bluuebunny/crossref_metadata_2025 huggingface dataset:
Neural crest development is regulated by the transcription factor Sox9
Authors: Cheung Martin , Briscoe James
Published: October 2003

The neural crest is a transient migratory population of stem cells derived from the dorsal neural folds at the border between neural and non-neural ectoderm. Following induction, prospective neural crest cells are segregated within the neuroepithelium and then delaminate from the neural tube and migrate into the periphery, where they generate multiple differentiated cell types. The intrinsic determinants that direct this process are not well defined. Group E Sox genes (Sox8, Sox9 and Sox10)are expressed in the prospective neural crest and Sox9 expression precedes expression of premigratory neural crest markers. Here, we show that group E Sox genes act at two distinct steps in neural crest differentiation. Forced expression of Sox9 promotes neural-crest-like properties in neural tube progenitors at the expense of central nervous system neuronal differentiation. Subsequently, in migratory neural crest cells, SoxE gene expression biases cells towards glial cell and melanocyte fate, and away from neuronal lineages. Although SoxE genes are sufficient to initiate neural crest development they do not efficiently induce the delamination of ectopic neural crest cells from the neural tube consistent with the idea that this event is independently controlled. Together, these data identify a role for group E Sox genes in the initiation of neural crest development and later SoxE genes influence the differentiation pathway adopted by migrating neural crest cells.
Segment boundary formation inDrosophilaembryos
Authors: Larsen Camilla W. , Hirst Elizabeth , Alexandre Cyrille , Vincent Jean-Paul
Published: October 2003

In Drosophila embryos, segment boundaries form at the posterior edge of each stripe of engrailed expression. We have used an HRP-CD2 transgene to follow by transmission electron microscopy the cell shape changes that accompany boundary formation. The first change is a loosening of cell contact at the apical side of cells on either side of the incipient boundary. Then, the engrailed-expressing cells flanking the boundary undergo apical constriction, move inwards and adopt a bottle morphology. Eventually,grooves regress, first on the ventral side, then laterally. We noted that groove formation and regression are contemporaneous with germ band retraction and shortening, respectively, suggesting that these rearrangements could also contribute to groove morphology. The cellular changes accompanying groove formation require that Hedgehog signalling be activated, and, as a result, a target of Ci expressed, at the posterior of each boundary (obvious targets like stripe and rhomboid appear not to be involved). In addition, Engrailed must be expressed at the anterior side of each boundary,even if Hedgehog signalling is artificially maintained. Thus, there are distinct genetic requirements on either side of the boundary. In addition,Wingless signalling at the anterior of the domains of engrailed (and hedgehog) expression represses groove formation and thus ensures that segment boundaries form only at the posterior.
The optimization of electric power assisted steering to improve vehicle performance
Authors: Liu Zhao , Yang Jiajun , Liao Daoxun
Published: October 2003

To investigate the e ects of electric power assisted steering (EPAS) on a vehicle's steering performance, this paper explicitly proposes several indices, such as steering sensitivity and road feel, for evaluation of steering performance. Then a single-track car model with lateral and yaw movement is considered and an EPAS model is built. The mathematical expressions for the steering performance indices are deduced and a stability criterion is established by integrated analysis of the vehicle's dynamics and the EPAS dynamics. By means of frequency domain analysis, the e ect of each parameter of EPAS on steering performance indices is analysed. Finally, an optimizing model used to improve these steering indices is put forward and the result is given.
Structural optimization concept for the design of an aluminium control arm
Authors: Lee Dong-Chan , Lee Jeong-Ick
Published: October 2003

The development of a mechanical structure needs to be started from the conceptual design with low cost, high performance and quality. In this regard, the structural and topological shape of the system has a great e ect on the performance in terms of rigidity, strength and mass reduction. In this paper, optimization design methodologies in the design stages of an aluminium control arm for a suspension are presented. First, using topology optimization, the optimal layout and the reinforcement structure are obtained, and then the detail designs are carried out using shape optimization for the structural rigidity and strength. In comparison with a steel control arm, the mass reduction is 50 per cent and the structural rigidity and strength are improved up to 40 per cent.
Analysis of automotive disc brake cooling characteristics
Authors: Voller G P , Tirovic M , Morris R , Gibbens P
Published: October 2003

