We consider the stability of parallel server systems under the longest queue first (LQF) rule. We show that when the underlying
graph of a parallel server system is a tree, the standard nominal traffic condition is sufficient for the stability of that
system under LQF when interarrival and service times have general distributions. Then we consider a special parallel server
system, which is known as the X-model, whose underlying graph is not a tree. We provide additional “drift” conditions for
the stability and transience of these queueing systems with exponential interarrival and service times. Drift conditions depend
in general on the stationary distribution of an induced Markov chain that is derived from the underlying queueing system.
We illustrate our results with examples and simulation experiments. We also demonstrate that the stability of the LQF depends
on the tie-breaking rule used and that it can be unstable even under arbitrary low loads. 相似文献
This study compared the influence of various surface treatments on the shear bond strength between resin cement and lithium disilicate glass-ceramics. A series of 120 lithium disilicate ceramic samples were prepared to compare the effect of different surface treatments on the shear strength of a luting cement bonded to two all-ceramic systems. IPS Empress 2 and IPS e.max Press ceramic samples were fabricated according to the manufacturer's instructions. The ceramic samples were divided into the following 6 surface treatment groups for each ceramic system: 1—no treatment (C), 2—airborne-particle abrasion (A), 3—acid etching (E), 4—airborne-particle abrasion + acid etching (AE), 5—Nd:YAG laser (L), 6—Nd:YAG laser + acid etching (LE). Resin cement was then bonded to the treated ceramic surfaces and light polymerized. The shear bond strengths of the specimens were measured using a universal testing machine. Two-way ANOVA and Tukey HSD (α = 0.05) test were used to determine differences in shear bond strength between the groups. The ANOVA revealed significant differences between the treatment groups and ceramic types (p < 0.05). The shear bond strengths of IPS Empress 2 were significantly higher than those of IPS e.max Press. 相似文献
The objective of this paper is to present and to validate a new hybrid coupling (HC) algorithm for modeling of fluid-structure interaction (FSI) in incompressible, viscous flows. The HC algorithm is able to avoid numerical instability issues associated with artificial added mass effects, which are often encountered by standard loosely coupled (LC) and tightly coupled (TC) algorithms, when modeling the FSI response of flexible structures in incompressible flow. The artificial added mass effect is caused by the lag in exchange of interfacial displacements and forces between the fluid and solid solvers in partitioned algorithms. The artificial added mass effect is much more prominent for light/flexible structures moving in water, because the fluid forces are in the same order of magnitude as the solid forces, and because the speed at which numerical errors propagate in an incompressible fluid. The new HC algorithm avoids numerical instability issues associated with artificial added mass effects by embedding Theodorsen’s analytical approximation of the hydroelastic forces in the solution process to obtain better initial estimates of the displacements. Details of the new HC algorithm are presented. Numerical validation studies are shown for the forced pitching response of a steel and a plastic hydrofoil. The results show that the HC algorithm is able to converge faster, and is able to avoid numerical instability issues, compared to standard LC and TC algorithms, when modeling the transient FSI response of a plastic hydrofoil. Although the HC algorithm is only demonstrated for a NACA0009 hydrofoil subject to pure pitching motion, the method can be easily extended to model general 3-D FSI response and stability of complex, flexible structures in turbulent, incompressible, multiphase flows. 相似文献
Plane strain analytical solutions to estimate purely elastic, partially plastic and fully plastic deformation behavior of
rotating functionally graded (FGM) hollow shafts are presented. The modulus of elasticity of the shaft material is assumed
to vary nonlinearly in the radial direction. Tresca’s yield criterion and its associated flow rule are used to formulate three
different plastic regions for an ideal plastic material. By considering different material compositions as well as a wide
range of bore radii, it is demonstrated in this article that both the elastic and the elastoplastic responses of a rotating
