Static Zone Research Articles

Growth of a short fatigue crack – A long term simulation using a dislocation technique

In this study, the growth of a short edge crack during more than 14 000 cycles of fatigue loading is investigated in detail. An edge crack, in a semi-infinite body with no pre-existing obstacles present, is modelled in a boundary element approach by a distribution of dislocation dipoles. The fatigue cycles are fully reversed ( R = - 1 ) , and the load range is well below the threshold for long fatigue cracks. The developing local plasticity consists of discrete edge dislocations that are emitted from the crack tip. The movements of discrete dislocations are restricted to slip along preferred slip planes. The present model is restricted to a 2D plane strain problem with a through-thickness crack, assuming no 3D irregularities. A remote load is applied perpendicular to the crack extension line, and the material parameters are those of a BCC crystal structure. The competition between influence of the global loading on and local shielding of the crack tip governs the crack growth. The growth rate increases in discrete steps with short periods of retardation, from approximately the size of Burgers vector, b, up to 25 b per cycle as the length of the crack is tripled. The plastic zone changes from having an elongated, slender form to include a low angle grain boundary, which, eventually, divides into two parts. The crack growth is found to change from constant acceleration to constant growth rate as the event of the low-angle grain boundary split is approached. The results are compared to long crack characteristics, for which linear elastic fracture mechanics and Paris law can be used to predict fatigue crack growth. The exponent in Paris law varies between 1 and 0 in the present study, i.e. smaller than typical values for ductile BCC materials. The ratio between static and cyclic plastic zone sizes is found to increase during crack growth, and the angle of the general plastic zone direction increases, showing a tendency towards long crack values. The characteristics of the simulated crack growth, found in the present study, are typical for below-threshold growth, with slow acceleration, constant growth rate, and, eventually, either arrest or transition to long crack growth behaviour, as reported in the literature.

Modelling of paste flows subject to liquid phase migration

AbstractParticulate pastes undergoing extrusion can exhibit differential velocities between the solid and liquid phases, termed liquid phase migration (LPM). This is observed experimentally but understanding and predictive capacity for paste and extruder design is limited. Most models for LPM feature one‐dimensional analyses. Here, a two‐dimensional finite element model based on soil mechanics approaches (modified Cam‐Clay) was developed where the liquid and the solids skeleton are treated separately. Adaptive remeshing routines were developed to overcome the significant mesh distortion arising from the large strains inherent in extrusion.Material data to evaluate the model's behaviour were taken from the literature. The predictive capacity of the model is evaluated for different ram velocities and die entry angles (smooth walls). Results are compared with experimental findings in the literature and good qualitative agreement is found.Key results are plots of pressure contributions and extrudate liquid fraction against ram displacement, and maps of permeability, liquid velocity and voids ratio. Pore liquid pressure always dominates extrusion pressure.The relationship between extrusion geometry, ram speed and LPM is complex. Overall, for a given geometry, higher ram speeds give less migration. Pastes flowing into conical entry dies give different voids ratio distributions and do not feature static zones. Copyright © 2007 John Wiley & Sons, Ltd.

The Crystal Structure and Mutational Binding Analysis of the Extracellular Domain of the Platelet-activating Receptor CLEC-2

The human C-type lectin-like molecule CLEC-2 is expressed on the surface of platelets and signaling through CLEC-2 causes platelet activation and aggregation. CLEC-2 is a receptor for the platelet-aggregating snake venom protein rhodocytin. It is also a newly identified co-receptor for human immunodeficiency virus type 1 (HIV-1). An endogenous ligand has not yet been identified. We have solved the crystal structure of the extracellular domain of CLEC-2 to 1.6-A resolution, and identified the key structural features involved in ligand binding. A semi-helical loop region and flanking residues dominate the surface that is available for ligand binding. The precise distribution of hydrophobic and electrostatic features in this loop will determine the nature of any endogenous ligand with which it can interact. Major ligand-induced conformational change in CLEC-2 is unlikely as its overall fold is compact and robust. However, ligand binding could induce a tilt of a 3-10 helical portion of the long loop region. Mutational analysis and surface plasmon resonance binding studies support these observations. This study provides a framework for understanding the effects of rhodocytin venom binding on CLEC-2 and for understanding the nature of likely endogenous ligands and will provide a basis for rational design of drugs to block ligand binding.

