Stability and accuracy of finite element methods for flow acoustics. I: general theory and application to one‐dimensional propagation

The dispersion properties of finite element models for aeroacoustic propagation based on the convected scalar Helmholtz equation and on the Galbrun equation are examined. The current study focusses on the effect of the mean flow on the dispersion and amplitude errors present in the discrete numerical solutions. A general two‐dimensional dispersion analysis is presented for the discrete problem on a regular unbounded mesh, and results are presented for the particular case of one‐dimensional acoustic propagation in which the wave direction is aligned with the mean flow. The magnitude and sign of the mean flow is shown to have a significant effect on the accuracy of the numerical schemes. Quadratic Helmholtz elements in particular are shown to be much less effective for downstream—as opposed to upstream—propagation, even when the effect of wave shortening or elongation due to the mean flow is taken into account. These trends are also observed in solutions obtained for simple test problems on finite meshes. A similar analysis of two‐dimensional propagation is presented in an accompanying article. Copyright © 2005 John Wiley & Sons, Ltd.     

On Formal Modeling and Validation of Wireless Sensor Network Protocols

Formal verification is becoming more and more important in the field of wireless networks (WSN). The general purpose formal method called Event-B is the latest incarnation of the B Method: it is a proof based approach with a formal notation and refinement technique for modeling and verifying systems. Refinement enables implementation level features to be proven correct with respect to an abstract specification of the system. This paper proposes an initial attempt to model and verify consistency and correctness of a WSN operation in its different layers. Several formal models are introduced for this type of networks. In the first time, coloured Petri net are used to elaborate network layer models, then each one will be detailed by an Event-B formalism, while proofs are carried out using the RODIN platform which is an integrated development framework for Event-B.

     

Numerical and experimental analysis of a gear system with teeth defects

The modelling of a one-stage spur gear transmission by a two degrees of freedom system produces two modes: rigid body and elastic. The time varying meshing stiffness is the main internal excitation source for the transmission and governs the behaviour of the elastic mode. Deterioration of one or several teeth, which affects the gear mesh stiffness, is considered in this work. The beginning of crack or spalling are modelled respectively by tooth having localised and distributed defect and are taken into account in the model. Simulation results are analysed by cepstrum and spectrum techniques. It is found that cepstrum and spectrum techniques are very efficient for localised and distributed defects, respectively. Series of tests are made in the experimental setup. Spectrum and cepstrum analysis of the recorded responses, with and without defects, are compared with numerical results and confirms their usefulness in gear monitoring .     

Characterization of Asphaltenes and Resins Separated from Water-in-Oil Emulsions

Knowledge of the properties and behavior of asphaltenes and resins is indispensable for the design of preventive and curative measure for emulsion problems created by the presence of asphaltene, resins, and other organic and inorganic solids. In order to understand the phenomena of water-oil emulsions formed in Kuwaiti oil fields and determine the factors involved in the stabilization of these emulsions, the role of asphaltenes, resins and wax separated from various samples of oil field emulsions formed in Burgan oil field have been evaluated. Physicochemical properties of asphaltenes, resins, wax, and de-asphalted de-resined (DADR) oil samples have been studied via FT-IR, ¹H, and ¹³C NMR, elemental analysis, and differential scanning calorimetry (DSC). These emulsion samples contain different amounts of water ranges from 24 to 35%, asphaltene content ranges from 0.9 to 1.7%, and resin content from 3.7 to 4.6%. IR-FT spectra were performed to identify the various functional groups which have an effect on the stability of water-oil emulsions. The freezing behavior of an emulsion was characterized by differential scanning calorimetry to determine whether the water in the emulsion is free water or emulsified water.     

Highly Enantioselective Synthesis of 3‐Hydroxy‐2‐methylpropanoic Acid Esters through Ruthenium‐SYNPHOS®‐Catalyzed Hydrogenation: Useful Building Blocks for the Synthetic Community

Both enantiomers of 3‐hydroxy‐2‐methylpropanoic acid tert‐butyl ester were prepared with high enantioselectivity (up to 94 %) through a ruthenium‐SYNPHOS®‐promoted asymmetric hydrogenation reaction using an atom‐economic transformation from simple and inexpensive precursors.     