The aim of this investigation was to study automotive disc brake cooling characteristics experimentally using a specially developed spin rig and numerically using finite element (FE) and computational fluid dynamics (CFD) methods. All three modes of heat transfer (conduction, convection and radiation) have been analysed along with the design features of the brake assembly and their interfaces. The spin rig proved to be very valuable equipment; experiments enabled the determination of the thermal contact resistance between the disc and wheel carrier. The analyses demonstrated the sensitivity of this mode of heat transfer to clamping pressure. For convective cooling, heat transfer coefficients were measured and very similar results were obtained from spin rig experiments and CFD analyses. The nature of radiative heat dissipation implies substantial e ects at high temperatures. The results indicate substantial change of emissivity throughout the brake application. The influence of brake cooling parameters on the disc temperature has been investigated by FE modelling of a long drag brake application. The thermal power dissipated during the drag brake application has been analysed to reveal the contribution of each mode of heat transfer.
Visualization study of natural gas direct injection combustion
Authors: Huang Z , Shiga S , Ueda T , Nakamura H , Ishima T , Obokata T , Tsue M , Kono M
Published: October 2003

A visualization study of natural gas direct injection combustion was carried out by using a high speed video camera. The results show that the distribution of the stratified mixture di ers with the injection mode, with parallel and single injection tending to form a higher degree of mixture stratification than opposed injection. Flame propagates toward the downstream direction in the cases of parallel and single-injection combustion, and flame propagates outward from the centre of the combustion chamber in the case of opposed injection combustion. A characteristic of turbulent combustion with a wrinkled flame front is presented in natural gas direct injection combustion. Super-lean combustion can be realized owing to the formation of an ignitable stratified mixture with the optimum setting of the fuel injection timing.
A novel approach to disturbance rejection in idle speed control towards reduced idle fuel consumption
Authors: Manzie C , Watson H C
Published: October 2003

Idle speed control remains one of the most challenging problems in the automotive control field owing to its multiple-input, multiple-output structure and the step nature of the disturbances applied. In this paper a simulation model is described for a 4.0 l production engine at idle which includes the standard bypass air valve and spark advance dynamics, as well as the e ects of operating point on cycle-by-cycle combustion-generated torque variations. A model predictive control scheme is then developed for the idle bypass valve and spark advance. The idle speed control algorithm is based on rejecting the torque disturbance using model predictive control for the bypass valve duty cycle while minimizing the transient e ects of the disturbance by adjusting the spark advance. Simulation results are presented to demonstrate the effects of different elements of the controller such as levels of spark offset from minimum spark advance for best torque at idle and feedforward load previews. Compensation of the effects of cyclic variation in combustion torque is also implemented in the controller and its benefits are discussed.
Development of a low-cost piezo film-based knock sensor
Authors: Pipitone E , D'Acquisto L
Published: October 2003

It is well known that spark advance is a key parameter in spark ignition engine management. Increasing fuel cost and emission regulation strictness require a higher engine efficiency, which can be improved by an accurate regulation of the spark advance. Under high load conditions, an optimal spark advance choice leads the engine to run next to the knock limit, so the management and control system needs to be equipped with a knock sensor in order to preserve the engine from damage. The authors developed a low-cost knock sensor whose sensing element is a thin washer of polyvinylidine fluoride (PVDF), a fluoropolymer characterized by a great piezoelectric e ect if polarized. The sensor has been tested on a spark ignition CFR engine (the standard single-cylinder test engine used by ASTM for octane number determination of spark ignition engine fuel) and compared with a commercial accelerometer and a pressure sensor, in terms of knocking detection capability, measured knock intensity (KI) and signal-to-noise ratio (SNR). Knocking tests have also been carried out on a Renault series production engine. The collected data show that PVDF ensures a reliable detection of knock, a precise measurement of knock energy and accurate information about the frequency content of the perceived vibration. The sensor worked for several hours without depolarizing and, above all, owing to the great piezoelectric e ect of PVDF, the use of a charge amplifier was unnecessary. PVDF proved to have great potential as a knock detector in spark ignition engines at a very low cost.
A fully analytical treatment of heat release in diesel engines
Authors: Hawley J G , Wallace F J , Khalil-Arya S
Published: October 2003

The paper details a wholly analytical approach to heat release modelling. The approach is based on the diffusion combustion process which is specific to diesel engines operating under very high injection pressures. The combustion is subsequently controlled mainly by two items, the instantaneous fuel mass present in the cylinder charge and the local density of turbulent kinetic energy. Analytical solutions are developed for each. An extensive test series was undertaken, covering the limiting torque curve (LTC) between 1250 and 4000 r/min and utilizing a high-pressure common rail diesel engine to generate the necessary validation data. The validation exercise has shown the flexibility of this heat release modelling approach to be extremely accurate and suitable for further development.
Reduction of emissions in an automotive direct injection diesel engine dual-fuelled with natural gas by using variable exhaust gas recirculation
Authors: Pirouzpanah V , Sarai R Khoshbakhti
Published: October 2003