Using Events as a Mapping Concept that Complement Existing ROS Methods

The Recreation Opportunity Spectrum (ROS) methods have used static measures such as spatial zones for settings and activities. However, visitor perceptions, activities, and experiences are dynamic and not always localized within a single and static ROS zone. The ROS framework could be enhanced by methods that document common movements across time and space during peoples' recreation engagements. This paper adds movement as a third pillar to the ROS conceptualization. The purpose of this paper is to demonstrate the improved utility of event mapping methods over traditional ROS mapping techniques by comparing an ROS map with an event map derived from research in the Spruce Knob-Seneca Rocks National Recreation Area. Visitors were interviewed on-site using a structured instrument (n = 176). Six common events were mapped and compared with the ROS map. Results suggest that vectors, lines, points, and other symbols complement ROS polygons and better address movement during recreational events.

Dynamic zone topology routing protocol for MANETs

AbstractThe limited scalability of the proactive and reactive routing protocols have resulted in the introduction of new generation of routing in mobile ad hoc networks, called hybrid routing. These protocols aim to extend the scalability of such networks beyond several hundred to thousand of nodes by defining a virtual infrastructure in the network. However, many of the hybrid routing protocols proposed to date are designed to function using a common pre‐programmed static zone map. Other hybrid protocols reduce flooding by grouping nodes into clusters, governed by a cluster‐head, which may create performance bottlenecks or a single point of failures at each cluster‐head node. We propose a new routing strategy in which zones are created dynamically, using a dynamic zone creation algorithm. Therefore, nodes are not restricted to a specific region. Additionally, nodes perform routing and data forwarding in a cooperative manner, which means that in the case failure, route recalculation is minimised. Routing overheads are also further reduced by introducing a number of GPS‐based location tracking mechanisms, which reduces the route discovery area and the number of nodes queried to find the required destination. Copyright © 2006 AEIT

Velocity profiling inside a ram extruder using magnetic resonance (MR) techniques

The extrusion of soft-solid materials is an important industrial process. Magnetic resonance imaging (MRI) has been used to acquire in situ velocity profiles during the ram extrusion of two opaque materials representative of those often extruded in industry; a mixture of 5 cSt poly-dimethylsiloxane (PDMS) added to a commercial soap (Dove TM ) (a soft solid), and a stiff biscuit dough (a paste). Radial velocity profiles have been acquired in both the axial and radial directions at different positions within the barrel (25 mm diameter) and die land, during extrusion through two 8 mm diameter cylindrical dies (length 8 and 16 mm), and through a multi-holed die. At a height of 24 mm above the die entrance, both materials exhibit plug flow with pure slip at the barrel wall. Nearer to the die, the flow has converged towards the centre of the barrel and static zones are observed at the barrel walls. The shape of the velocity profiles was found to be independent of ram speed (0.6–1.2 mm/s), and die length. The shape of the velocity profiles is material dependent, with greater shear effects evident in the biscuit dough and compression effects evident in the soap material.

Effect of Fines and Porous Catalyst on Hydrodynamics of Trickle Bed Reactors

The use of fine particles as a diluent of trickle bed reactor packings is a common method of ensuring good liquid distribution and catalyst wetting in laboratory scale tests, where the laboratory tests are operated at space velocities that correspond to industrial operation. In this paper, the effect of diluting the bed of industrial catalyst particles with fines upon the residence time distribution (RTD) of liquid flowing through the bed is investigated. It was determined that the dispersion coefficient decreased by up to 50% for the bed with fines, compared to that of an undiluted bed. The effect of using porous catalyst in the RTD experiment was also investigated, because many of the previously reported studies involved the case of a diluted bed of nonporous particles. The RTD for porous particles showed a more-pronounced tail, because of the holdup of liquid in the catalyst pores. A model with dynamic and static zones and intrapellet diffusion was fitted to the results, showing good agreement. This enabled the role of the static and dynamic zones, and effective diffusivity in a pellet upon RTD, to be investigated. The validated model may be used in predicting RTD in larger industrial-scale reactors.

