Showing papers by "Worcester Polytechnic Institute published in 1996"
TL;DR: It is demonstrated that ischemia‐related and induced SDs increase significantly ischemic lesion volume in vivo, supporting the hypothesis for a causative role of SD in extending focal ischema injury.
Abstract: This study investigated the role of spontaneous and induced spreading depression (SD) on the evolution of focal ischemia in vivo. We induced focal ischemia in 12 rats using the middle cerebral artery suture occlusion (MCAO) method. Chemical stimulation of nonischemic ipsilateral cortex by potassium chloride application (KCl group; n = 7) and saline (NaCl group; n = 5) was performed at 15, 30, 45, and 60 minutes following MCAO, and SD was detected electrophysiologically. Ischemic lesion volumes assessed over 15-minute intervals, evaluated by continuous apparent diffusion coefficient (ADC) of water mapping, demonstrated that the ischemic region increased significantly during 15-minute time epochs with a single SD episode (36.5 +/- 12.9 mm3, mean +/- SD) or multiple SD episodes (39.8 +/- 22.3) compared with those without SD (13.9 +/- 11.5) (p = 0.0009). Infarct volume at postmortem 24 hours after MCAO was significantly larger in the KCl group, with more total SDs (237.8 +/- 13.8) than the NaCl group (190.5 +/- 12.6) (p = 0.0001). This study demonstrates that ischemia-related and induced SDs increase significantly ischemic lesion volume in vivo, supporting the hypothesis for a causative role of SD in extending focal ischemic injury.
244 citations
TL;DR: In this article, the authors proposed two new sources emitting in the mid-IR: type-II and type-I interband cascade lasers, and performed detailed gain calculations. And they showed that high radiative efficiencies are expected, since the phonon processes which dominate relaxation in the intersub-band quantum cascade laser are effectively eliminated.
Abstract: The authors propose two novel sources emitting in the mid-IR: type-II and type-I interband cascade lasers, and perform detailed gain calculations. High radiative efficiencies are expected, since the phonon processes which dominate relaxation in the intersub-band quantum cascade laser are effectively eliminated.
174 citations
TL;DR: In this paper, the authors introduced new variational principles and bounds describing the effective conductivity tensor for anisotropic two-phase heat conducting composites with interfacial surface resistance between phases.
Abstract: New variational principles and bounds are introduced, describing the effective conductivity tensor for anisotropic two-phase heat conducting composites with interfacial surface resistance between phases. The new upper bound is given in terms of the two-point correlation function, component volume fractions and moment of inertia tensor for the surface of each heterogeneity. The new lower bound is given in terms of the interfacial surface area, component volume fractions and a scale-free matrix of parameters. This matrix corresponds to the effective conductivity associated with the same geometry but with non-conducting inclusions. The bounds are applied to theoretically predict the occurrence of size effect phenomena. We identify a parameter R$\_{\text{cr}}$ that measures the relative importance of interfacial resistance and contrast between phase resistivities. The scale at which size effects occur is determined by this parameter. For isotropic conducting spheres in a less conducting isotropic matrix we show that for monodisperse suspensions of spheres of radius R$\_{\text{cr}}$ the effective conductivity equals that of the matrix. For polydisperse suspensions of spheres it is shown that, when the mean radius lies below R$_{\text{cr}}$, the effective conductivity lies below that of the matrix.
126 citations
Patent•
30 Oct 1996TL;DR: In this paper, an impedance spectroscopy tissue status monitoring and measurement system is described, where an electrical current source is responsive to the synthesizer and generates electrical currents for transmission through tissue.
Abstract: An impedance spectroscopy tissue status monitoring and measurement system is disclosed. The system uses a synthesizer to generates electrical signals of selected frequencies. An electrical current source is responsive to the synthesizer and generates electrical currents for transmission through tissue. Electrodes or inductive coils of the system apply the electrical current to the tissue and sense voltages generated in the tissue in response to the electrical current. A controller determines the spectral response of the tissue by detecting magnitude and phase information of the electrical energy transmitted through the tissue. The information is then used to determine volumes of compartments within the tissue and ionic concentrations of compartmental fluids. Capacitive effects derived from the phase information are used to determine cell membrane functionality within the tissue. From this analysis, status, specifically, ischemia, may be determined on an absolute basis.
126 citations
TL;DR: This work reparameterize the cumulative-link generalized linear model to accelerate the convergence of Cowles’ algorithm even further and provides a proposal density based on the Dirichlet distribution which is more natural than the truncated normal density used in the competing algo.