An experimental study was conducted to determine the performance and exhaust emission characteristics of an automotive direct injection dual-fuelled diesel engine. Natural gas was used such that 65 per cent of engine brake power was supplied from compressed natural gas and the rest was supplied from diesel fuel. The objective of this work is to investigate the possibility of decreasing exhaust emission with the lowest performance sacrifice. At part loads, a dual-fuelled engine inevitably suffers from lower thermal efficiency and higher carbon monoxide (CO) emission. This is mainly due to leaner mixture and incomplete combustion, which is a consequence of the smaller amount of pilot fuel. To resolve these problems, the e ects of cooled exhaust gas recirculation (EGR) were investigated. The experimental results show that the application of EGR, at higher loads with 10 per cent EGR and at part loads with 15 per cent EGR, can considerably reduce NO x and other exhaust emissions such as unburned hydrocarbons, CO and soot. Results show that the performance parameters almost remain at the baseline engine level.
Measurement of oxygen storage capacity in automotive catalysts
Authors: Khossusi T , Douglas R , McCullough G
Published: October 2003

There is considerable disagreement in the literature on available oxygen storage capacity, and on the reaction rates associated with the storage process, for three-way automotive catalysts. This paper seeks to address the issue of oxygen storage capacity in a clear and precise manner. The work described involved a detailed investigation of oxygen storage capacity in typical samples of automotive catalysts. The capacity has also been precisely defined and estimates have been made of the specific capacity based on catalyst dimensions. A purpose-built miniature catalyst test rig has been assembled to allow measurement of the capacity and the experimental procedure has been developed to ensure accurate measurement. The measurements from the first series of experiments have been compared with the theoretical calculations and good agreement is seen. A second series of experiments allowed the e ect of temperature on oxygen storage capacity to be investigated. This work shows very clearly the large variation of the capacity with temperature.
Theoretical and experimental study of atomization from an annular liquid sheet
Authors: Cao Jianming
Published: October 2003

A temporal linear instability analysis is conducted to examine the e ects of both inner and outer gas velocities and various flow parameters on the break-up process of an annular viscous liquid sheet. The results show that a gas stream of high speed applied to the inner interface is more e ective to promote the sheet instability than to the outer one. Compared with a single gas stream, a co-flowing gas stream with a high speed improves the atomization performance. With the same or di erent gas velocities between the two sides of the liquid sheet, the gas density and the liquid surface tension have di erent e ects on the liquid sheet instability. Experiments measured liquid sheet wavelengths and break-up lengths. Computations with a typical nozzle and the flow conditions are comparable with the experiment.
On the design and tuning of the controllers in a power-split continuously variable transmission for agricultural tractors
Authors: Savaresi S M , Taroni F , Prevedi F , Bittanti S
Published: October 2003

In this work, a power-split continuously variable transmission (CVT) used in high-power tractors is considered. Power-split CVTs are characterized by the combination of a traditional discontinuous mechanical transmission and a continuously variable transmission. This provides, at the same time, smooth variations of the transmission ratio and high efficiency of the overall transmission system. The CVT considered in this paper is a hydrostatic power-split unit conceived for top-power agricultural tractors. The goal of this paper is to present a case study on the design and tuning of the control system structure for this type of CVT. This structure consists of three main parts: a servo controller on the internal current of the valve of the hydraulic transmission; a servo controller on the hydraulic transmission ratio; a synchronizer which coordinates the hydraulic and the mechanical parts of the CVT. The design, tuning and testing of this set of automatic controllers is presented.
Automotive door design using structural optimization and design of experiments
Authors: Lee Kwon-Hee , Shin Jung-Kyu , Song Se-Il , Yoo Yung-Myun , Park Gyung-Jin
Published: October 2003

Currently, the ultralight steel auto body (ULSAB) concept is receiving more attention owing to various benefits in automotive body design. One of the tasks of the ULSAB is constructing a door using tailor welded blanks (TWBs). In TWBs, two or more gauges of steel panels (parts) are welded together before the stamping process. In this research, the domains and thicknesses of the gauges in a front door structure are determined by a series of optimization methods such as topology, size and shape optimizations and design of experiments (DOE). The door is designed to have better performance compared with an existing structure in terms of stiffness and natural frequency. The final design is discussed and compared with the current design.
Vibration and squeal of a disc brake: Modelling and experimental results
Authors: Ouyang H , Cao Q , Mottershead J E , Treyde T
Published: October 2003

This paper presents a method for analysing the unstable vibration of a car disc brake, and numerical results are compared with squeal frequencies from an experimental test. The stationary components of the disc brake are modelled using many thousands of solid and special finite elements, and the contacts between the stationary components and between the pads and the disc are considered. The disc is modelled as a thin plate and its modes are obtained analytically. These two parts (stationary and rotating) of the disc brake are brought together with the contact conditions at the disc/pads interface in such a way that the friction-induced vibration of the disc brake is treated as a moving load problem. Predicted unstable frequencies are seen to be close to experimental squeal frequencies. The numerical simulation indicates that the stability can be improved by shifting the centre of the piston line pressure towards the trailing side of the pad.
Experimental and modelling aspects of flow boiling heat transfer for application to internal combustion engines
Authors: Robinson K , Hawley J G , Campbell N A F
Published: October 2003

