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排序方式: 共有469条查询结果,搜索用时 15 毫秒
111.
The linear stability of boundary-layer flow over a viscoelastic-layer wall is considered. A companion matrix technique is used to formulate the stability problem as a linear matrix eigenvalue problem for complex frequency and all the eigenvalues may be determined without any initial guess values. The eigenvalues are compared with those obtained with an accurate shooting method. The instability character of the boundary-layer flow is further investigated with the purpose of finding the conditions under which the instability of the flow could become absolute. The mapping technique of Kupferet al. (1987) is used to identify the occurrence of absolute instability eigenvalues. Absolute instabilities are discovered for cases of soft damped wall over certain ranges of Reynolds number. The effects of wall material stiffness, damping coefficient, thickness of layer, and Reynolds number on the occurrence of absolute instability are examined and presented. 相似文献
112.
Li Pan Xudong Chen Swee Ping Yeo 《Applied Physics A: Materials Science & Processing》2010,101(1):143-146
In this paper, we demonstrate how subwavelength images of nanoscale structures can be obtained from the measurements of electromagnetic
fields scattered by the objects, via an enhanced inverse scattering approach, which takes into account the multiple-scattering
effect and allows the detectors to be placed in the far field. Specifically, the method is a combination of the transverse
electric (TE) incidence and the subspace-based optimization method (SOM). It is observed that a wide continuous range of integer
values of leading singular values can yield satisfactory reconstruction results, and even if the antennas for radiating and
receiving the electromagnetic wave can only be arranged on one side of the scatterers, due to the limitation in the real world,
the proposed method is still capable of achieving high resolution for the reconstructed patterns with rapid convergence rates
and robust immunity to high noise corruption (33%). 相似文献
113.
The present paper presents a hybrid meshfree-and-Cartesian grid method for simulating moving body incompressible viscous flow problems in 3D space. The method combines the merits of cost-efficient and accurate conventional finite difference approximations on Cartesian grids with the geometric freedom of generalized finite difference (GFD) approximations on meshfree grids. Error minimization in GFD is carried out by singular value decomposition (SVD). The Arbitrary Lagrangian–Eulerian (ALE) form of the Navier–Stokes equations on convecting nodes is integrated by a fractional-step projection method. The present hybrid grid method employs a relatively simple mode of nodal administration. Nevertheless, it has the geometrical flexibility of unstructured mesh-based finite-volume and finite element methods. Boundary conditions are precisely implemented on boundary nodes without interpolation. The present scheme is validated by a moving patch consistency test as well as against published results for 3D moving body problems. Finally, the method is applied on low-Reynolds number flapping wing applications, where large boundary motions are involved. The present study demonstrates the potential of the present hybrid meshfree-and-Cartesian grid scheme for solving complex moving body problems in 3D. 相似文献
114.
115.
Lysophosphatidic acid (LPA) is a phospholipid growth factor that acts through G-protein-coupled receptors. Previously, we demonstrated an altered profile of LPA-dependent cAMP content during the aging process of human diploid fibroblasts (HDFs). In attempts to define the molecular events associated with the age-dependent changes in cAMP profiles, we determined the protein kinase A (PKA) activity, phosphorylation of cAMP-response element binding protein (CREB), and the protein expression of CRE-regulatory genes, c-fos and COX-2 in young and senescent HDFs. We observed in senescent cells, an increase in mRNA levels of the catalytic subunit a of PKA and of the major regulatory subunit Ialpha. Senescence-associated increase of cAMP after LPA treatment correlated well with increased CREB phosphorylation accompanying activation of PKA in senescent cells. In senescent cells, after LPA treatment, the expression of c-fos and COX-2 decreased initially, followed by an increase. In young HDFs, CREB phosphorylation decreased following LPA treatment, and both c-fos and COX-2 protein levels increased rapidly. CRE-luciferase assay revealed higher basal CRE-dependent gene expression in young HDFs compared to senescent HDFs. However, LPA-dependent slope of luciferase increased more rapidly in senescent cells than in young cells, presumably due to an increase of LPA-induced CREB phosphorylation. CRE-dependent luciferase activation was abrogated in the presence of inhibitors of PKC, MEK1, p38MAPK, and PKA, in both young and senescent HDFs. We conclude that these kinase are coactivators of the expression of CRE-responsive genes in LPA-induced HDFs and that their changed activities during the aging process contribute to the final expression level of CRE-responsive genes. 相似文献
116.