Hydrodynamic properties of a novel ‘open wall’ monolith reactor

The hydrodynamic behavior of high-porosity ceramic acicular mullite (ACM) monoliths operated in gas–liquid Taylor flow was investigated by cold-flow tracer residence time distribution (RTD) experiments and compared with the hydrodynamic behavior of classical cordierite monoliths in terms of static liquid fraction, maldistribution and mass exchange between the dynamic liquid zone and the static liquid zone. The piston-dispersion-exchange (PDE) model was successfully fitted to the experimental data. The multiphase fluid mechanics were indeed different for ACM and cordierite monoliths: in contrast to the classical cordierite system, the mass exchange between static liquid and dynamic liquid was much higher and the liquid phase could access the open volume of the permeable ACM monolith wall. On the other hand, the larger velocity maldistribution, larger static liquid fraction and the liquid interaction between adjacent ACM microchannels resulted in less plug flow behavior.

Optimization of ERT Surveys for Monitoring Transient Hydrological Events Using Perturbation Sensitivity and Genetic Algorithms

A simple yet powerful algorithm is presented for the optimal allocation of electrical resistivity tomography (ERT) electrodes to maximize measurement quality. The algorithm makes use of a definition of the sensitivity of an ERT array to a series of subsurface perturbations. An objective function that maximizes the average sensitivity of a survey comprised of a large number of arrays is defined. A simple genetic algorithm is used to find the optimal ERT survey if there is a limited time allowed for survey. We further show that this approach allows for user definition of the sensitivity distribution within the targeted area. Results show clear improvement in the sensitivity distribution. The total sensitivity of the optimized survey compared with typically used surveys composed of one array type. This improved sensitivity will allow for more accurate monitoring of static and transient vadose zone processes. Furthermore, the algorithm presented may be fast enough to allow for real‐time optimization during time‐lapse surveys.

Optimization of ERT Surveys for Monitoring Transient Hydrological Events Using Perturbation Sensitivity and Genetic Algorithms

A simple yet powerful algorithm is presented for the optimal allocation of electrical resistivity tomography (ERT) electrodes to maximize measurement quality. The algorithm makes use of a definition of the sensitivity of an ERT array to a series of subsurface perturbations. An objective function that maximizes the average sensitivity of a survey comprised of a large number of arrays is defined. A simple genetic algorithm is used to find the optimal ERT survey if there is a limited time allowed for survey. We further show that this approach allows for user definition of the sensitivity distribution within the targeted area. Results show clear improvement in the sensitivity distribution. The total sensitivity of the optimized survey compared with typically used surveys composed of one array type. This improved sensitivity will allow for more accurate monitoring of static and transient vadose zone processes. Furthermore, the algorithm presented may be fast enough to allow for real-time optimization during time-lapse surveys.

Dynamic dc voltage band in vertical transport of superlattices

By the numerical method, we show a transition process from static to dynamic electric-field domain formation in semiconductor superlattices. During this transition, there can be noticed a sawtooth-like zone in which static and dynamic electric-field domain zones appear alternatively with increasing voltage. Therefore, a dynamic dc voltage band emerges from each sawtooth-like branch of the current–voltage characteristic. These results are qualitatively in agreement with experiment.

Mathematical modeling of zone drawing process

To provide guidelines and a basic understanding of static and continuous zone drawing processes, we propose two different mathematical models in terms of the processing conditions and material parameters. Although the models are not finely tuned, because of assumptions made, they are still useful for the analysis of the process and for predicting the processibility.

Relation between dry granular flow regimes and morphology of deposits: formation of levées in pyroclastic deposits

Experiments on dry granular matter flowing down an inclined plane are performed in order to study the dynamics of dense pyroclastic flows. The plane is rough, and always wider than the flow, focusing this study on the case of laterally unconfined (free boundary) flows. We found that several flow regimes exist depending on the input flux and on the inclination of the plane. Each flow regime corresponds to a particular morphology of the associated deposit. In one of these regimes, the flow reaches a steady state, and the deposit exhibits a levée/channel morphology similar to those observed on small pyroclastic flow deposits. The levées result from the combination between lateral static zones on each border of the flow and the drainage of the central part of the flow after the supply stops. Particle segregation features are created during the flow, corresponding to those observed on the deposits of pyroclastic flows. Moreover, the measurements of the deposit morphology (thickness of the channel, height of the levées, width of the deposit) are quantitatively related to parameters of the dynamics of the flow (flux, velocity, height of the flow), leading to a way of studying the flow dynamics from only measurements of the deposit. Some attempts to make extensions to natural cases are discussed through experiments introducing the polydispersity of the particle sizes and the particle segregation process.