A detailed programme of work has been undertaken to quantify the suitability of predictive methods for accurate determination of the levels of boiling heat transfer within an internal combustion (IC) engine cooling gallery simulator. An extensive array of experimental data has been obtained as the basis for the predictive validation. Working on the principle of superposition, the convective component of heat transfer has been represented by the established Dittus-Boelter correlation which has been extensively modified to account for developing boundary layers, surface roughness and nearwall viscous effects. The boiling component has been represented by the Chen model, modified for binary fluids and subcooling. For the IC engine cooling application it is concluded that the application of the Chen approach must be complemented by a convective heat transfer model that accurately represents the complex thermo-fluid situation being experienced within a developing flow.
Theoretical evaluation of the effects of crank offset on the reduction of engine friction
Authors: Cho Myung-Rae , Oh Dae-Yoon , Moon Tae-Seon , Han Dong-Chul
Published: October 2003

This study discusses the effects of crankshaft offsets to the piston thrust side on engine friction. An analytical model to interpret some key friction parts of an engine, such as crankshaft bearings, pistons and piston rings, is considered, and the effects of a crankshaft offset on the moving parts is calculated using numerical analysis. Analytical results show that the crankshaft offset has some influence mainly on the side force upon the piston and effects variation in the piston sliding speed. The crankshaft offset can reduce significantly friction loss of the piston skirt, whereas friction loss in other parts is negligible. The optimum offset to minimize skirt friction loss depends on the operating conditions. Upon calculation and measurement it is determined that reduction in friction loss occurs mainly at low engine speed and low engine load. When the speed and load increase, benefit is confined to the lowest offsets, and at higher offsets the friction increases. Analytical and experimental results indicate that crank offset is effective in reducing engine friction and improving fuel economy in the low and medium engine speed region.
Design of overlapped crankshafts. Part 3: Holes in crankpin and journal
Authors: Warrior N A , Sime A P , Hyde T H , Fessler H
Published: October 2003

In this paper, the boundary element (BE) technique is used to analyse the effects of 12 different holes in the crankpin and journal on the crankpin and journal fillet stress distributions. Significant reductions in journal fillet peak stresses can be achieved under radial bending, but crankpin fillet maxima are slightly increased. The angular position of the SCF is changed from a single peak in the longitudinal plane of symmetry for a solid crankshaft to two peaks symmetrically positioned relative to this longitudinal plane of symmetry for bored crankshafts. Under torsion, there is a change in fillet stress distribution, but little variation in SCF. Under both load cases, the stiffness of the crankshaft is slightly reduced.
Composition and size of diesel particulate emissions from a commercial European engine tested with present and future fuels
Authors: Lapuerta M , Hernández J J , Ballesteros R , Durán A
Published: October 2003

The search for alternative fuels, such as methyl ester from vegetable oils or water-oil emulsions, has become even more pertinent in the current context of fossil fuel shortage, the diesel vehicle population explosion and the new environmental policies. In this paper, the results of experimental research on particulate emissions from a typical indirect injection (IDI) diesel engine running under five different operating conditions and tested with fourteen types of conventional and alternative fuel are reported. The chemical analysis of the emitted particulate matter showed, for example, that the total mass of particulate emissions decreases when using biofuels owing to a reduction in their insoluble fraction. Moreover, composition analysis made it possible to identify the hydrocarbons adsorbed on the soot surface, to quantify their proportion with respect to the total particulate mass and to distinguish between their origins. The results of surface analysis using scanning electron microscopy showed for all the fuels that the number of particles detected per filter surface increased with load. This analysis also showed the effect of some fuel specifications (aromatic and sulphur content) on certain parameters related to the particle size distribution obtained from the filter images.
Analysis of variations in diesel engine idle vibration
Authors: Ajovalasit M , Giacomin J
Published: October 2003

The variations in diesel engine idle vibration caused by fuels of diffierent composition and their contributions to the variations in steering wheel vibrations were assessed. The time-varying covariance method (TV-AutoCov) and time-frequency continuous wavelet transform (CWT) techniques were used to obtain the cyclic and instantaneous characteristics of the vibration data acquired from two turbocharged four-cylinder, four-stroke diesel engine vehicles at idle under 12 diffierent fuel conditions. The analysis revealed that TV-AutoCov analysis was the most effective for detecting changes in cycle-to-cycle combustion energy (22.61 per cent), whereas changes in the instantaneous values of the combustion peaks were best measured using the CWT method (2.47 per cent). On the other hand, both methods showed that diesel idle vibration was more affected by amplitude modulation (12.54 per cent) than frequency modulation (4.46 per cent). The results of this work suggest the use of amplitude modulated signals for studying the human subjective response to diesel idle vibration at the steering wheel in passenger cars.
NO/NO2 Concentration of direct injection stratified combustion under constant volume condition fuelled by compressed natural gas and gasoline
Authors: Huang Z , Shiga S , Ueda T , Nakamura H , Ishima T , Obokata T , Tsue M , Kono M
Published: October 2003

