On power-law fluids with the power-law index proportional to the pressure

In this short note we study special unsteady flows of a fluid whose viscosity depends on both the pressure and the shear rate. Here we consider an interesting dependence of the viscosity on the pressure and the shear rate; a power-law of the shear rate wherein the exponent depends on the pressure. The problem is important from the perspective of fluid dynamics in that we obtain solutions to a technologically relevant problem, and also from the point of view of mathematics as the analysis of the problem rests on the theory of spaces with variable exponents. We use the theory to prove the existence of solutions to generalizations of Stokes’ first and second problem.     

Determination of Ciprofloxacin in Human Plasma and Urine by Precolumn Derivatization High Performance Liquid Chromatography with Fluorescence Detection

A sensitive, simple and selective high‐performance liquid chromatographic (HPLC) method was developed for the determination of ciprofloxacin in biological fluids. The method is based on the reaction between the drug and 4‐chloro‐7‐nitrobenzofurazan (NBD‐Cl) in borate buffer of pH 9.0 to yield a highly fluorescent derivative that is measured at 535 nm after excitation at 464 nm. The calibration curves were linear over the concentration ranges of 25–3000 and 50–3000 ng·mL^?1 for plasma and urine, respectively. The mean recovery of ciprofloxacin from plasma and urine was 98.37% and 98.40%, respectively. The method was found to be sensitive, precise, accurate, and reproducible. All of the validation parameters were within the acceptance range.     

Aspects of trapping efficiency and matrix effects in the development of a restricted‐access‐media‐based trap‐and‐elute liquid chromatography with mass spectrometry method

Online restricted access media with liquid chromatography and tandem mass spectrometry for the direct analysis of small molecules in biological fluids represents an interesting alternative to time‐demanding traditional sample preparation techniques. In this study, important considerations concerning the development of a restricted access media with liquid chromatography and tandem mass spectrometry method for the analysis of dansylated estrogens in biological matrix are presented. Parameters influencing peak tailing and trapping efficiency were evaluated. The key factors included the ion strength of the mobile phase, a loading flow rate of the sample onto the trap column, and selection of a proper stationary phase of the trap column for a given set of analytes. These parameters have proven to be essential for minimizing any unwanted chromatographic peak tailing. The bulk derivatization of the analytes in the biological fluids and its relationship to the observed matrix effects was evaluated as well.     

Influence of power-law index on transitional Reynolds numbers for flow over a semi-circular cylinder

In this work, the governing partial differential equations (continuity and Cauchy’s momentum equations) describing the flow of power-law type non-Newtonian fluids across a semi-circular cylinder (oriented with its curved surface in the upstream direction) have been solved numerically. In particular, consideration has been given to the delineation of the critical Reynolds numbers denoting the onset of flow separation from the surface of the cylinder and the onset of the laminar vortex shedding regime. This information is germane to establish the scaling of the macroscopic characteristics like drag coefficient and Strouhal number on the governing parameters, namely, Reynolds number and power-law index. The present results clearly suggest that the transitional Reynolds numbers show a strong dependence on the type (shear-thinning and shear-thickening) of fluid behavior as well as on the severity of the shear-dependence of the viscosity. With reference to the behavior seen in Newtonian fluids, the flow remains not only attached to the surface up to higher Reynolds numbers, but shear-thinning behavior also delays the onset of the laminar vortex shedding regime. As expected, shear-thickening fluids, of course, display the opposite characteristics.     

Stability of a Rayleigh-Bénard Poiseuille flow for yield stress fluids—Comparison between Bingham and regularized models

A linear stability analysis of a Rayleigh-Bénard Poiseuille flow is performed for yield stress fluids whether we use the Bingham or regularized models. A fundamental difference between those models is that the effective viscosity is not defined in the plug zone for the Bingham model, while it is defined in the whole domain for the regularized models. For these models, the viscosity depends highly on a parameter ? near the axis and increases drastically in an intermediate region. The convergence of the critical conditions between the simple and the Bingham models is not obtained. However, we show that the Bercovier and Papanastasiou models can tend to the exact Bingham results.     

平面P波在充满流体的任意形状孔洞上的散射

采用复变函数的保角映射方法和波函数展开法,根据孔洞与内部流体在界面上的应力和位移连续的边界条件,得到了充满流体的任意形状的孔洞对入射平面P波的散射问题的理论解,以椭圆形孔洞为例,着重分析了椭圆的长短轴之比、流体的存在与否以及入射频率对散射幅值的影响,结果表明:(1)圆形孔洞的分波波谱的峰值分布均匀,而椭圆形孔洞则不均匀;(2)散射P波的波谱主要集中于前进方向和背向一侧,而散射S波的波谱主要集中于与传播方向垂直的一侧;(3)入射波与孔洞的作用面积越大,散射P波的波谱也越大,而散射S波的波谱也越小;(4)当孔洞为圆形时,流体对散射P波和S波的波谱影响最小,即此时流体与孔壁的动力相互作用最小.     

Orientations of special water dipoles that accelerate water molecules exiting from carbon nanotube

One-dimensional ordered water molecules entering and exiting from a carbon nanotube with an appropriate radius are studied with molecular dynamics simulations.It can be found that a water molecule near the nanotube end is more likely to be expelled from the nanotube if its dipole is almost perpendicular to the nanotube axis.The key to this observation is that those water molecules axe closer to the wall of the nanotube away from the equilibrium position of the Lennar-Jones (LJ) potential.Thus,the interaction energy for those water molecules is relatively high.There are two particular structures of the perpendicular water molecule depending on the dipole direction of the adjacent water molecule in the nanotube.Although the probabilities of these structures are quite small,their contributions to the net flux across the nanotube end are approximately equal to the predominant structures.The present findings further show the possibility of controlling the water flow by regulating the dipole directions of the water molecules inside the nanochannels.     

Influence of DC electric field on the Lennard-Jones potential and phonon vibrations of carbon nanotube on catalyst

Influence of DC electric field on carbon nanotube (CNT) growth in chemical vapor deposition is studied. Investigation of electric field effect in van der Waals interaction shows that increase in DC electric field raises the magnitude of attractive term of the Lennard-Jones potential. By using a theoretical model based on phonon vibrations of CNT on catalyst, it is shown that there is an optimum field for growth. Also it is observed that CNT under optimum electric field is longer than CNT in the absence of field. Finally, the relation between optimum DC electric field and type of catalyst is investigated and for some intervals of electric field, the best catalyst is introduced, which is very useful for experimental researches.     

基于改进粒子群优化算法的氩原子团簇结构优化

采用改进粒子群优化算法(IPSO)结合Lennard-Jones势对氩原子团簇结构进行优化,得到了氩原子团簇的稳态结构能量。以氩原子团簇Arn（n = 2-14）为例,验证了该方法的有效性。结果表明,应用本文提出的方法可得到对称性良好的团簇结构。与基本粒子群优化算法(BPSO)及遗传算法(GA)相比,改进粒子群优化算法具有更好的收敛特性,能较快地得到氩原子团簇结构的最优解。