Partial thiosulfate oxidation by steady‐state continuous culture in a bioreactor‐settler system

AbstractThe design, characterization, and performance of a bioreactor‐settler system that allows the partial biological oxidation of reduced sulfur compounds is described. The design incorporates the physical separation of the aeration from the bioreactor. The reactor has a dynamic flow zone and a static zone. Steady state thiosulfate‐oxidizing cultures were established at different O2/S2O32− molar ratios. At O2/S2O32− molar ratios > 1.0, sulfate was the main product of the biological oxidation of thiosulfate, while below O2/S2O32− molar ratios of about 1.0, formation of elemental sulfur was observed. The maximum sulfur formation (60%) was obtained at a molar ratio of 0.98. The elemental sulfur settled in the bottom of the bioreactor and was removed daily from the system. A recovery efficiency of 70% was obtained. The settled sulfur–biomass particles consisted of 90% sulfur and 6% biomass and had an average size of 150 µ m and a sedimentation rate of 7.50 m h−1. The hydraulic retention time (HRT of 2 min) of the medium in the aerator vessel showed an influence over the system performance due to the oxidation of thiosulfate and intermediates under the high oxygen concentration found in this vessel. Copyright © 2003 Society of Chemical Industry

The Ka‘ storm (November 2000): Imaging precipitable water using GPS

We use a network of GPS receivers on the Big Island of Hawaii to examine in detail the precipitable water field of the Ka‘ storm, which generated record rainfall and flash floods over much of the southern and eastern portions of the island. With GPS stations distributed from sea level to the highest points on the island, the spatial and temporal variations in precipitable water as the storm passed over the network are investigated. The results highlight the role of the topography in controlling the location of convection, revealing relatively static zones of high precipitable water where onshore winds and terrain anchor the storm. Using the observed correlation between the precipitable water and rainfall, estimates of rainfall based on the GPS data are generated. These estimates compare well with most of the high‐rainfall sites but tend to overestimate rainfall for the low‐rainfall areas. Rainfall predictions from a mesoscale spectral model of the storm consistently underestimate the observed heavy values. Comparison with the GPS, however, suggests that this underestimate is probably due to a correlated underestimate of precipitable water, indicating that significant improvements in predictions could be expected if the precipitable water field can be better constrained.

Settling behaviour of polymer flocculated water-treatment sludge II: effects of floc structure and floc packing

On the basis of experiments carried out using an image analysis system and static zone settlement, the alum sludge floc structure and the floc packing status with varied polymer dosages were examined in greater detail to reveal the link with the settling behaviour manifested in the “CML30 method”. The results of image analysis lead to the findings of the floc size with a pattern in initially great increase with low polymer dose and finally a plateau being reached at higher amount of polymer addition. Meanwhile, a polymer dosage in the range of 2.0–20.0 mg/l (for sludge SS=4,595 mg/l) can result in the increase of the floc fractal dimension ( D F ) up to an average value of 1.72 (raw sludge D F being 1.06), indicating greater compactness of the floc solids. These floc features control the settling behaviour of the “CML30 method”, especially in small polymer dosages. However, in large polymer doses, the controlling factor is derived from the floc ‘packing’ status (referring to the inter–floc contact manner). This ‘packing’ status may associate with viscosity of settling medium (supernatant) and the excess polymer in bulk solution. All these affect the sludge floc networked structure in overdosing range.

Highway-Rail Quad Gate Operation Time by System Simulation of Dynamic Dilemma Zone

A simulation model incorporating determination of a dynamic dilemma zone and algorithms to determine gate operation parameters for train-vehicle collision avoidance at highway-rail intersections (HRI) is developed, validated, and applied to two crossings on the Chicago-St. Louis High Speed Passenger Rail Corridor. A dynamic dilemma zone, as opposed to a static dilemma zone, is a road segment on approach to an intersection, which varies in length based on fluctuations in vehicle speeds and the number of vehicles within a road segment. Findings from the system simulation indicate that the proposed operating policy of control devices at highway-rail intersections is appropriate to provide advance warning and theoretically, detection parameters for eliminating high risk of a collision between vehicles within a dynamic dilemma zone and a train. Application to intelligent transportation system safety algorithms for HRI should be considered.