The concentrations of NO, NO2 and NOx for direct injection (DI) combustion were analysed on the basis of experimental data by using a rapid compression machine fuelled with compressed natural gas (CNG) and gasoline. The analysis showed that the maximum values of NO and NO2 for stratified combustion would shift to a smaller overall equivalence ratio than that of homogeneous mixture combustion, and this phenomenon was more obvious for gasoline stratified combustion. The NOx concentration drops to a very low level at the stoichiometric equivalence ratio but still remains relatively high at an equivalence ratio of 0.6 in the case of stratified combustion. The ratio of NO2/NOx increased with decreasing equivalence ratio. A high ratio of NO2/NOx was observed in the constant-volume combustion experimental study which differs from that of an engine. This was caused by the long residence time of NO at high temperature after combustion, leading to the conversion of more NO into NO2.
The Marketization of New Zealand Schools: Assessing Fiske and Ladd
Authors: Woodfield Alan , Gunby Philip
Published: October 2003

Edward Fiske and Helen Ladd's review of market-based educational reforms in New Zealand are assessed in light of recent developments. We agree that predicted benefits were overstated, that there were both losers and winners, and that educational nirvana did not result. In our view, however, the main impact was to make schools' problems more transparent, creating discomforting pressures and attempts to undermine this transparency. We examine responses to changes in zoning laws, the effects of socioeconomic status on observed outcomes, signalling and value-added behavior, and school accountability. We find that educational reforms produce substantial short-term changes, largely on the demand-side.
Globalization and the Politics of International Finance: The Stiglitz Verdict
Authors: Basu Kaushik
Published: October 2003

There are many “outsiders” who have written critiques of the global financial system. Insiders typically do not; they have too much to lose. What makes Stiglitz's book, Globalization and Its Discontents, unusual is that it is an insider's critique. This book sketches the unfairness of the global system and how institutions, like the IMF, perpetuate the asymmetries of power and wealth. Though the book provides a morally compelling account of recent global crises, and its plea that we must think about radical change is hard to dismiss, it does not break new analytical ground nor offer concrete suggestions of alternatives.
Parametric Analysis of Axially Loaded RHS Gap K-Joints by Means of 2-D and 3-D F.E. Models
Authors: Mistakidis Euripidis S. , Baniotopoulos Charalambos C.
Published: October 2003

In the present paper the behaviour of steel structural RHS gap K-joints under static loading is studied by applying the Finite Element Method. The analysis is performed by the development of both 2-D and 3-D models of gap K-joints, thus investigating the influence of certain geometrical parameters into the strength and the overall stability of the joint. The geometry of the analyzed joints is in agreement with the general requirements of the EC3. The obtained results are compared to those obtained using the EC3–Annex K calculation formulas and the respective conclusive remarks are presented in the last part of the paper.
Simulation of Impact Effects on Large Space Structures Using Advanced SE Method
Authors: Horr A. M. , Safi M.
Published: October 2003

One of the important parameters in analysis and design of large space structures is to account for the possible forces induced by flying object impact and the subsequent progressive collapse. Conventional finite element methods have been used successfully in linear and nonlinear dynamic analyses of large space structures. During the past three decades, problems of contact mechanics have also been considered by some authors, and special attention has been devoted to high speed impact. Impact wave propagation in space structures may be analyzed using the conventional finite element method. However, in order to guarantee stability and accuracy of the solution, the number of elements used to model the structure may be very large indeed; more precisely, accurate results can be obtained only after a substantial computational effort. In this respect, the spectral element method (SEM) is developed for analyzing the wave propagation problems in large space structures. Using a computer program, the proposed formulation has been used to derive the impact global dynamic responses of a large dome structure.
Theoretical Analysis of the Structural Performance of Space Trusses Commonly Used in Brazil
Authors: Souza A. S. C. , Gonçalves R. M. , Maiola C. H. , Malite M.
Published: October 2003

