Propagation in the earth-ionosphere waveguide by the multiscaling method

Summary A problem of propagation of modulated high-frequency signals in a waveguide of nonconstant height and nonconstant wall impedance is analysed. A multiple-scale method is used, which reduces the three-dimensional problem to a twodimensional problem for each mode. The modal propagation problem is analysed by the ray method, giving rise to phase and group velocities, and showing the possibility of focusing effects due to variation in height and wall impedance. Mode coupling effects are shown and mode coupling coefficients calculated. This work was done while the author was visiting at the Department of Mathematics, Rensselaer Polytechnic Institute, Troy, New York, and was supported by NSF Grant No. GU 2605.     

Consideration of constraints within the finite element method by means of matrix operators

The following paper describes the incorporation of different constraints into a finite element system by means of matrix operators in conjunction with consecutive corresponding transformations. Instead of increasing the number of equations-as e.g. the Lagrange Multiplier Method^{10, 14} does-the Matrix Operator Method yields a set of reduced magnitude which can be solved more efficiently. The method will be developed for two classes of constraints: (i) stiff coupling of previously known subdomains and (ii) contact problem between two bodies. The assembly rules to obtain the system matrices are deduced. An application is given by a three-dimensional example of structural analysis in mechanical engineering.     

Passing of instability points by applying a stabilized Newton–Raphson scheme to a finite element formulation: Comparison to arc-length method

In the vicinity of limit and bifurcation points the global stiffness matrix of a finite element formulation becomes ill-conditioned and at the critical point singular. This disturbs the convergence behavior of the standard Newton–Raphson scheme as well as the arc-length method. The stabilization procedure suggested solves the numerical defects and is thus able to pass critical points. Bifurcation points are passed on the primary path. Branch switching to the secondary path is done automatically. The stabilization procedure and the imperfection force are derived based on the eigenvalues and -vectors of the structure.     

Delamination of FRP-reinforced concrete by means of an extended finite element formulation

The present analysis focusses on the finite element modeling of delamination tests in FRP-reinforced concrete blocks using a continuous–discontinuous XFEM approach, which takes into direct consideration the mechanical properties of concrete, adhesive and FRP. Both FRP and adhesive layers are considered as linear elastic materials, while the constitutive behavior of the concrete is governed by an elasto-damaging constitutive law. As soon as a critical damage threshold is reached in the concrete, additional degrees of freedom, representative of the displacement discontinuity corresponding to the delamination process, are added. The proposed approach only requires the use of material parameters of direct mechanical meaning, which can be obtained by standard testing procedures. Hence the use of one-dimensional bond–stress–slip laws, whose parameters must be identified through a cumbersome procedure, appears to be unnecessary. Validation of the proposed approach is carried out by comparison of the computed results with two sets of experimental results. In both cases, after adoption of a simple damage criterion, structural behavior, slip, strain and stress profiles along the reinforcement are satisfactorily reproduced. Moreover, bond–stress–slip laws, which agree with those assumed in previous works based on the use of interface elements, were obtained retrospectively.     

Delamination of FRP-reinforced concrete by means of an extended finite element formulation

The present analysis focusses on the finite element modeling of delamination tests in FRP-reinforced concrete blocks using a continuous–discontinuous XFEM approach, which takes into direct consideration the mechanical properties of concrete, adhesive and FRP. Both FRP and adhesive layers are considered as linear elastic materials, while the constitutive behavior of the concrete is governed by an elasto-damaging constitutive law. As soon as a critical damage threshold is reached in the concrete, additional degrees of freedom, representative of the displacement discontinuity corresponding to the delamination process, are added. The proposed approach only requires the use of material parameters of direct mechanical meaning, which can be obtained by standard testing procedures. Hence the use of one-dimensional bond–stress–slip laws, whose parameters must be identified through a cumbersome procedure, appears to be unnecessary. Validation of the proposed approach is carried out by comparison of the computed results with two sets of experimental results. In both cases, after adoption of a simple damage criterion, structural behavior, slip, strain and stress profiles along the reinforcement are satisfactorily reproduced. Moreover, bond–stress–slip laws, which agree with those assumed in previous works based on the use of interface elements, were obtained retrospectively.     

Uncovering the hidden content of layered documents by means of photoacoustic imaging

We introduce a novel diagnostic approach for the uncovering of hidden content in layered documents, based on a photoacoustic imaging set‐up with optimised acoustic detection parameters. By exploiting the laser‐induced ultrasound following the intense absorption of pulsed optical radiation by printed ink, this traditional biomedical imaging method is proven capable of revealing text characters buried under several layers of stacked paper sheets at high spatial resolution and excellent contrast levels. Such a remarkable performance benefits from the effective combination of strong light scattering by paper fibrils and the virtually unobstructed propagation of the generated ultrasonic waves through multiple layers, enabling thus the accurate recording of text, even in double‐sided prints, with minimum shadowing artefacts. The imaging effectiveness, the implementation simplicity, the robustness, and the relatively low‐cost features offered by the proposed photoacoustic modality are anticipated to contribute in its wide adoption by the preservation of cultural heritage community, as a powerful diagnostic tool for the in‐depth investigation of complex multilayered objects.     

Simultaneous imaging and chemical attack of a single living cell within a confluent cell monolayer by means of scanning electrochemical microscopy

Epithelial cell monolayers from rat kidney were imaged by scanning electrochemical microscopy (SECM) with sub-micrometer resolution in both lateral and vertical direction. Platinum disk ultra-microelectrodes (UMEs) with effective electrode radii between 200 and 600 nm were operated in the constant-height mode. The quality of the recorded SECM images compare favorably with those of phase contrast and confocal laser scanning microscopy. Besides the acquisition of SECM images, the UME was used to selectively attack a single living cell within the monolayer ensemble. Hydroxide ions were locally generated in the vicinity of a single target cell by the UME. The increase in pH induced cell necrosis that was subsequently imaged by SECM. It could be clearly demonstrated that the single target cell was selectively affected, whereas the adjacent reference cells remained unchanged.     

Efficacy of capsicum oleoresin nanocapsules formulation by the modified emulsion-diffusion method

In this study, we investigated the effect of high pressure homogenizer on the physico-chemical properties of capsicum oleoresin loaded nanoemulsion (NE) or nanocapsules (NCs) based on the emulsion-diffusion method. According to the application stage of high pressure process at principle emulsion-diffusion method, NCs was prepared by conventional-emulsion-diffusion method (CED), modified-emulsion-microfluidization-diffusion method (MEMD) and modified-emulsion-diffusion-microfluidization method (MEDM). The nanocapsules of MEMD showed homogeneous and the smallest particle size as compared with CED. In addition, MEMD presented the surface tension at the value 36.5 mN/m. The encapsulated capsicum oleoresin was generated the bright color and suppressed the dark red color. Furthermore, MEMD gave the high encapsulation efficiency of capsicum oleoresin around 95% and showed the slow release rate. On the other hand, MEDM presented the non-homogeneous and agglomerate of the particle, low percentage of encapsulation efficiency and the high initial release rate when compared with CED and MEMD methods. According these results, it was supposed that the microfluidization was interesting technique to ameliorate the physical properties and efficiency of NCs. However, it was depending on the appropriate combination of microfluidization based on the emulsion-diffusion method.     

Application of the Newton method to the so-called integral equation method in transonic flow

Summary A suitable approximation of the Newton method and the complete Newton method are applied to the so-called integral equation method in transonic flow. Results of example calcultion are compared. At first the methods are explained using the example of an ordinary differential equation of first order occurring in the field of hypersonic flow with a connection to the method of Picard-Lindelöf.