

Polyethylene glycol matrix reduces the rates of photochemical and thermal release of nitric oxide from S-nitroso-N-acetylcysteine

S -nitrosothiols have many biological activities and may act as nitric oxide (NO) carriers and donors, prolonging NO half-life in vivo. In spite of their great potential as therapeutic agents, most S -nitrosothiols are too unstable to isolate. We have shown that the S -nitroso adduct of N -acetylcysteine (SNAC) can be synthesized directly in aqueous and polyethylene glycol (PEG) 400 matrix by using a reactive gaseous (NO/O₂) mixture. Spectral monitoring of the S–N bond cleavage showed that SNAC, synthesized by this method, is relatively stable in nonbuf-fered aqueous solution at 25°C in the dark and that its stability is greatly increased in PEG matrix, resulting in a 28-fold decrease in its initial rate of thermal decomposition. Irradiation with UV light (λ= 333 nm) accelerated the rate of decomposition of SNAC to NO in both matrices, indicating that SNAC may find use for the photogeneration of NO. The quantum yield for SNAC decomposition decreased from 0.65 ± 0.15 in aqueous solution to 0.047 ± 0.005 in PEG 400 matrix. This increased stability in PEG matrix was assigned to a cage effect promoted by the PEG microenvironment that increases the rate of geminated radical pair recombination in the homolytic S–N bond cleavage process. This effect allowed for the storage of SNAC in PEG at −20°C in the dark for more than 10 weeks with negligible decomposition. Such stabilization may represent a viable option for the synthesis, storage and handling of S -nitrosothiol solutions for biomedical applications.     

Raman scattering studies of the high-pressure stability of pentaerythritol tetranitrate, C(CH2ONO2)4

High-pressure Raman scattering studies have been performed on a crystalline energetic material, pentaerythritol tetranitrate C(CH2ONO2)4 (PETN), an important secondary explosive. In situ, ambient-temperature investigations employed diamond anvil cell techniques and nitrogen as a quasi-hydrostatic-pressure-transmitting medium. The pressure-induced alterations in the profiles of the Raman lines, including positions, bandwidths, and intensities, were studied in a compression sequence up to about 31.3 GPa and in a subsequent decompression to ambient conditions. The observed changes of the Raman spectra implied that PETN gradually densified and compressed smoothly up to the highest investigated pressures. Compression below 12 GPa gradually shifted all Raman peaks to higher frequencies without significantly changing their relative intensities or bandwidths. At higher pressures, the peak intensities of the Raman spectra decreased considerably and the bands broadened significantly. The Raman spectrum of the material quenched from 31.3 GPa to ambient conditions indicated that no pressure-driven permanent reconstructive modification or decomposition of the PETN structure occurred. That is, the spectral changes were completely reversible upon compression and subsequent decompression to ambient conditions.     

A high pressure,high temperature study of 1,1-diamino-2,2-dinitro ethylene

We report a synchrotron energy-dispersive X-ray diffraction study of the novel high explosive 1,1-diamino-2,2-dinitroethylene at high pressures and high temperatures. Pressure was generated using a Paris–Edinburgh cell to employ larger sample volumes. High temperatures were created using a resistive graphite cylinder surrounding the sample. The PT phase diagram was explored in the 3.3 GPa pressure range and in the ～ 400°C temperature range. We believe that the sample commenced in the α-phase and then ended up in an amorphous phase when the temperature increased beyond 280°C near 2 GPa, which we believe to be the γ-phase. Further pressure and temperature cycling suggests that the sample transformed reversibly into and out of the amorphous phase near the phase line.     

Separation of streamlines for spatially periodic flow at non-zero Reynolds numbers

The flow generated by a small rotating circular cylinder at the center of a corrugated outer cylinder is considered. By using a Stokes expansion, the first order correction in the Reynolds numberR is found for the creeping flow solution. An approximate critical Reynolds numberR _c is found at which separation appears, and it is expressed in terms of the boundary parameters. Separation is found to occur in the concave regions of the boundary skewed opposite to the direction of rotation of the inner cylinder. By partially solving for the second order correction in the Stokes expansion, it is found that an increase inR causes an increase in the torque exerted on the outer boundary.This work was supported in part by a grant from NSERC.     

Giant Pressure‐Driven Lattice Collapse Coupled with Intermetallic Bonding and Spin‐State Transition in Manganese Chalcogenides

Materials with an abrupt volume collapse of more than 20 % during a pressure‐induced phase transition are rarely reported. In such an intriguing phenomenon, the lattice may be coupled with dramatic changes of orbital and/or the spin‐state of the transition metal. A combined in situ crystallography and electron spin‐state study to probe the mechanism of the pressure‐driven lattice collapse in MnS and MnSe is presented. Both materials exhibit a rocksalt‐to‐MnP phase transition under compression with ca. 22 % unit‐cell volume changes, which was found to be coupled with the Mn²⁺(d⁵) spin‐state transition from S=5/2 to S=1/2 and the formation of Mn?Mn intermetallic bonds as supported by the metallic transport behavior of their high‐pressure phases. Our results reveal the mutual relationship between pressure‐driven lattice collapse and the orbital/spin‐state of Mn²⁺ in manganese chalcogenides and also provide deeper insights toward the exploration of new metastable phases with exceptional functionalities.     

广义Bethe树上奇偶马尔可夫链场上的若干极限性质

通过构造两个非负鞅证明了一个强极限定理,然后把它应用到本文所定义的广义Bethe树上的奇偶马尔可夫链场上,从而获得了此马氏链场上的一类强极限定理.