Fluid pressure penetration for advanced FEA of metal-to-metal seals

This numerical study investigates the behaviour of the contact faces in the metal-to-metal seal of a typical pressure relief valve in the commercial FE-package ANSYS. The valve geometry is simplified to an axisymmetric problem, which comprises a simple representative geometry consisting of only three components. A cylindrical nozzle, which has a valve seat on top, contacts with a disk, which is preloaded by a compressed linear spring. Analysis considerations include the effects of the Fluid Pressure Penetration (FPP) across the valve seat which exists at two different scales. In-service observations show that there is certain limited fluid leakage through the valve seat at operational pressures about 90% of the set pressure, which is caused by the fluid penetrating into surface asperities at the microscale. At the macroscale, non-linear FE-analysis using the FPP technique available in ANSYS revealed that there is also a limited amount of fluid penetrating into gap, which is caused primarily by the global plastic deformation of the valve seat. Accurate prediction of the fluid pressure profile over the valve seat is addressed in this study by considering the FPP interaction on both scales. The shape of this pressure profile introduces an additional component of the spring force, which needs to be considered to provide a reliable sealing

No approximate complex fermion coherent states

Whereas boson coherent states with complex parametrization provide an elegant, and intuitive representation, there is no counterpart for fermions using complex parametrization. However, a complex parametrization provides a valuable way to describe amplitude and phase of a coherent beam. Thus we pose the question of whether a fermionic beam can be described, even approximately, by a complex-parametrized coherent state and define, in a natural way, approximate complex-parametrized fermion coherent states. Then we identify four appealing properties of boson coherent states (eigenstate of annihilation operator, displaced vacuum state, preservation of product states under linear coupling, and factorization of correlators) and show that these approximate complex fermion coherent states fail all four criteria. The inapplicability of complex parametrization supports the use of Grassman algebras as an appropriate alternative.Comment: Argumentation made cleare

Pattern formation in annular convection

This study of spatio-temporal pattern formation in an annulus is motivated by two physical problems on vastly different scales. The first is atmospheric convection in the equatorial plane between the warm surface of the Earth and the cold tropopause, modeled by the two dimensional Boussinesq equations. The second is annular electroconvection in a thin semetic film, where experiments reveal the birth of convection-like vortices in the plane as the electric field intensity is increased. This is modeled by two dimensional Navier-Stokes equations coupled with a simplified version of Maxwell's equations. The two models share fundamental mathematical properties and satisfy the prerequisites for application of O(2)-equivariant bifurcation theory. We show this can give predictions of interesting dynamics, including stationary and spatio-temporal patterns

Discussion of “Learning from Failure of a Long Curved Veneer Wall: Structural Analysis and Repair” by Paulo B. Lourenço and Pedro Medeiros