Space trusses built in Brazil use basically two connection systems. In system 1, the node is formed by the superimposition of tubular section bars with stamped ends connected by a single bolt. The system 2 is a node consisting of welded steel plates to which bars with stamped ends or bars with end plates are connected. The authors have been engaged in theoretical and experimental studies to characterize the behaviour of these systems, including modifications to improve structural performance and establishing analytical models consistent with experimental behaviour. The theoretical analyses by finite element method have been performed utilizing the ANSYS and LUSAS software programs, using three distinct approaches: (i) analyses of isolated bars with section variations at the ends, (ii) analyses of the global behaviour of the structure considering the eccentricities on the connection, section variations in the bars and nonlinear behaviour, and (iii) numerical analyses of the local behaviour of the connection and its influence on global response and collapse modes.
Neural Networks for the Analysis and Design of Domes
Authors: Kaveh A. , Dehkordi M. Raissi
Published: October 2003

In this paper, efficient neural networks are trained for the analysis, design and prediction of the displacements of domes using the Backpropagation and Radial Basis Functions neural networks. The performance of these networks is compared when applied to domes. Programs are developed for accurate distribution of applied forces and wind load to the nodal points of domes. Training and testing pairs are prepared by ANSYS software.
Data Parallel Fuzzy Genetic Algorithm for Cost Optimization of Large Space Steel Structures
Authors: Sarma Kamal C. , Adeli Hojjat
Published: October 2003

A data parallel fuzzy genetic algorithm is presented for cost optimization of large space steel structures subjected to commonly-used design codes such as the American Institute of Steel Construction (AISC) Allowable Stress Design (ASD) and Load and Resistance Factor Design (LRFD) specifications. The algorithm is implemented on a distributed shared-memory multiprocessor, Origin 2000, using the OpenMP API (Application Programming Interface), and evaluated through application to two large space structures including a 144-storey moment resisting space building structure with more than 20,000 members.
Influence of Separation on the Compression Wave Generated by a Train Entering a Tunnel
Authors: Howe M. S. , Iida M.
Published: October 2003

An analytical model is proposed for calculating the influence of separation behind the nose of a high-speed train on the compression wave produced when the train enters a tunnel. The viscous drag associated with the back flow over the train and over the adjacent tunnel wall produces an aeroacoustic dipole whose strength increases linearly with the distance of the train nose into the tunnel and accounts for the linearly growing tail of the compression wave observed in model scale and field measurements. A detailed comparison with experiment suggests also that the contribution to the compression wave pressure rise from the large vortex ejected from the tunnel portal when the nose passes into the tunnel is in practice likely to be small, contrary to previous predictions that have modelled the exit flow as a uniform parallel jet exhausting from the tunnel.
Effects of Vortex Generating Tabs on Noise Sources in an Ideally Expanded Mach 1.3 Jet
Authors: Hileman J. , Samimy M.
Published: October 2003

The flow and acoustic fields of an ideally expanded Mach 1.3 axisymmetric jet with delta tabs were examined to explore the effects of the tabs on noise sources. This work continues research that was performed on a baseline (no-tab) jet. Noise measurements were made at an angle of 30° to the downstream jet axis to allow a direct comparison to previous work, and to relate the sound generation mechanisms to the large structures that were visualized with temporally resolved flow visualization. Additional acoustic measurements were made at 60° and 90° locations. Three cases were examined: a baseline jet, a single delta tab jet, and a dual delta tab jet. Both tab jets were operated at the same pressure ratio as the baseline jet, which was ideally expanded. Power spectra and average acoustic waveform measurements were made for a variety of azimuthal locations; apparent noise origins were estimated with a 3-D microphone array; and temporally resolved flow visualization was used to examine the dynamic flow structure of the jet's mixing-layer. The results confirm that the tabs generate strong streamwise vortices that have a significant effect on both the flow and acoustic fields of the jet. The tabs cause significant deformation in the cross-stream plane of the mixing-layer, as well as regulating the formation and roll-up of vortices due to Kelvin Helmholtz instability. With the addition of tabs, the noise field becomes azimuthally dependent and the region of noise generation moves dramatically upstream. It appears that the tabs are directly responsible for an increase in noise over a range of Strouhal numbers between 0.8 and 2.5 through generated streamwise vortices and they are indirectly responsible for the modification of the noise generating mechanisms at Strouhal numbers below 0.6 through the induced spanwise vortex roll-ups.
Hybrid RANS-LES Modeling for Cavity Aeroacoutics Predictions
Authors: Arunajatesan Srinivasan , Sinha Neeraj
Published: October 2003

A new hybrid RANS-LES turbulence modeling approach suited for modeling cavity aeroacoustics is presented. The model combines a one-equation LES subgrid model and a two equation RANS turbulence model in a physically consistent manner. The model takes into account the extent of resolution of the flow features by the mesh and computes an eddy viscosity for the turbulence model to reflect this resolution. It is shown that the model replicates the behavior of the underlying RANS and LES models in the coarse and fine mesh limits, respectively. Predictions of a surface pressures due to a supersonic flow over a cavity are compared with experimental measurements to demonstrate the model capabilities.
Accessing the Literature: Using Bibliographic Databases to Find Journal Articles. Part 2
Authors: Marlborough Helen S
Published: October 2003

