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.     

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.     

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)     

Acoustic pulse interaction with a submerged functionally graded material hollow cylinder

A detailed study is undertaken to analyze the two-dimensional transient fluid-structure interaction of a plane acoustic pressure pulse with an arbitrarily thick, isotropic, functionally graded, hollow cylinder of infinite length, submerged in and filled with non-viscous compressible fluids. A laminate approximate model is adopted to deal with the assumed power-law variation of the constituents’ volume fractions across the thickness of the inhomogeneous cylinder. The problem solution is obtained by employing the classical method of modal expansion in conjunction with the powerful Transfer matrix solution technique and Durbin’s numerical Laplace inversion algorithm. Detailed numerical examples for the transient responses of water-filled and submerged thick-walled TiC-Al FGM cylinders with ceramic or metal rich material compositional gradient profiles under wideband and narrowband Gaussian incident shock loadings are presented and discussed. Many of the interesting dynamic features in the transient shell-shock interaction are addressed through appropriate plots of the internal/external pressure field as well as the induced dynamic stress concentrations within the shell material. Also, the response curves for the FGM cylinders are compared with those of equivalent bi-laminate shells containing comparable total volume fractions of constituent materials. A limiting case is considered and the validity of the work is established by comparison with the data obtained with the aid of a commercial finite element package.     

A new method for the analysis and assessment of fatigue crack closure. I: modeling

In this paper of a two-paper series a new method is proposed for the analysis and assessment of fatigue crack closure phenomena. Based on the proposed model, the combined effect of residual plastic stretch, asperity mismatch, and corrosion debris on the closure behavior of a fatigue crack can be simulated by a hypothetical rigid insert located in an ideal crack wake. The formulation of the model results in a set of equations which can predict the closure load as well as the load-CMOD characteristics using the residual CMOD at zero load as a unique experimental input. The model is verified using different sets of experimental data reported in the literature.     

Phase-Transfer-Catalyzed Michaelis-Becker Synthesis of Dialkyl Methyl Phosphonates

A liquid–liquid phase-transfer-catalyzed (PTC) Michaelis-Becker reaction was adopted in the preparation of dialkyl methyl phosphonate (R = Me, iPr, nBu, and iBu). This was performed by the reaction of an appropriate dialkyl hydrogen phosphonate with methyl iodide in the presence of benzyl triethyl ammonium chloride and sodium hydroxide as PTC and base, respectively. A liquid–liquid two-phase system (H₂O/CH₂Cl₂) introduced a suitable situation for the preparation of dialkyl methyl phosphonates with bulky alkyl groups (R = iPr, nBu, and iBu), but with R = Me, the hydrolysis of dimethyl hydrogen phosphonate (reagent) reduced the yield to 22%. In this case, a solid–liquid PTC-free system was successfully applied and yield of over 80% was obtained.     

Chemically and thermally stable,emissive carbon dots as viable alternatives to semiconductor quantum dots for emissive nematic liquid crystal–nanoparticle mixtures with lower threshold voltage

Dispersions of chemically and thermally robust carbon dots (2.5 ± 0.5 nm in core diameter) were prepared and investigated by polarised optical microscopy, electro-optic measurements including dynamic tests and numerical simulations as well as fluorescence confocal microscopy. The carbon dots were prepared by a straightforward thermal decomposition method from citric acid and hexadecylamine, and they show typical excitation wavelength-dependent photoluminescence behaviour. All dispersions, ranging from 0.5 to 5.0 wt.%, showed lower values for isotropic–nematic phase transition temperature and broader isotropic–nematic biphasic temperature intervals with increasing carbon dot content in comparison to the neat material. Doping of the nematic host with the carbon dots resulted in lower values for the apparent threshold voltage and the elastic constants, but higher values for the rotational viscosity. At 2.5 wt.% and higher, carbon dots residing at the confining interfaces in planar cells induce an increasing initial pre-tilt of up to 8° at lower temperatures. Fluorescence confocal microscopy confirmed this, where the luminescence of the carbon dots permitted visualisation of the distribution of the carbon dots in the bulk with a noticeable, in some cases even pattern-like, segregation to the confining interfaces.     

Selective ion-imprinted polymers for preconcentration and determination of silver in water and hair by electrothermal atomic absorption spectrometry

A new Ag(I)-ion imprinted polymer (IIP) was prepared by formation of 2-(4-hydroxypent-3-en-2-ylideneamine) phenol complex for selective extraction and preconcentration of silver ions. Polymerization was performed with ethylene glycol dimethacrylate as crosslinking monomer and methacrylic acid as functional monomer in the presence of 2,2-azobis(isobutyronitrile) as initiator via bulk polymerization method. The Ag(I)-imprinted polymeric particles were characterized by scanning electron microscopy and infrared spectroscopy. The template silver ion was removed from the polymeric matrix using 2.0 M HNO₃. Optimum pH for maximum sorption was 6.0. Maximum sorbent capacity and enrichment factor for Ag(I) were 12.5 mg/g and 50, respectively. The relative standard deviation and detection limit of the method were evaluated as ±4% and 2.3 ng/L, respectively. The developed IIP has also been tested for preconcentration and recovery of Ag(I) ions from water and hair samples.     

Computational studies of the purine-functionalized graphene sheets

We performed a computational work to investigate the properties of functionalized graphene sheets (S) by adenine (A) and guanine (G) purine nucleobases. To achieve the purpose of this work, we examined the functionalization of armchair and zigzag tips of the S model by each of the A and G purines. The results indicated that the optimized properties for the investigated hybrid structures are different depending on the tip of functionalization and the used purine nucleobase. Moreover, the atomic level properties of the investigated structures were investigated by evaluating quadrupole coupling constants (C_Q) for the atoms of the optimized structures. The remarkable trend of the C_Q parameters is that the changes of atomic properties are many more significant for the functionalization of the zigzag-tip by the G nucleobase, which is in agreement with the results of the optimized properties.