Hostile Takeover: The Effects of Work Stress

Guided by family/work border theory, this phenomenological study explored the effects of stress while attempting to balance work and home through the lens of six high school female principals from the Central and Northern parts of California. This qualitative study used a phenomenological approach to examine the effects stress had on the physical and mental health of participants. Phenomenological interviews provided a rich description of the lived experiences of female high school principals. A thorough analysis of data exposed six distinct themes: (a) work became the priority, (b) coping mechanisms, (c) implications of being a woman, (d) lonely at the top, (e) blurred boundaries, and (f) the need for more support. Study participants’ experiences provide insight into the primary sources of stress, how stress affected their physical and mental health, the effect their stress had on their home domain, and how participants coped with the stress. Findings from this study address ways stress can be minimized to better balance work and home

Effect of a finite external heat transfer coefficient on the Darcy-Benard instability in a vertical porous cylinder

The onset of thermal convection in a vertical porous cylinder is studied by considering the heating from below and the cooling from above as caused by external forced convection processes. These processes are parametrised through a finite Biot number, and hence through third-kind, or Robin, temperature conditions imposed on the lower and upper boundaries of the cylinder. Both the horizontal plane boundaries and the cylindrical sidewall are assumed to be impermeable; the sidewall is modelled as a thermally insulated boundary. The linear stability analysis is carried out by studying separable normal modes, and the principle of exchange of stabilities is proved. It is shown that the Biot number does not affect the ordering of the instability modes that, when the radius-to-height aspect ratio increases, are displayed in sequence at the onset of convection. On the other hand, the Biot number plays a central role in determining the transition aspect ratios from one mode to its follower. In the limit of a vanishingly small Biot number, just the first (non-axisymmetric) mode is displayed at the onset of convection, for every value of the aspect ratio. (C) 2013 American Institute of Physic

Unstable Mixed Convection Flow in a Horizontal Porous Channel with Uniform Wall Heat Flux

Buoyancy-induced instability of the horizontal flow in a plane-parallel porous channel is analysed. A model of momentum transfer is adopted where a quadratic form-drag contribution is taken into account. The basic fluid flow is parallel and stationary. Due to the uniform wall heating and the effect of the buoyancy, the velocity and the vertical temperature gradient depend on the vertical coordinate. The dynamics of small-amplitude perturbations on the basic mixed convection flow is studied numerically. Transverse, longitudinal and general oblique rolls are investigated. It is proved that the longitudinal rolls are the normal modes triggering the instability at the lowest Darcy\u2013Rayleigh numbers. The condition of neutral stability is studied for different values of the form-drag parameter and of the P\ue9clet number

Influence of the public transportation system on the air quality of a major urban center. A case study: Milan, Italy

A sampling campaign was conducted in the city of Milan, Italy before and during a transportation strike in January 2004. This strike provided a unique opportunity to investigate the influence of public transportation on the air quality in a major metropolitan area. Twenty-four air samples were collected each day around the city on January 2nd, 7th and 9th. The samples were analyzed for methane, carbon monoxide, non-methane hydrocarbons (NMHCs), halocarbons and alkyl nitrates. Significant differences in the mixing ratios were observed among the three days of sampling, with January 2nd showing the lowest concentrations as a result of decreased activity in the city during the holiday season. January 9th showed the highest NMHC concentrations because of increased vehicular activity in the city due to a public transportation strike. This paper investigates the correlation between the increased number of vehicles and decreased air quality because of a reduction in public transportation. Computer simulations were able to reproduce measurements of ozone production during the January 2004 strike and a July 2005 strike. The measurements and simulations suggest that reduced VOC emissions due to the existence of public transportation lowers peak ozone by 11-33% during the summer months. © 2008 Elsevier Ltd. All rights reserved

Yang-Mills fields on CR manifolds

We study pseudo Yang-Mills fields on a compact strictly pseudoconvex CR manifold.Comment: 52 page

When does the onset of convection in an inclined porous layer become subcritical?

We consider the onset of convective instability in an inclined porous layer heated from below. Linearised stability theory tells us that there always exists a band of wavenumbers within which small-amplitude disturbances will grow, but this is true only when the inclination of the layer is less than 31.49032∘. At higher inclinations such disturbances always decay. However, it is also widely known that nonlinear convection may be computed for larger inclinations. This paper provides an initial explanation for how these two facts may be reconciled. It is generally assumed that the onset of convection in an inclined layer is supercritical, and, while this is certainly true when the layer is horizontal, there is no reason to assume that it remains so for other inclinations. The present paper, then, is a combined weakly-nonlinear and numerical investigation of the effect of inclination on the manner of onset. The weakly nonlinear analysis shows that the transition from a supercritical onset to a subcritical one takes place when the inclination is 24.247627∘, and this is confirmed using a detailed and focussed set of nonlinear numerical simulations

Three-dimensional convective planforms for inclined Darcy-Bénard convection.

