Numerical study of convective heat transfer and fluid flow around two side by side square cylinders using k - \omega - \overline{{\upsilon^{2} }} - f turbulence model

The effect of spacing between two identical square cylinders placed side by side on the fluid flow and heat transfer is numerically investigated using $ k - \omega - \overline{{\upsilon^{2} }} - f $ turbulence model. The present study is performed at Pr = 0.7 and Re = 10,000, 21,000 for different scaled gap spacing between cylinders in the range of Gl = 0.5–6. It should be noted all geometrical lengths such as Gl are scaled with cylinders side. In order to show the accuracy of $ k - \omega - \overline{{\upsilon^{2} }} - f $ model, part of the results such as various flow patterns (flip-flop, in-phase and anti-phase) and global quantities are compared with the available numerical and experimental results and also a Large Eddy Simulation study of the present work. Based on this comparison, a close agreement is observed. The local and averaged flow and thermal quantities are also compared for two side by side square and circular cylinders and some significant similarities and differences are presented. Progressive increasing and decreasing of the distance between cylinders indicates that the hysteresis phenomenon appears for the gap spacing in the range of Gl = 1–2.5. In the hysteresis range, two different patterns are observed for each distance in the aforementioned range. Also in this range, two different values are found for different quantities such as lift and drag coefficients, Strouhal number and Nusselt number.     

A new image encryption method: parallel sub-image encryption with hyper chaos

A new image encryption scheme, based on a total shuffling and parallel encryption algorithm is proposed in this paper. Two chaotic systems have been used in the encryption algorithm to confuse the relationship between the plain-image and the cipher-image. To make the encryption procedure more confusing and complex, the plain-image is first divided into 4 sub-images and then the position of each sub-image is changed pseudo-randomly according to a logistic map. Next, a total shuffling matrix is used to shuffle the position of pixels in the whole image and then sub-images are encrypted simultaneously in a parallel manner. The experimental results on USC data base demonstrate that the proposed encryption algorithm has a low time complexity and has the advantages of large key space and high security. Moreover, the robustness of this locally encryption method is much more in contrast with other encryption schemes and the distribution of gray values has a random-like behavior in the encrypted image.     

Calculation of chemical shielding in C-doped zigzag BN nanotubes

Abstract  

Chemical shielding tensors at the sites of various ¹¹B and ¹⁵N nuclei were calculated to study the electronic structure properties of carbon-doped boron nitride nanotubes. The chemical shielding tensors were converted to isotropic and anisotropic chemical shielding parameters. The results reveal the significant effect of C-doping on the chemical shielding tensors at the sites of those ¹¹B and ¹⁵N nuclei located in the nearest neighborhood of the C-doped ring. The isotropic and anisotropic chemical shielding tensors of those B atoms directly bonded to C atoms reach values observed for B atoms placed at the B-mouth of the nanotube.     

Study of hydrogen bonds in crystalline 5-nitrouracil. Density functional theory calculations of the O-17, N-14, and H-2 nuclear quadrupole resonance parameters

Hydrogen bond interaction properties of backbone uracil was studied in crystalline structure of 5-nitrouracil. To this aim the electric field gradient tensors were calculated at the level of density functional theory in two single (non-hydrogen bonded) and cluster (hydrogen-bonded four-molecule) models of 5-nitrouracil. The electric field gradient tensors at the sites of O-17, N-14, and H-2 nuclei were converted to the experimentally measurable nuclear quadrupole resonance spectroscopy parameters, quadrupole coupling constant and asymmetry parameter. The results indicated different hydrogen bond interaction properties at the sites of various nuclei and also the protective role of ?NO₂ group for contribution of O1 to hydrogen bond interactions in comparison with uracil. The density functional theory calculations were performed using GAUSSIAN 98 package employing B3LYP method and 6-311G** and 6-311++G** basis sets.     

Diastereoselective synthesis of fused [1,3]oxazines from ethyl pyruvate,activated acetylenes and N-heterocycles

The 1:1 zwitterionic intermediates obtained from the reaction between isoquinoline, quinoline, pyridine, or N-methylimidazole with activated acetylenes are trapped by ethyl pyruvate to produce highly functionalized fused [1,3]oxazines in good yields.     

Synthesis and characterization of tetraprolinium silicotungstic acid tetra‐hydrate,a new organic‐inorganic hybrid based on polyoxometallates

A new solvated polyoxometallate bearing an amino acid, ( I ), has been prepared for the first time by the reaction of 0.16 mmol α‐H₄SiW₁₂O₄₀.15H₂O with 0.78 mmol of L‐proline in hydrochloric acid at pH 3. The structure of ( I ) was substantially characterized by some physical approaches, such as elemental analysis, infrared spectroscopy, and ¹H‐NMR. Our findings proved that ( I ) could be formulated as [L‐C₅H₁₀NO₂]₄[SiW₁₂O₄₀].4H₂O. It is believed that the electrostatic attractions and hydrogen bonding between tungstosilisic acid and the prolinium cation affected the structure of ( I ). It is evident that the polyoxometallates moieties were arranged in good order to form large holes in the structural backbone. These cavities were occupied by the prolinium cations and water molecules. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Development of 68Ga ethyl cysteinate dimer for PET studies

In this work development of ⁶⁸Ga-ethyl cysteinate dimer (⁶⁸Ga-ECD) a ⁶⁸Ga tracer for possible cerebral blood flow based on ^99mTc ECD homolog is reported. ⁶⁸Ga-ECD was prepared using generator-based ⁶⁸GaCl₃ and ECD at optimized conditions. Quality control, stability, partition co-efficient and the biodistribution of the tracer (by tissue counting and PET/CT in rats) was studied. Significant metabolism of the lipophilic tracer into water soluble metabolite(s) led to urinary excretion of the tracer, un-comparable to that of homologous ^99mTc-compound. Cardiac uptake of the complex suggests formation of a possible lipophil cationic complex and/or metabolite.