Abstract: Albert and Chib introduced a complete Bayesian method to analyze data arising from the generalized linear model in which they used the Gibbs sampling algorithm facilitated by latent variables. Recently, Cowles proposed an alternative algorithm to accelerate the convergence of the Albert-Chib algorithm. The novelty in this latter algorithm is achieved by using a Hastings algorithm to generate latent variables and bin boundary parameters jointly instead of individually from their respective full conditionals. In the same spirit, we reparameterize the cumulative-link generalized linear model to accelerate the convergence of Cowles’ algorithm even further. One important advantage of our method is that for the three-bin problem it does not require the Hastings algorithm. In addition, for problems with more than three bins, while the Hastings algorithm is required, we provide a proposal density based on the Dirichlet distribution which is more natural than the truncated normal density used in the competing algo...
99 citations
TL;DR: In this paper, a one-point collocation analysis is performed on the two-dimensional pseudohomogeneous model of a fixed bed heat-exchanger, based on a single radial collocation point whose position depends on the wall Biot number.
Abstract: A one-point collocation analysis is performed on the two-dimensional pseudohomogeneous model of a fixed bed heat-exchanger. The analysis is based on a single radial collocation point whose position depends on the wall Biot number. The resulting improved equation for the overall heat transfer coefficient U is 1 U = 1 h w + R t 3 k r B i + 3 B i + 4 which gives an error less than 3.8% in the exact asymptotic value of U over the entire range of Bi. A formula is also given for U′, the overall heat transfer coefficient based on the difference between tube wall and bed center temperatures. The synthesis of vinyl acetate in a fixed bed reactor was simulated. Use of the conventional one-dimensional model with the improved equation for U, combined with the equation for U′ to obtain bed center temperatures, gave predictions of acetylene conversion and bed center temperature in good agreement with those of the two-dimensional model. A criterion for the neglect of radial temperature gradients in a fixed bed reactor is derived which is more stringent than criteria currently available in the literature.
98 citations
TL;DR: The PACS-Unlicensed B (PACS-UB) as mentioned in this paper is a version of PACS using time division duplexing, optimized for private, indoor wireless PBX applications and cordless telephony.
Abstract: The Personal Access Communications System (PACS) is an American National Standards Institute common air interface standard developed for the 1.9 GHz PCS band in the United States. PACS uses frequency division duplexing technology and is optimized to support low-mobility pedestrian outdoor usage and wireless local loop applications in a medium-range environment. PACS-Unlicensed B (PACS-UB) is a version of PACS using time division duplexing. PACS-UB has been optimized for private, indoor wireless PBX applications and cordless telephony. Both modes of operation are supported using the same portable hardware and the same signaling protocol.
94 citations
Patent•
06 May 1996TL;DR: An external fixator system comprises a clamp adapted to couple a fixator pin to a connecting rod as mentioned in this paper, and a bolt is inserted through a hole passing through both sides of the slot.
Abstract: An external fixator system comprises a clamp adapted to couple a fixator pin to a connecting rod. The clamp includes a slot for transversely receiving the body of the connecting rod. The slot preferably includes a region of reduced width, providing interference between the clamp and the connecting rod as it is inserted. This causes the clamp to snap onto the rod. A bolt is inserted through a hole passing through both sides of the slot. The bolt includes a head at one end formed in the shape of a hook adapted to hook the shaft of a fixator pin, and a thread at the opposite end. The bolt is rotatably mountable in the hole such that the fixator pin can be retained at a range of angles relative to the connecting rod. The clamp is attachable to a fixator pin and a connecting bar between two previously-installed clamps without disassembly of the system.
91 citations
TL;DR: In this paper, the mutual diffusion coefficient of Triton X-100 micelles and probe diffusion coefficient Dp of polystyrene latex spheres diffusing through micelle solutions were measured using quasi-elastic light scattering spectroscopy.
Abstract: Quasi-elastic light scattering spectroscopy was used to measure the mutual diffusion coefficient Dm of Triton X-100 micelles, and the probe diffusion coefficient Dp of polystyrene latex spheres diffusing through micelle solutions. The solution ionic strength has a substantial effect on aggregation number N and hydration δ of Triton X-100 micelles, N and δ both increasing with increasing salt concentration. This behavior may be qualitatively described as arising from a salt-induced change in the location of the water/Triton X-100 cloud curve.