Research in primary dental care, re-certification, continuing professional development, lifelong learning, peer review and quality healthcare are all informed by the published literature. Dental practitioners can find out about reliable and up-to-date information available in the published literature by searching bibliographic databases. This concluding article alerts readers to some of the limitations and pitfalls of database-searching.
Simulation of heat transfer during rotational molding
Authors: Greco A. , Maffezzoli A. , Vlachopoulos J.
Published: October 2003

In rotational molding, polymer powders are subjected to heating, melting, cooling, and subsequent solidification in biaxially rotating molds. Heat transfer phenomena during rotational molding are significantly affected by the presence of endothermic and exothermic transitions. In this paper instead of using the traditional moving interface method, a new approach is presented which is applicable to semicrystalline materials like linear low‐density polyethylene. Melting is described by a statistical model and crystallization by a kinetic model. The model parameters are determined from differential scanning calorimetry measurements. The one‐dimensional unsteady heat conduction equation is solved by a finite difference method. The numerical predictions are in good agreement with experimental data. The overall heat transfer model can be used for process optimization purposes. © 2003 Wiley Periodicals, Inc. Adv Polym Techn 22: 271–279, 2003; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/adv.10055
Rotational foam molding of polypropylene with control of melt strength
Authors: Pop‐Iliev Remon , Liu Fangyi , Liu Guobin , Park Chul B.
Published: October 2003

Polypropylene (PP) has not been used extensively in rotational foam molding because it has been traditionally considered as nonfavorable for foaming applications because of its relatively weak melt strength and melt elasticity at elevated temperatures. However, because of the unique end‐use properties of PP, PP foams have recently grown in importance. An experimental study was conducted to identify feasible processing strategies for producing PP foams with satisfactory morphologies in dry‐blending‐based rotational foam molding. The obtained results revealed that cell coalescence plays a key role in the production of PP foams in rotational foam molding. If it is efficiently suppressed, the cell morphology of the PP foams improves dramatically. To suppress cell coalescence, it would be necessary to preserve the melt strength of PP during processing. One way of doing this is maintaining the temperature of the PP melt as low as possible. This can be accomplished by either lowering the decomposition temperature of the chemical blowing agent by using an activator such as zinc oxide and/or reducing the temperature of the oven. © 2003 Wiley Periodicals, Inc. Adv Polym Techn 22: 280–296, 2003; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/adv.10056
Influence of melt drawing on the morphology of one‐ and two‐step processed LDPE/thermoplastic starch blends
Authors: Rodriguez‐Gonzalez F. J. , Virgilio N. , Ramsay B. A. , Favis B. D.
Published: October 2003

In this study the morphology of LDPE/TPS blends prepared by a one‐step extrusion process is compared to that obtained by reprocessing of the original blends. The influence of composition and melt drawing is examined. A novel methodology based on the form factor of the dispersed particle was used to estimate the equivalent spherical particle size of dispersed thermoplastic starch (TPS). This approach allows for the quantitative comparison of average dispersed phase particles regardless of their shape. Blends prepared in the one‐step extrusion process show increased levels of anisotropy as a consequence of a combination of coalescence and particle deformation during melt drawing. Reprocessed materials demonstrate morphologies that are highly stable to a wide range of hot stretch ratio conditions. The TPS particles of reprocessed blends show no coalescence and a low degree of deformation. This phenomenon is explained by plasticizer evaporation resulting from the second processing step. The TPS is transformed from a highly deformable phase to one resembling a partially cross‐linked material. These data indicate that the one‐step processing of LDPE/TPS blends can be used to generate a wide range of highly elongated morphological structures. A two‐step approach, analogous to typical compounding and shaping operations and involving controlled glycerol removal in the second step can be used to prepare a wide range of highly stable, more isotropic, dispersed particle morphologies. © 2003 Wiley Periodicals, Inc. Adv Polym Techn 22: 297–305, 2003; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/adv.10057
Investigation of mold plate separation in thin‐wall injection molding
Authors: Chen Shia‐Chung , Liaw Wei‐Liang , Su Pao‐Lin , Chung Ming‐Hsiu
Published: October 2003