FGM hollow shaft are affected significantly by the material nonhomogeneity. 相似文献
Two 5-hydroxyisophthalate complexes of nickel(II), formulated as [Ni(μ-Hhip)(2-hepy)2]n (1) and [Ni2(μ-Hhip)2(dap)4]n (2) (H3hip = 5-hydroxyisophthalic acid, 2-hepy = 2-(2-hydroxyethyl)pyridine, dap = 1,3-diaminopropane), have been synthesized and characterized by chemical and spectroscopic methods. The molecular structures of the complexes have been determined by single-crystal X-ray diffraction analysis. The Ni(II) centres have distorted octahedral geometries in both crystals. Furthermore, both complexes have 1D chain structures in which the individual chains are linked together via hydrogen bonds to give 3D frameworks. Evaluation of the complexes by the agar diffusion method showed that they have weak antibiotic activities against the tested microorganisms. 相似文献
A two‐step solution processing approach has been established to grow void‐free perovskite films for low‐cost high‐performance planar heterojunction photovoltaic devices. A high‐temperature thermal annealing treatment was applied to drive the diffusion of CH3NH3I precursor molecules into a compact PbI2 layer to form perovskite films. However, thermal annealing for extended periods led to degraded device performance owing to the defects generated by decomposition of perovskite into PbI2. A controllable layer‐by‐layer spin‐coating method was used to grow “bilayer” CH3NH3I/PbI2 films, and then drive the interdiffusion between PbI2 and CH3NH3I layers by a simple air exposure at room temperature for making well‐oriented, highly crystalline perovskite films without thermal annealing. This high degree of crystallinity resulted in a carrier diffusion length of ca. 800 nm and a high device efficiency of 15.6 %, which is comparable to values reported for thermally annealed perovskite films. 相似文献
The natural radioactivity levels and some radiological parameters of Turkish Portland cements (PC) originated in various regions were determined in this study. The activity concentration of cement samples for 226Ra, 232Th, and 40K were measured using a gamma-ray spectrometer with high purity germanium radiation detector. The PC samples had activity concentrations of 33.0, 16.7, and 239.5 Bq kg−1 for 226Ra, 232Th, and 40K, respectively. The mean value of radium equivalent value (Raeq) was found to be 75.4 Bq kg−1. The radium equivalent values in the cement samples were lower than the acceptable level of 370 Bq kg−1. The calculated radiological parameters were found to be below the acceptance levels.
In this study, production and optimisation of rosmarinic acid, a phenolic acid and an economically important metabolite, was investigated in the callus cultures established from the mature seeds of Satureja hortensis L. (summer savory) plant. Gamborg's B5 basal medium, supplemented with indol butyric acid (IBA) (1.00 mg L(-1)), N6-benzyl aminopurine (6-BA) (1.00 mg L(-1)) and sucrose (2.5%, w/v), was employed for the establishment and maintenance of the callus cultures. Applications were individually prepared by preparing the media containing different IBA/6-BA combinations and sucrose concentrations. All of the applications were carried out in the continuous dark. In the applications, where the effects of IBA/6-BA combinations on the growth and rosmarinic acid accumulation were assayed (1-15 applications), the highest biomass yield was obtained from the medium supplemented with 1.00 mg L(-1) IBA and 5.00 mg L(-1) 6-BA. In the case of the rosmarinic acid accumulation, an opposite relationship was determined between the growth and rosmarinic acid production. While the highest biomass yield was obtained from the medium containing 1.00 mg L(-1) IBA and 5.00 mg L(-1) 6-BA, the highest rosmarinic acid accumulation was obtained from the medium supported with 1.00 mg L(-1) IBA and 1.00 mg L(-1) 6-BA. In the applications where the effects of sucrose concentrations on the growth and rosmarinic acid accumulation were examined, the highest biomass yield was obtained from the medium which is supplemented with 5.0% (w/v) sucrose. In this category, the highest rosmarinic acid accumulation was obtained from the medium which is supported with 3.0% (w/v) sucrose. According to the experiments carried out with the wild S. hortensis, it is found to have 25.02+/-1.21 mg g(-1) rosmarinic acid. No differentiation was observed in any callus during the course of this study. 相似文献
The European Physical Journal C - A generalized theory of electroweak interaction is developed based on the underlying geometrical structure of the tangent bundle with symmetries arising from... 相似文献