The authors, in an interesting and valuable paper, describe the failure of the brick-masonry veneer façade of a multiuse public hall in Gondomar, Portugal. Damage occurred within 2 years of construction. The veneer was a single leaf of continuous brick masonry tied to a RC structural wall, forming a cavity of nominal width 0.07 m that was partly filled with foamed polyurethane. The veneer wall was 242 m in length and 15 m in height, without movement joints, and extended around most of the elliptical perimeter of the building with portions facing north, east, and south. From their site investigations and technical analysis, the authors attributed the failure primarily to effects of “the irreversible expansion of clay brick,” apparent both from cracking and from extensive out-of-plane deformation of the wall, which had widened the cavity to as much as 0.13 m. Effects were greater on parts of the wall facing south. The failure analysis made use of a power law proposed by Wilson et al. (2003) to describe how expansive strain develops in fired-clay ceramics with time. Here, the discussers comment on recent fundamental work on moisture expansion in brick, and in particular on its temperature dependence, matters of direct relevance to the paper under discussion. The discussers’ comments support and extend the conclusions of the authors, with which the discussers broadly agree. Irreversible moisture expansion occurs as a result of slow chemical reactions between components of the fired-clay ceramic and environmental moisture (Hamilton and Hall 2012). The magnitude of the expansion varies strongly with brick mineralogy and kiln firing history, but a predictive model for expansion based on these factors does not yet exist. However, in general, highly crystalline engineering ceramics produced at high kiln temperatures expand less than low-fired ceramics with a higher amorphous content. The penalty is that high-fired ceramics tend to be more brittle and prone to cracking. It is now established that the expansive reaction continues indefinitely, although at a diminishing rate over all timescales; therefore, there is no well-defined time at which it ceases. Recent reanalysis of published data (Hall et al. 2011; Hall and Hoff 2012) shows that the equation e=at1/4 accurately describes expansion strain e over periods of time t as long as 65 years. It follows from this equation that expansive strain at 16 years is double the value at 1 year and three times the 1-year value at 81 years. The persistence of the expansion reaction, albeit at a diminishing rate, emphasizes the need to incorporate appropriate movement joints in masonry design. The authors mention the possibility of using a “poor mortar” to accommodate some of the expansive strain. The use of weak mortars undoubtedly explains the absence of expansion damage in some much-older buildings with thick brick walls. However, the discussers consider that in thin brick veneers, such as those used in Gondomar, a weak mortar is potentially dangerous. It is unfortunate both for design and for failure analysis that the test procedures generally used to characterize clay brick do not provide values of the expansivity a that are needed to apply the equation e=at1/4 . Accelerated steam tests, such as EN772-19 cited by the authors, are at best semiquantitative. In our view, it is essential to determine the expansivity from measurements of expansion strain made over an appropriate period of time under controlled conditions (Hall and Hoff 2012). The discussers also draw attention to the important practical matter of the temperature dependence of the moisture expansivity (Hall et al. 2013). The fact that moisture expansion is the direct consequence of a chemical rehydroxylation reaction (Hamilton and Hall 2012) ensures that the expansivity increases notably with temperature. Available data indicate that the activation energy (which controls the temperature dependence) is about 70 kJ/mol. This means, for example, that the expansivity a of any brick material is about 60% greater at a temperature of 30°C than it is at 10°C. Thus, if a limit expansion strain (say, 1×10−3) is reached in a particular material in 50 years at 10°C, the same strain is attained in the same material in only 7 years at 30°C. It seems likely that its strong temperature dependence explains why moisture expansion is perceived differently in different geographical regions [e.g., McNeilly (1985)] and generally receives more attention in regions with warmer climates, such as Australia, southern Asia, and Brazil. However, in any particular region, the magnitude of expansion and the associated damage within individual buildings are influenced by local temperature variations, in particular variations due to solar heat gain. In the Gondomar structure, deformation and cavity expansion were greatest in parts of the structure with a southern aspect, where the summer temperatures of the veneer are highest. The influence of aspect here is presumably exacerbated by the open situation of the building and by the insulation of the cavity where large temperature gradients might be expected. The discussers believe this large gradient acting over a thin veneer may partly explain such dramatic damage over a short period of time. A thicker brick cladding would probably fare better. Undoubtedly, there are also seasonal modulations of the expansion. The discussers have shown elsewhere how related thermal effects in the rehydroxylation of archaeological ceramics may be calculated (Hall et al. 2013)

Study of mechanical aspects of leak tightness in a pressure relief valve using advanced FE-analysis

This paper presents a numerical study involving the deformation of contact faces in the metal-to-metal seal in a typical pressure relief valve. The valve geometry is simplified to an axisymmetric problem, which comprises a simple geometry consisting of only 3 components. A cylindrical nozzle, which has a valve seat on top, contacts with a disk, which is preloaded by a compressed linear spring. All the components are made of AISI type 316N(L) steel defined using the multilinear kinematic hardening model based on monotonic and cyclic tests at 20°C. In-service observations show that there is a limited fluid leakage through the valve seat at operational pressures about 90% of the set pressure, which is caused by the fluid penetrating into surface asperities at the microscale. Nonlinear FEA in ANSYS using the fluid pressure penetration (FPP) technique revealed that there is a limited amount of fluid penetrating into gap, which is caused by the plastic deformation of the valve seat at the macroscale. Prediction of the fluid pressure distribution over the valve seat just before the valve lift is addressed in this study considering the FPP interaction on multiscale. This is the principal scope, since it allows adjustment of the valve spring force in order to improve the leak tightness

Factors affecting ammonium uptake in streams - an inter-biome perspective

The Lotic Intersite Nitrogen experiment (LINX) was a coordinated study of the relationships between North American biomes and factors governing ammonium uptake in streams. Our objective was to relate inter-biome variability of ammonium uptake to physical, chemical and biological processes. 2. Data were collected from 11 streams ranging from arctic to tropical and from desert to rainforest. Measurements at each site included physical, hydraulic and chemical characteristics, biological parameters, whole-stream metabolism and ammonium uptake. Ammonium uptake was measured by injection of \u275~-ammonium and downstream measurements of 15N-ammonium concentration. 3. We found no general, statistically significant relationships that explained the variability in ammonium uptake among sites. However, this approach does not account for the multiple mechanisms of ammonium uptake in streams. When we estimated biological demand for inorganic nitrogen based on our measurements of in-stream metabolism, we found good correspondence between calculated nitrogen demand and measured assimilative nitrogen uptake. 4. Nitrogen uptake varied little among sites, reflecting metabolic compensation in streams in a variety of distinctly different biomes (autotrophic production is high where allochthonous inputs are relatively low and vice versa). 5. Both autotrophic and heterotrophic metabolism require nitrogen and these biotic processes dominate inorganic nitrogen retention in streams. Factors that affect the relative balance of autotrophic and heterotrophic metabolism indirectly control inorganic nitrogen uptake

