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51.
In the beginning of the 1980’s Large Eddy BreakUp (LEBU) devices, thin plates or airfoils mounted in the outer part of turbulent boundary layers, were shown to be able to change the turbulent structure and intermittency as well as reduce turbulent skin friction. In some wind-tunnel studies it was also claimed that a net drag reduction was obtained, i.e. the reduction in skin-friction drag was larger than the drag on the devices. However, towing-tank experiments with a flat plate at high Reynolds numbers as well as with an axisymmetric body showed no net reduction, but instead an increase in total drag. Recent large-eddy simulations have explored the effect of LEBUs on the turbulent boundary layer and evaluations of the total drag show similar results as in the towing tank experiments. Despite these negative results in terms of net drag reduction, LEBUs manipulate the boundary layer in an interesting way which explains why they still attract some interest. The reason for the positive results in the wind-tunnel studies as compared to drag measurements are discussed here, although no definite answer for the differences can be given. 相似文献
52.
In this study, we shall suggest analytical expressions for two-center nuclear attraction integrals over STO’s with a one-center
charge distribution by using Fourier transform method. The derivation is based on partial-fraction decompositions and Taylor
expansions of rational functions. Analytical expressions obtained by this method are expressed in terms of Gegenbauer, and
binomial coefficients and linear combinations of STO’s. Finally, it is relatively easy to express the Fourier integral representations
of two-center nuclear attraction integrals with a one-center charge distribution mentioned above as finite and infinite of
series of STO’s and irregular solid harmonics which may be considered to be limiting cases of STO’s. 相似文献
53.
Emin Erdem Eylem Yildirim Sari Rafet Kilinçarslan Nilgün Kabay 《Transition Metal Chemistry》2009,34(2):167-174
Azo compounds were prepared by coupling of benzenediazonium chloride ions with 1-amino-2-hydroxy-4-naphthalene sulfonic acid
under alkaline conditions, and Schiff bases, L1–3 were then obtained by the condensation of 1-amino-2-hydroxy-3-(phenylazo)-4-naphthalene sulfonic acid, 1-amino-2-hydroxy-3-(4-ethylphenylazo)-4-naphthalene sulfonic acid, and 1-amino-2-hydroxy-3-(4-nitrophenylazo)-4-naphthalene sulfonic acid with salicylaldehyde. New copper(II), nickel(II), and zinc(II) complexes of the
Schiff base ligands were also prepared and characterized by spectroscopic methods, magnetic measurements, elemental, and thermogravimetric
analysis. 相似文献
54.
Ultrasound-assisted emulsification-microextraction (USAEME) procedure was developed for the determination of selected polychlorinated biphenyls (PCBs) in 10 mL of water samples by gas chromatography-mass-selective detection. After determination of the most suitable solvent and extraction time, several other parameters including solvent volume, centrifugation time and ionic strength of the sample were optimized using a 23 factorial experimental design. The optimized USAEME procedure used 200 μL of chloroform as extraction solvent, 10 min of extraction with no ionic strength adjustment at 25 °C and 5 min of centrifugation at 4000 rpm. The limits of detection ranged from 14 ng L−1 (for PCB153) to 30 ng L−1 (for PCB101). Recoveries of PCBs from fortified distilled water are over 80% for three different fortification levels between 0.1 and 5 μg L−1 and relative standard deviations of the recoveries are below 10%. The performance of the proposed method was compared with those involving traditional liquid-liquid extraction (LLE) and solid phase extraction (SPE) on the real water samples (i.e., tap and well water as well as domestic and industrial wastewaters, etc.) and comparable efficiencies were obtained. The proposed USAEME procedure has been demonstrated to be viable, simple, rapid and easy to use for residue analysis of PCBs in water samples. 相似文献
55.