Rohde RD Agnew HD Yeo WS Bailey RC Heath JR 《Journal of the American Chemical Society》2006,128(29):9518-9525
A general method for the non-oxidative functionalization of single-crystal silicon(111) surfaces is described. The silicon surface is fully acetylenylated using two-step chlorination/alkylation chemistry. A benzoquinone-masked primary amine is attached to this surface via Cu(I)-catalyzed Huisgen 1,3-dipolar cycloaddition ("click" chemistry). The benzoquinone is electrochemically reduced, resulting in quantitative cleavage of the molecule and exposing the amine terminus. Molecules presenting a carboxylic acid have been immobilized to the exposed amine sites. X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FTIR), cyclic voltammetry (CV), and contact angle goniometry were utilized to characterize and quantitate each step in the functionalization process. This work represents a strategy for providing a general platform that can incorporate organic and biological molecules on Si(111) with minimal oxidation of the silicon surface. 相似文献
117.
Effect of acid on the aggregation of poly(ethylene xide)-poly(propylene oxide)-poly(ethylene oxide) block copolymers 总被引:1,自引:0,他引:1
Yang B Guo C Chen S Ma J Wang J Liang X Zheng L Liu H 《The journal of physical chemistry. B》2006,110(46):23068-23074
The acid effect on the aggregation of poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) block copolymers EO(20)PO(70)EO(20) has been investigated by transmission electron microscopy (TEM), particle size analyzer (PSA), Fourier transformed infrared, and fluorescence spectroscopy. The critical micellization temperature for Pluronic P123 in different HCl aqueous solutions increases with the increase of acid concentration. Additionally, the hydrolysis degradation of PEO blocks is observed in strong acid concentrations at higher temperatures. When the acid concentration is low, TEM and PSA show the increase of the micelle mean diameter and the decrease of the micelle polydispersity at room temperature, which demonstrate the extension of EO corona and tendency of uniform micelle size because of the charge repulsion. When under strong acid conditions, the aggregation of micelles through the protonated water bridges was observed. 相似文献
118.
The non-isothermal degradation of poly(3-hydroxybutyrate) (PHB) and silver sulfide/poly(3-hydroxybutyrate) (Ag2S/PHB) nanocomposites was investigated using thermogravimetric (TG) analysis. In the composite materials, Ag2S caused the degradation of PHB at a lower temperature as opposed to that of neat PHB. Moreover, an increase Ag2S loading in the PHB decreased the onset temperature (Tonset) of thermal degradation, whereas it was raised upon augmenting the heating rate. From Kissinger plots, the observed trend of the degradation activation energy, Ed, was attributed to polymer-particle surface interactions and the agglomeration of Ag2S. The thermal degradation rate constant, k, was linearly related to the Ag2S loading in PHB. Thus, the Ag2S nanoparticles effectively catalyzed the thermal degradation of PHB in the Ag2S/PHB nanocomposites. Differential scanning calorimetry (DSC) data also supported the catalytic property of Ag2S. 相似文献
119.
Ballmann J Yeo A MacKay BA van Rijt S Patrick BO Fryzuk MD 《Chemical communications (Cambridge, England)》2010,46(46):8794-8796
The side-on end-on dinitrogen complex [PhP(CH(2)SiMe(2)NPh)(2)Ta](μ-H)(2)(μ-η(2):η(1)-N(2)) reacts with CS(2) with complete cleavage of both C=S double bonds and the formation of [PhP(CH(2)SiMe(2)NPh)(2)Ta](μ-S)(2)(μ-CH(2)), which has two bridging sulfides and a bridging methylene unit. Further reaction with H(2) produces CH(4) and the disulfide complex. 相似文献
120.