Non-modal instabilities of two-dimensional disturbances in plane Couette flow of a power-law fluid

Instabilities of fluid flows have traditionally been investigated by normal mode analysis, i.e. by linearizing the equations of flow and testing for unstable eigenvalues of the linearized problem. However, the results of eigenvalue analysis agree poorly in many cases with experiments, especially for shear flows. In this paper we study the instabilities of two-dimensional Couette flow of a polymeric fluid in the framework of non-modal stability theory rather than normal mode analysis. A power-law model is used to describe the polymeric liquid. We focus on the response to external excitations and initial conditions by examining the pseudospectra structures and the transient energy growths. For both Newtonian and non-Newtonian flows, the results show that there can be a rather large transient growth even though the linear operator of Couette flow has no unstable eigenvalue. The effects of non-Newtonian viscosity on the transient behaviors are examined in this study. The results show that the "shear-thinning/shear-thickening" effect increases/decreases the amplitude of responses to external excitations and initial conditions. (C) 2010 Elsevier B.V. All rights reserved.

Modal correction for fiber-coupling efficiency in free-space optical communication systems through atmospheric turbulence

High-speed free-space optical communication systems have recently used fiber-optical components. The coupling efficiency with which the received laser beam can be coupled into a single-mode fiber is noticeably limited by atmospheric turbulence due to the degradation of its spatial coherence. Fortunately, adaptive optics (AO) can alleviate this limitation by partially correcting the turbulence-distorted wavefront. The coupling efficiency improvement provided by Zernike modal AO correction is numerically evaluated. It is found that the first 3-20 corrected polynomials can considerably improve the fiber-coupling efficiency. The improvement brought by AO is compared with that brought by a coherent fiber array. Finally, a hybrid technique that integrates AO and a coherent fiber array is proposed. Results show that the hybrid technique outperforms each of the two above-mentioned techniques. (C) 2009 Elsevier GmbH. All rights reserved.

Selecting ionization path by dynamic stark shift with strong laser pulse

Multiphoton ionization of NO via intermediate Rydberg states with ultra-short laser pulses is investigated with time-resolved photoelectron spectroscopy in combination with fermosecond pump-probe technology. The Rydberg states of NO, which are characterized by obvious ac-Stark shift in ultra-strong laser field, can be tuned in resonance to ionize NO molecule at one's will with identical laser pulses, i.e., one can 'select' resonance path to ionization. The results shown in this Letter demonstrate that the states holding notable dynamic Stark shift provide us another dimension to chemical control with strong laser field. (C) 2003 Elsevier Science B.V. All rights reserved.

Configuration-dependent diffusion can shift the kinetic transition state and barrier height of protein folding

We show that diffusion can play an important role in protein-folding kinetics. We explicitly calculate the diffusion coefficient of protein folding in a lattice model. We found that diffusion typically is configuration- or reaction coordinate-dependent. The diffusion coefficient is found to be decreasing with respect to the progression of folding toward the native state, which is caused by the collapse to a compact state constraining the configurational space for exploration. The configuration- or position-dependent diffusion coefficient has a significant contribution to the kinetics in addition to the thermodynamic free-energy barrier. It effectively changes (increases in this case) the kinetic barrier height as well as the position of the corresponding transition state and therefore modifies the folding kinetic rates as well as the kinetic routes. The resulting folding time, by considering both kinetic diffusion and the thermodynamic folding free-energy profile, thus is slower than the estimation from the thermodynamic free-energy barrier with constant diffusion but is consistent with the results from kinetic simulations. The configuration- or coordinate-dependent diffusion is especially important with respect to fast folding, when there is a small or no free-energy barrier and kinetics is controlled by diffusion.Including the configurational dependence will challenge the transition state theory of protein folding.

The relationship between chemical bond properties and Stokes shift of Eu2+ in some silicate host lattices

The covalency of each bond in divalent europium doped hosts CaSiO3, SrSiO3, BaSiO3, Sr2LiSiO4F, Ba5SiO4Cl6 and Ba5SiO4Br6 were calculated by using the complicate crystal chemical bond theory. The relationship between the Stokes shift and the bond properties of Eu2+ in these crystals was discussed. The result demonstrates that, in the isostructural crystals that being doped with Eu2+, there is a more precise connection between the magnitude of Stokes shift and the mean covalency of the dopant site.