General Enantioselective Synthesis of Butyrolactone Natural Products via Ruthenium‐SYNPHOS®‐Catalyzed Hydrogenation Reactions

Enantioselective syntheses of several paraconic acids have been achieved using catalyzed asymmetric hydrogenation of β‐keto esters with SYNPHOS^® as a ligand. This strategy allowed the short synthesis of biologically active (−)‐methylenolactocin 1 , (−)‐protolichesterinic acid 2 , (−)‐phaseolinic acid 3 and (+)‐roccellaric acid 4 .     

Energetic and exergetic parametric study of a SOFC-GT hybrid power plant

A parametric study is conducted on a hybrid SOFC-GT cycle as part of a national program aiming to improve the efficiency of the actual gas turbine power plants and to better undertake the future investigations. The proposed power plant is mainly constituted by a Gas Turbine cycle, a SOFC system, and an ammonia water absorption refrigerating system. An external pre-reformer is installed before the SOFC. Heat recovery systems are adopted to valorize the waste heat at the SOFC and GT exhausts. The gas from the SOFC exhaust is also used as additional supply for the combustion chamber. An extraction is performed on the gas Turbine in order to feed the SOFC cycle by thermal heat flux at medium pressure.The equations governing the electrochemical processes, the energy and the exergy balances of the power plant components are established. Numerical simulation using EES software is performed. The influences of key operating parameters, such as humidity, pre-reforming fraction, extraction fraction from the Gas Turbine and fuel utilization on the performances of the SOFC-GT hybrid system are analyzed. Obtained results show that the integration of the SOFC enhances significantly the hybrid overall cycle efficiency. The increase of the ambient temperature and humidity reduces the system efficiencies. The utilization factor has a negative effect on the SOFC temperature and voltage. That leads to a decrease in the power plant performances. While the pre-reforming fraction, has a positive effect on the indicated parameters.     

Role of Zinc Oxide and Nickel Nanoparticles on EMI Shielding Behavior of Chopped Carbon Fibre/Electrospun Fibrous Cobalt Epoxy Composites in X and Ku Band Microwave Frequencies

Silicon - This work investigated the microwave scattering parameters along with mechanical properties of a wave shielding magnetic nickel nanoparticles Ni-chopped carbon fibre CCF-Electrospun...     

Synthesis of Mg-stabilized nitrogen-rich alpha-sialons along Si3N4–0.5Mg3N2:3AlN line via field-assisted sintering

The present study reports for the first time the synthesis and evaluation of magnesium (Mg)-doped nitrogen-rich (N-rich) sialon ceramics exploring the possible existence of Mg-stabilized single-phase alpha-sialon (on the Mg-alpha-sialon plane). Mg-stabilized N-rich sialons, with the general formula of Mg_xSi_12−2xAl_2xN₁₆ having x value in the range of 0.2–2.2 for the composition laying along the Si₃N₄–0.5Mg₃N₂:3AlN line, were synthesized at 1500°C, using nano-size starting powder precursors and field-assisted (or spark plasma) sintering technique. Consolidated sialon ceramic samples were characterized for their microstructure, phase stability regime, and physical and mechanical properties. Although a relatively low synthesis temperature was adopted, well densified sialon samples were achieved; however, the densification of the samples became difficult with a higher x value (containing high Mg₃N₂/AlN content). Contrary to what was expected to take place, a single-phase Mg-doped sialon was not obtained near the N-rich line (on Mg-alpha-sialon plane). Such distinctive behavior in Mg-doped sialons was supposed to be due to the formation of a highly stable Mg-containing aluminum nitride polytype phase, which consumed most of the high-temperature transient liquid phase. Mg-doped N-rich sialon sample having the maximum amount of alpha phase depicted a remarkable hardness (HV₁₀) of 21.4 GPa and a fracture toughness of 3.5 MPa m^1/2.