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.     

Optimized intermolecular potential for aromatic hydrocarbons based on anisotropic united atoms. III. Polyaromatic and naphthenoaromatic hydrocarbons

In this third article of the series, a new anisotropic united atoms (AUA) intermolecular potential parameter set has been proposed for the carbon force centers connecting the aromatic rings of polyaromatic hydrocarbons to predict thermodynamic properties using both the Gibbs ensemble and NPT Monte Carlo simulations. The model uses the same parameters as previous AUA models used for the aromatic CH force centers. The optimization procedure is based on the minimization of a dimensionless error criterion incorporating various thermodynamic data of naphthalene at 400 and 550 K. The new model has been evaluated on a series of polyaromatic and naphthenoaromatic hydrocarbons over a wide range of temperatures up to near-critical conditions. Vaporization enthalpy, liquid density, and normal boiling temperature are reproduced with good accuracy. The new potential parameters have also been tested successfully on toluene, 1,3,5-trimethylbenzene, styrene, m-xylene, n-hexylbenzene, and n-dodecylbenzene to demonstrate their transferability to alkylbenzenes.     

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.     

Effect of surface resistances on the diffusion of binary mixtures in the silicalite single crystal membrane

Diffusion of methane and argon mixtures through the silicalite single-crystal membrane is studied using the dual-control volume-grand canonical molecular dynamics method to understand how surface resistances alter selectivity and permeance. Comparison of results from intracrystalline transport and entrance simulations for binary mixtures of CH4 and Ar shows that the selectivity of silicalite membranes toward Ar is enhanced in the presence of the surface resistances. In both cases, however, diffusion of faster Ar molecules was inhibited by slower diffusing CH4 molecules, whereas diffusion of the latter remained unaffected. This behavior was explained by the difference between the magnitudes of surface resistances for two molecules, which is much smaller for Ar because of its smaller permeant-crystal interaction size. We find that selectivity of the membrane at the surface depends strongly on total feed pressure and temperature, whereas this dependence is weak for intracrystalline diffusion. Furthermore, we show that the selectivity at the surface diminishes with crystal thickness until a certain thickness is reached, whereas the intracrystalline selectivity remains constant with increasing thickness. Finally, a study of diffusion of C2H6 and CF4 mixtures shows that the diatomic ethane molecules diffuse faster inside the zeolite channels, but their desorption is hindered to a larger extent than that of a spherical molecule with larger diameter and lower heat of adsorption. This observation indicates that the difference in molecular geometry is also a significant factor to explain the exit effect.     

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.     

Suppression of plasma turbulence during optimized shear configurations in JET

Correlation of density turbulence suppression and reduced plasma transport is observed in the internal transport barrier (ITB) region of JET tokamak discharges with optimized magnetic shear. The suppression occurs in two stages. First, low frequency turbulence and ion transport are reduced across the plasma core by a toroidal velocity shear generated by intense auxiliary heating. Then with the ITB formation, high frequency turbulence and electron transport are reduced locally within the steep pressure gradient region of the ITB.     

Molecular Simulation of Fibronectin Adsorption onto Polyurethane Surfaces

Poly(ethylene glycol)-based polyurethanes have been widely used in biomedical applications; however, they are prone to swelling. A natural polyol, castor oil, can be incorporated into these polyurethanes to control the degree of the swelling, which alters mechanical properties and protein adsorption characteristic of the polymers. In this work, we modeled poly(ethylene glycol) and castor oil copolymers of hexamethylene diisocyanate-based polyurethanes (PEG-HDI and CO-HDI, respectively) and compared their mechanisms for fibronectin adsorption using molecular mechanics and molecular dynamics simulations. Results showed that the interplay between the hydrophobic residues concentrated at the N-terminal end of the protein, the surface roughness, and the hydrophilicity of the polymer surface determine the overall protein adsorption affinity. Incorporating explicit water molecules in the simulations results in higher affinity for fibronectin adsorption to more hydrophobic surface of CO-HDI surfaces, emphasizing the role that water molecules play during adsorption. We also observed that the strain energies that are indicative of flexibility and consequently entropy are significantly affected by the changes in the patterns of β-sheet formation/breaking. Our study lends supports to the view that while castor oil controls the degree of swelling, it increases the adsorption of fibronectin to a limited extent due to the interplay between its hydrophobicity and its surface roughness, which needs to be taken into account during the design of polyurethane-based biomaterials.     

Monte Carlo simulation of water adsorption in hydrophobic MFI zeolites with hydrophilic sites

The effect of strong and weak hydrophilic sites, Al atoms with associated extraframework Na cations and silanol nests, respectively, in high-silica MFI zeolites on water adsorption was investigated using Monte Carlo simulations. For this purpose, a new empirical model to represent potential energy interactions between water molecules and the MFI framework was developed, which reproduced the hydrophobic characteristics of a siliceous MFI-type zeolite, silicalite-1, with both the vapor-phase adsorption isotherm and heats of adsorption at 298 K being in good agreement with experimental data. The proposed model is also compatible with previous hydrocarbon potential models and can be used in the adsorption simulations of VOC-water mixtures. Adsorption simulations revealed that strongly hydrophilic Al sites in Na-ZSM-5 zeolites coordinate two water molecules per site at low coverage, which promotes water clustering in the vicinity of these sites. However, weakly hydrophilic silanol nests in silicalite-1 are in coordination with a single water molecule per site, which does not affect the adsorption capacity significantly as expected. However, even in the presence of 0.125 silanol nest per unit cell, the increase in the heat of adsorption at low coverage is drastic.     

Molecular simulation of adsorption and diffusion of chlorinated alkenes in ZSM-5 zeolites

In this work, adsorption and diffusion of trichloroethylene (TCE) and tetrachloroethylene (PCE) in ZSM-5-type zeolites were studied using molecular simulation methods. Grand canonical Monte Carlo technique was to calculate adsorption isotherms and heats of vaporization of TCE and PCE in zeolite. The results demonstrated that the Pnma-P2(1)2(1)2(1) symmetry transition of the zeolite framework has no significant effect on the TCE adsorption capacity of the silicalite, but it causes an increase of the PCE adsorption capacity. Simulations using a silicalite framework with Pnma symmetry showed that the adsorption capacity of the silicalite was limited to five molecules per unit cell. However, when a framework with P2(1)2(1)2(1) symmetry was used in the simulations, the capacity reached to eight molecules per unit cell, which is the actual adsorption capacity. To calculate intracrystalline diffusion coefficients of these compounds, molecular dynamics simulations were performed at different temperatures and loadings. The results show that the zeolite symmetry has a significant impact on diffusion coefficients of the sorbate molecules.