Study on the structure change and oxygen vacation shift for Ce1-xSmxO2-y solid solution

A series of solid electrolytes Ce1-xSmxO2-y (x=0similar to0.6) were prepared by sol-gel method. XRD measurement showed that single-phase solid solution was formed in all investigated ranges at 160 degreesC, which is a significantly lower synthesis temperature compared to traditional solid state reaction. High temperature X-ray, ESR, and Raman scattering were used to characterize the samples. ESR measurement showed that ESR with sample irradiated by high-energy particle is an effective way to study the defect structure. These changes in the Raman spectrum are attributed to O vacancies, which are introduced into the lattice when tetravalent Ce4+ is substituted by trivalent Sm3+.

A novel structure analysis method for lanthanide complex - None paramagnetic shift tri-lanthanide mixture method

A new structure analysis method for lanthanide complexes was proposed, that is, none paramagnetic shift tri-lanthanide mixture method, It was found that the paramagnetic induced shift could be cancelled by mixing three kinds of paramagnetic lanthanide ions in appropriate proportion. As a result, the chelating sites would he seen simplely from the half widths and the relative distances between lanthanide ion and the ligand nucleus could be calculated from the relaxation time (T-1) or the half width. Care should be addressed that the analysis method is suitable for the systems in which intramolecular arrangements and intermolecular ligand exchanges are relatively fasten NMR time scale used.

LANTHANIDE-INDUCED SHIFT AND RELAXATION RATE STUDIES OF THE AQUEOUS COMPLEXATION OF CITRATE

The aqueous complexation of lanthanide ions with citrate in pH 7.4 solution has been investigated with use of the lanthanide-induced shift and paramagnetic relaxation rate enhancement methods. The results show that citrate coordinates via hydroxyl and central carboxylate groups with lanthanide ions and forms 1:2 (Ln/cit) isostructural complexes through the lanthanide series. A new possible coordination geometry deduced from our experimental data is suggested and discussed.

CHARACTERIZATION OF A POTASSIUM-PROMOTED COBALT MOLYBDENUM ALUMINA WATER-GAS SHIFT CATALYST

A series of potassium-promoted CoMo/Al2O3 has been investigated by means of X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and temperature-programmed reduction (TPR). CoMoO4 was found in the CoMo/Al2O3 catalyst by XRD and is destroyed by the presence of potassium. The reducibility of molybdenum is enhanced by potassium in the CoMoK/Al2O3 catalyst and is easier to reduce to Mo(IV) during sulfidation. In the oxidic state catalyst cobalt is increased on the surface by the addition of potassium. After sulfidation this phenomena disappeared, the distribution of cobalt remains at a constant level and is unaffected by the potassium content. The addition of potassium leads to a monotonical decrease of the molybdenum dispersion with the impregnating amount of potassium in the oxidic state catalyst but is more complicated after sulfidation. Potassium is well dispersed on the surface in both the oxidic and sulfided state. The activity in the water-gas shift reaction was correlated with the potassium content of CoMoK/Al2O3.

Diet shift of upland buzzards (Buteo hemilasius): evidence from stable carbon isotope ratios

We measured the stable carbon isotope ratios for muscle of the upland buzzards (Buteo hemilasius), plateau pika (Ochotoma curzoniae), root vole (Microtus oeconomus), plateau zokor (Myospalax fontanierii) and passerine bird species at the Haibei Alpine Meadow Ecosystem Research Station (HAMERS), and provided diet information of upland buzzards with the measurement of stable carbon isotopes in tissues of these consumers. The results showed that δ~(13)C values of small mammals and passerine bird species ranged from -25.57‰ to -25.78‰ (n = 12), and from -24.81‰ to -22.51% (n = 43), respectively, δ~(13)C values of the upland buzzards ranged from -22.60‰ to -23.10‰ when food was not available. The difference in δ~(13)C values (2.88‰±0.31‰) between upland buzzards and small mammals was much larger than the differences reported previously, 1‰-2‰, and showed significant difference, while 1.31‰±0.34‰ between upland buzzard and passerine bird species did not differ from the previously reported trophic fractionation difference of 1‰-2‰. Estimation of trophic position indicated that upland buzzards stand at trophic position 4.23, far from that of small mammals, i.e., upland buzzards scarcely captured small mammals as food at the duration of food shortage. According to isotope mass balance model, small mammals contributed 7.89% to 35.04% of carbon to the food source of the upland buzzards, while passerine bird species contributed 64.96% to 92.11%. Upland buzzards turned to passerine bird species as food during times of shortage of small mammals. δ~(13)C value, a useful indicator of diet, indicates that the upland buzzards feed mainly on passerine bird species rather than small mammals due to "you are what you eat" when small mammal preys are becoming scarce.