M. Emin Günay Nam?k Özdemir Melih Uçak Bekir Çetinkaya 《Journal of organometallic chemistry》2009,694(14):2179-3680
Deprotonation of 1,3-diorganyl imidazolinium salts, 1, with N,C-type palladacyclic acetate dimer 2 afforded novel NHC coordinated complexes 3 along with ring opening hydrolysis products 4, which may coordinate to palladium center via NH group to give 5a. The hydrolysis necessitates the study of NHC complex formation in anhydrous media. The new compounds were characterized by spectroscopic methods and three of them (3c, 4c, 5a) by X-ray single-crystal diffraction studies. 相似文献
56.
The Capacitated Minimum Spanning Tree Problem is NP-hard and several heuristic solution methods have been proposed. They can be classified as classical ones and metaheuristics. Recent developments have shown that metaheuristics outperform classical heuristics. However, they require long computation times and there are difficulties in their parameter calibration and coding phases. This explains the popularity of the Esau–Williams (EW) heuristic in practice, and its use in many successful metaheuristics and second-order greedy methods. In this work, we are concerned with the EW heuristic and we propose simple new enhancements. Based on our computational experiments, we can say that they considerably improve its accuracy with minor increase in computation time, and without harming its simplicity. 相似文献
57.
The objective of this study is to remove the phenol from aqueous solution by using the neutralized red mud in batch adsorption technique. The study was carried out as functions of contact time, pH, initial phenol concentration, red mud dosage and effect of salt addition. The experiments demonstrated that maximum phenol removal was obtained in a wide pH range of 1-9 and it takes 10 h to attain equilibrium. The adsorption data was analyzed using the Langmuir and the Freundlich isotherm models and it was found that the Freundlich isotherm model represented the measured sorption data well. The influence of addition of salt on phenol removal depends on the relative affinity of the anions for the red mud surface and the relative concentrations of the anions. 相似文献
58.
In a seminal paper, Alcubierre showed that Einstein’s theory of general relativity appears to allow a super-luminal motion. In the present study, we use a recent eternal-warp-drive solution found by Alcubierre to study the effect of Hawking radiation upon an observer located within the warp drive in the framework of the quantum tunneling method. We find the same expression for the Hawking temperatures associated with the tunneling of both massive vector and scalar particles, and show this expression to be proportional to the velocity of the warp drive. On the other hand, since the discovery of gravitational waves, the quasinormal modes (QNMs) of black holes have also been extensively studied. With this purpose in mind, we perform a QNM analysis of massive scalar field perturbations in the background of the eternal-Alcubierre-warp-drive spacetime. Our analytical analysis shows that massive scalar perturbations lead to stable QNMs. 相似文献
59.
In the present paper we explore the Hawking radiation as a quantum tunneling effect from a rotating 5 dimensional Myers-Perry black hole (5D-MPBH) with two independent angular momentum components. First, we investigate the Hawking temperature by considering the tunneling of massive scalar particles and spin-1 vector particles from the 5D-MPBH in the Painlevé coordinates and then in the corotating frames. More specifically, we solve the Klein-Gordon and Proca equations by applying the WKB method and Hamilton-Jacobi equation in both cases. Finally, we recover the Hawking temperature and show that coordinates systems do not affect the Hawking temperature. 相似文献
60.
The Multi-commodity Capacitated Multi-facility Weber Problem (MCMWP) is concerned with locating I-capacitated facilities in the plane in order to satisfy the demands of J customers for K commodities so that the total transportation cost is minimized. We propose a Lagrangean relaxation scheme and a subgradient-like algorithm based on the relaxation of the capacity and commodity bundle constraints. The resulting subproblem is a variant of the well-known Multi-facility Weber Problem and it can be solved by using column generation and branch-and-price on an equivalent set covering formulation, which is accurate but extremely inefficient. Therefore, we devise different strategies to increase the efficiency. They mainly benefit from the effective usage of the lower and upper bounds on the optimal value of the Lagrangean subproblem. On the basis of extensive computational tests, we can say that they increase the efficiency considerably and result in accurate Lagrangean heuristics. 相似文献