none2siWe investigate the onset of convection in an inclined Darcy-Bénard layer. When such a layer is unbounded in the spanwise direction it is generally known that longitudinal rolls comprise the most unstable planform. On the other hand, when a layer has a sufficiently small spanwise width, then transverse rolls form the most unstable planform. However, the layer remains stable to transverse roll disturbances when the inclination is above roughly 31 degrees from the horizontal. This paper considers the transition between these two extreme cases where the spanwise width takes moderate values and where rectangular cells are considered. It is found that the most unstable planform is quite strongly sensitive to the magnitude of the spanwise width and that there are large regions of parameter space within which three-dimensional convection patterns have the smallest critical Darcy-Rayleigh number.openRees D.A.S.; Barletta A.Rees D.A.S.; Barletta A

Use of titania powders in the laser sintering process: Link between process conditions and product mechanical properties

Selective Laser Sintering (SLS) of Titania powders is studied to understand how the initial material properties and the process conditions affect the degree of sintering/melting and the mechanical properties of the semi-3D artefact produced.Five samples with differing particle size were used to explore the feasibility of processing them by SLS. Laser power and scan speed were studied as process variables to assess and quantify the effect of their changes on the properties of product. The measured tensile strength was used in the equation for the strength of the powder’s agglomerates developed by Rumpf, which allowed estimating the size of the sintered necks. The sintering temperature of each powder sample was determined experimentally and used to predict the size of the sintered neck for the different powder grain size using different literature models; these values were then compared with the values obtained from the experiments

Selective laser sintering of ceramic powders with bimodal particle size distribution

This paper addresses the possibility of carrying out Selective laser sintering (SLS) using powders obtained as mixtures of particles of different size. The beam source used in the experiments was a CO2 laser tube with a nominal power of 40W. The materials used were model Glass beads and a real ceramic material characterized by irregular shape of the particles. Bimodal distributed powders were generated by mixing samples characterized by different narrow particle size distributions. Single layer sintered specimens were obtained with a laser scanning speed of 50 mm/s and 8W beam. The sintered specimens were studied by means of microphotography and were characterized in terms of bulk density and tensile strength.Results show that the strength of the sintered specimen is significantly dependent upon the amount of fines in the powder mixture, in spite of the limited effects on the specimen thickness and density. In particular, the highest strength of the sintered material are observed with the highest fraction of fines in the originating powder mixture. In order to estimate the value of the forces between particles of different size produced by the sintering action, the model developed by Liu et al. (2017), based on the Rumpf (1958) approach, was purposely adapted. The application of the model revealed that in our process conditions the connection between large and fines particles is significantly weaker than the force between particles of the same size. The model also indicates that the strength of the sintered materials from mixtures can potentially increase up to values significantly higher than those of the materials sintered starting from the unimodal powder components

The relevance of surface impurities on the effect of temperature on powder flow behavior

Cohesive interparticle forces may have a relevant role in several industrial process operations involving particulate materials, such as fluidization, granulation and drying, as well as storage and solids handling units. Several of these operations require process conditions which involve high temperatures which, in turn, may affect the intensity of interparticle forces such as van der Waals, capillary and electrostatic forces. The mean by which the system temperature can affect all these forces is the change of particle hardness, the generation of liquid phases, which determines the formation of liquid bridges, and the modification of the particle dielectric properties. A direct measure of interparticle forces is possible but can be affected by large fluctuations and require a great number of repetitions. Interparticle forces, instead, play in averaged ensembles in bulk properties such as powder cohesion. It is of interest, therefore, to have the possibility to measure powder cohesion at the process temperature and to observe possible changes due to temperature variations to infer possible changes at the particle level. Powder shear testing is one of the available methods able to measure powder cohesion and it has the great advantage of measuring well established physical properties and of being able to produce highly repeatable results. It has to be remarked, however that to date no established procedure exists to relate powder cohesion measured at the bulk level to powder fluidization properties. In this paper a systematic study on the effect of the process conditions on the fluidization quality of ceramic powders is presented. Tests were carried out on powders of industrial interests, characterized by different particle size distributions and by different amounts of surface impurities, ranging from easy-flowing to cohesive flow behaviour.Two different experimental facilities were used: a modified ring shear tester available at the University of Salerno and aX-ray high temperature fluidization facility available at University College of London. The first apparatus was used to characterize powder cohesion at different temperatures between ambient and 500°C. Experimental results have been interpreted in terms of possible changes in interparticle forces as a function of temperature. The powder samples without impurities show an increase of cohesion with temperature as a result of an increase of interparticle van der Waals forces. A larger increase of cohesion was observed in the case of the powder samples with chemical impurities. The behaviour can be explained only by considering a cooperative effect of both van der Waals and capillary forces. It is noteworthy that the amount of surface impurities that is able to determine significant changes of powder flow properties is still so small that no evidence of phase transition could be detected by means of sample thermal analysis.The same powders have been characterized in terms of fluidization quality by using the x-ray fluidization facility available at University College of London under the same temperature range. Moreover the changes by temperature on the flow properties of the bulk solid evaluated with the shear cell and the behaviour of particles under fluidization conditions are analysed and discussed. Though a direct quantification of the particle-particle interactions in fluidized beds and of their changes under process conditions is very difficult, this paper suggests a method by which powder rheology can be used to indirectly evaluate the effects of the interparticle forces on fluidization