The Orbit and Occultations of KH 15D

The unusual flux variations of the pre-main-sequence binary star KH 15D have been attributed to occultations by a circumbinary disk. We test whether or not this theory is compatible with newly available data, including recent radial velocity measurements, CCD photometry over the past decade, and photographic photometry over the past 50 years. We find the model to be successful, after two refinements: a more realistic motion of the occulting feature, and a halo around each star that probably represents scattering by the disk. The occulting feature is exceptionally sharp-edged, raising the possibility that the dust in the disk has settled into a thin layer, and providing a tool for fine-scale mapping of the immediate environment of a T Tauri star. However, the window of opportunity is closing, as the currently visible star may be hidden at all orbital phases by as early as 2008.Comment: To appear in ApJ [16 pages, 13 figures

Beef production from feedstuffs conserved using new technologies to reduce negative environmental impacts

End of project reportMost (ca. 86%) Irish farms make some silage. Besides directly providing feed for livestock, the provision of grass silage within integrated grassland systems makes an important positive contribution to effective grazing management and improved forage utilisation by grazing animals, and to effective feed budgeting by farmers. It can also contribute to maintaining the content of desirable species in pastures, and to livestock not succumbing to parasites at sensitive times of the year. Furthermore, the optimal recycling of nutrients collected from housed livestock can often be best achieved by spreading the manures on the land used for producing the conserved feed. On most Irish farms, grass silage will remain the main conserved forage for feeding to livestock during winter for the foreseeable future. However, on some farms high yields of whole-crop (i.e. grain + straw) cereals such as wheat, barley and triticale, and of forage maize, will be an alternative option provided that losses during harvesting, storage and feedout are minimised and that input costs are restrained. These alternative forages have the potential to reliably support high levels of animal performance while avoiding the production of effluent. Their production and use however will need to advantageously integrate into ruminant production systems. A range of technologies can be employed for crop production and conservation, and for beef production, and the optimal options need to be identified. Beef cattle being finished indoors are offered concentrate feedstuffs at rates that range from modest inputs through to ad libitum access. Such concentrates frequently contain high levels of cereals such as barley or wheat. These cereals are generally between 14% to 18% moisture content and tend to be rolled shortly before being included in coarse rations or are more finely processed prior to pelleting. Farmers thinking of using ‘high-moisture grain’ techniques for preserving and processing cereal grains destined for feeding to beef cattle need to know how the yield, conservation efficiency and feeding value of such grains compares with grains conserved using more conventional techniques. European Union policy strongly encourages a sustainable and multifunctional agriculture. Therefore, in addition to providing European consumers with quality food produced within approved systems, agriculture must also contribute positively to the conservation of natural resources and the upkeep of the rural landscape. Plastics are widely used in agriculture and their post-use fate on farms must not harm the environment - they must be managed to support the enduring sustainability of farming systems. There is an absence of information on the efficacy of some new options for covering and sealing silage with plastic sheeting and tyres, and an absence of an inventory of the use, re-use and post-use fate of plastic film on farms. Irish cattle farmers operate a large number of beef production systems, half of which use dairy bred calves. In the current, continuously changing production and market conditions, new beef systems must be considered. A computer package is required that will allow the rapid, repeatable simulation and assessment of alternate beef production systems using appropriate, standardised procedures. There is thus a need to construct, evaluate and utilise computer models of components of beef production systems and to develop mathematical relationships to link system components into a network that would support their integration into an optimal system model. This will provide a framework to integrate physical and financial on-farm conditions with models for estimating feed supply and animal growth patterns. Cash flow and profit/loss results will be developed. This will help identify optimal systems, indicate the cause of failure of imperfect systems and identify areas where applied research data are currently lacking, or more basic research is required

High Pressure Insulator-Metal Transition in Molecular Fluid Oxygen

We report the first experimental evidence for a metallic phase in fluid molecular oxygen. Our electrical conductivity measurements of fluid oxygen under dynamic quasi-isentropic compression show that a non-metal/metal transition occurs at 3.4 fold compression, 4500 K and 1.2 Mbar. We discuss the main features of the electrical conductivity dependence on density and temperature and give an interpretation of the nature of the electrical transport mechanisms in fluid oxygen at these extreme conditions.Comment: RevTeX, 4 figure