Quantum dynamics of hydride transfer catalyzed by bimetallic electrophilic catalysis: synchronous motion of Mg(2+) and H(-) in xylose isomerase.

Xylose isomerase exhibits a bridged-bimetallic active-site motif in which the substrate is bound to two metals connected by a glutamate bridge, and X-ray crystallographic studies suggest that metal movement is involved in the hydride transfer rate-controlling catalytic step. Here we report classical/quantal dynamical simulations of this step that provide new insight into the metal motion. The potential energy surface is calculated by treating xylose with semiempirical molecular orbital theory augmented by a simple valence bond potential and the rest of the system by molecular mechanics. The rate constant for the hydride-transfer step was calculated by ensemble-averaged dynamical simulations including both variational transition-state theory for determination of the statistically averaged dynamical bottleneck and optimized multidimensional tunneling calculations. The dynamics calculations include 25 317 atoms, with quantized vibrational free energy in 89 active-site degrees of freedom, and with 32 atoms moving through static secondary zone transition-state configurations in the quantum tunneling simulation. Our simulations show that the average Mg-Mg distance R increases monotonically as a function of the hydride-transfer progress variable z. The range of the average R along the reaction path is consistent with the X-ray structure, thus providing a dynamical demonstration of the postulated role of Mg in catalysis. We also predicted the primary deuterium kinetic isotope effect (KIE) for the chemical step. We calculated a KIE of 3.8 for xylose at 298 K, which is consistent with somewhat smaller experimentally observed KIEs for glucose substrate at higher temperatures. More than half of our KIE is due to tunneling; neglecting quantum effects on the reaction coordinate reduces the calculated KIE to 1.8.

Dynamic dilemma zone based on driver behavior and car-following model at highway–rail intersections

This study introduces the concept of a “dynamic dilemma zone” (DDZ) road segment. A DDZ as opposed to a static dilemma zone is a road segment on approach to an intersection which varies in length based on fluctuations in vehicle speeds and the number of vehicles within a road segment. This is in contrast to a static dilemma zone which is based on a constant approach speed and a single vehicle in the road segment. The “dilemma” refers to the interaction of vehicle kinematics and perception–reaction time, where a vehicle could not clear the ‘intersection’ or stop before entering it. Car-following theory is used within the DDZ approach to model driver–vehicle behavior in platoons. It is found that the values of gate delay and gate interval times in car-following situations are greater than those with a single vehicle approach. The determination of the DDZ aids in defining the conditions which contribute to the likelihood of vehicle clearance (or stopping) at four quadrant gate highway–rail intersections.

Band dispersion in chromatography—a universal expression for the contribution from the mobile zone

It is generally assumed that the dispersion which is covered by the C term of Van Deemter type equations arises from processes occurring in the static zone, while the dispersion covered by the A term arises from processes occurring in the mobile zone. It is also now widely accepted that the contribution to h, the reduced plate height, from mobile zone processes increases with a modest power of ν, the reduced flow velocity. A reassessment of data acquired since the 1960s suggests that this power falls with increasing velocity, but may be relatively high at reduced velocities, ν, in the range 1–30. Data for a wide variety of materials over a wide range of ν have been re-examined and are well fitted by an equation of the form: h= B/ ν+{1/ A+1/( Dν n )} −1+ Cν. With C≤0.02 in accordance with the theoretical value for slow equilibration in the static zone, n is found to be in the range 0.5–1.0 with the lower values applying to glass bead packings, and the higher values applying to porous spherical packing materials. The equation provides a decreasing power of velocity in the A term in agreement with experimental data. It is now clear that nearly all of the dispersion previously assigned to processes in the static zone actually occurs in the mobile zone. Accordingly, substantial improvements in column performance in LC may well be achieved by better packing of columns, or by designing structures such as monolithic beds and two dimensional designs on chips, which can provide more uniform structures than the beds of spherical particles widely used in current HPLC.