Precision injection molding of thin‐wall parts has become an important concern in 3C (computer, communication, and consumer electronics) plastics industry. Previous studies in precision injection molding control focused on the injection screw, the hydraulic system control and the associated operations.In the present study, the influence of relevant parameters, including injection speed, melt temperature, mold temperature, filling–packing switchover, and packing pressure, on the mold plate separation under different clamping pressures was investigated as part of precision molding control. A two‐cavity tensile test specimen mold equipped with four linear variable displacement transducers (LVDTs) on mold exteriors and across the parting surface was used for experiments. A PC‐based monitoring system was also built to detect the mold separation signals. Mold separation can also be identified from part weight variation and exhibits relevant correspondence with part weight. It was found that because of the high injection speed required for thin‐wall molding, mold separation is not negligible. In all situations, mold separation decreases with increasing clamping pressure. As melt temperature and mold temperature increase, mold separation increases, resulting in an increase of part weight. Similarly, when packing pressure and injection speed increase, mold separation also increases. Earlier switchover from filling to packing can decrease mold separation as well as part weight. Among all parameters, packing pressure exhibits the greatest influence on mold separation and the associated part weight change. Variation of melt temperature and mold temperature also changes the gate freezing time and the associated packing pressure distribution and mold separation. The influence also becomes larger when molding thin‐wall parts as compared with conventional injection molded parts because of the larger injection pressure required for molding. Simulations also show good correspondence with experimental measurements. © 2003 Wiley Periodicals, Inc. Adv Polym Techn 22: 306–319, 2003; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/adv.10058
Two‐phase flow in a gas‐injected capillary tube
Authors: Park Chan‐Seok , Baek Sang‐Youp , Lee Ki‐Jun , Kim Seong Woo
Published: October 2003

The characteristics of two‐phase flow in a gas‐assisted capillary tube were studied by experimental and numerical methods. Experiments were performed with five test fluids including two Newtonian fluids and three suspensions having yield stresses. The coating thickness of the fluids deposited on the tube wall was measured with the various fraction of the capillary volume occupied by the fluids. The fractional coverage increased monotonically as capillary number increased at low capillary number (Ca < 1), approaching an asymptotic value of 0.60 at high capillary number. The shape of the front and fractional coverage as a function of the capillary number could be obtained by numerical simulation based on a finite element method using Polyflow® software. It was found that the predicted results are in good agreement with the experimental results. © 2003 Wiley Periodicals, Inc. Adv Polym Techn 22: 320–328, 2003; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/adv.10059
Cure kinetics and properties of epoxy resins containing a phosphorous‐based flame retardant
Authors: Frigione M. , Maffezzoli A. , Finocchiaro P. , Failla S.
Published: October 2003

A fire retardant phosphorous‐containing bisphenol compound, bis(3‐diethyl phosphono‐4‐hydroxyphenyl)‐sulfide (DPHS), has been inserted in an epoxy network. In particular, a chain extension procedure has been used to incorporate DPHS into a standard bisphenol epoxy resin. Different amine catalysts have been tested to promote the reaction of DPHS with the epoxy oligomer. The cure kinetics of this chain‐extended oligomer, using an aliphatic amine (triethylene‐tetraamine, TETA) as hardener, has been studied with the aid of differential scanning calorimetry and compared with that of the standard stoichiometric epoxy/TETA formulation. For each formulation the activation energy of the curing reactions has been evaluated by the Kissinger method. The results of thermogravimetric and flammability tests indicate that DPHS promoted the formation of a char, leading to an improved fire resistance. Finally, a comparison of the thermal and mechanical properties of the standard stoichiometric formulation relative to those of the epoxy modified with DPHS, and cured with TETA, showed only marginal changes in the physical and mechanical properties. © 2003 Wiley Periodicals, Inc. Adv Polym Techn 22: 329–342, 2003; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/adv.10060
The effect of boron nitride particles and hot‐pressed boron nitride die on the capillary melt flow processing of polyethylene
Authors: Lee Sang Myung , Nam Gi Joon , Lee Jae Wook
Published: October 2003

The processability of polymers by extrusion is closely related to the interface between the polymer melt and the die wall. The wall surface energy affects the flow of polymer melts in extrusion die. For the purpose of improving the processability in such a process, the effects of boron nitride (BN) powders on the rheological properties and the extrusion processability of metallocene‐ catalyzed low‐density polyethylene (m‐LDPE) have been investigated in this study. It is known that BN powders act as a solid lubricant, and lower the surface energy. Two kinds of BN particle, differing in crystal and agglomerate size, have been used and compared. The initial agglomeration of particles had little effect on the processing performance once the samples were made by twin‐screw extrusion. However, crystal size and size distributions significantly influenced the processability. Small crystal size and uniform size distribution were found to be more effective. Also, this improvement of processability was due to the increased slip velocity, not due to rheological property changes. The effect of a hot‐pressed BN die on the instability of capillary flow was also studied. A hot‐pressed BN die was quite effective in delaying surface and sharkskin defects and postponing gross melt fracture compared to a tungsten carbide die. A synergistic effect of processability improvement could be obtained when both BN powder and a hot‐pressed BN die were used together. © 2003 Wiley Periodicals, Inc. Adv Polym Techn 22: 343–354, 2003; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/adv.10061