80 citations
TL;DR: In this paper, an axisymmetric model of the plume and backflow contamination from an ion-thruster plume is presented, which is applied to both propellant charge exchange ions and nonpropellant efflux ions produced within the beam and to their transport out of the beam to the regions surrounding a model spacecraft.
Abstract: An axisymmetric model of the plume and backflow contamination from an ion-thruster plume is presented. Components included are primary beam ions, neutral propellant efflux, thermal propellant ions created mainly by charge-exchange collisions between primary beam ions and neutral propellant, nonpropellant efflux sputtered from thruster components, and neutralizing electrons. The plasma hybrid particle-in-cell technique is applied to both propellant charge-exchange ions and nonpropellant efflux ions produced within the beam and to their transport out of the beam to the regions surrounding a model spacecraft. Simulation results for plume properties such as ion density, beam potential, and ion flow angle are compared with experimental data. It is shown that the charge-exchange ions formed in the beam are accelerated outward by strong radial beam electric fields and form two distinct energy populations—one with a significant backstreaming velocity component.
77 citations
TL;DR: In this article, scale-sensitive analyses using length-scale relations have been developed from fractal geometry (coastline, compass or Richardson methods), which have been applied to a series of turned surfaces with a range of feeds per revolution.
TL;DR: The Hit-and-Run sampler is elaborated on, a Monte Carlo approach that estimates the value of a high-dimensional integral with integrand h( x)f(x) by sampling from a time-reversible Markov chain over the suport of the density f.
TL;DR: In this paper, the optimal power-limited rendezvous with variable exhaust velocity was investigated for propulsion systems having both upper and lower bounds on thrust magnitude, and the mathematical structure of the solution of the optimal rendezvous problem associated with this propulsion model was found.
Abstract: Optimal power-limited rendezvous with variable exhaust velocity is investigated for propulsion systems having both upper and lower bounds on thrust magnitude. In this model the spacecraft thrusters have four admissible states, thrusting at the upper saturation level, unsaturated power-limited thrusting, thrusting at the lower saturation level, and unpowered (i.e., engine off.) A fifth chattering state is also possible. The mathematical structure of the solution of the optimal rendezvous problem associated with this propulsion model is found. Computer simulations of rendezvous with a satellite in circular orbit are presented. For rendezvous near circular orbit four classes of chattering solutions of two types can occur. It is assumed that multiple thrusters can be mounted on the spacecraft and that they can operate independently. Applying the efficiency condition presented in a previous paper, a logic for switching individual thrusters on and off for optimal fuel usage and computational efficiency is presented.
TL;DR: In this article, a generally pinching hysteretic restoring force model is used in stochastic equivalent linearization of single-degree-of-freedom structural systems, where the severity and rate of pinching are controlled by the hysterestic energy dissipation and the pinching level can be specified to match experimental data.
Abstract: Many types of structural systems that undergo cycles of inelastic deformation under severe natural hazard loadings exhibit 'pinching' of hysteresis loops. In this paper, a generally pinching hysteretic restoring force model-an extension of the Bouc-Wen differential hysteresis model-is used in stochastic equivalent linearization of single-degree-of-freedom structural systems. The severity and rate of pinching are controlled by the hysteretic energy dissipation and the pinching level can be specified to match experimental data. Under white noise excitations, estimates of reponse statistics from linearization are shown to compare favourably with those from Monte Carlo simulation. Numerical studies on the sensitivity of the accuracy of response statistics obtained by linearization to changes in the hysteresis parameters showed that, for a range of practical cases, the linearization method can be used in lieu of simulation and that, in low-frequency systems, some hysteresis parameters may be set to a constant value a priori to reduce the number of model parameters that needs to be estimated or identified, and to simplify further random vibration analysis and/or performance evaluation studies.
Posted Content•
TL;DR: This paper provides an algorithmic version of the Blow-up Lemma, which helps to find bounded degree spanning subgraphs in $\varepsilon$-regular graphs and can be parallelized and implemented in $NC^5$.
Abstract: Recently we have developed a new method in graph theory based on the Regularity Lemma. The method is applied to find certain spanning subgraphs in dense graphs. The other main general tool of the method, beside the Regularity Lemma, is the so-called Blow-up Lemma. This lemma helps to find bounded degree spanning subgraphs in $\varepsilon$-regular graphs. Our original proof of the lemma is not algorithmic, it applies probabilistic methods. In this paper we provide an algorithmic version of the Blow-up Lemma. The desired subgraph, for an $n$-vertex graph, can be found in time $O(nM(n))$, where $M(n)=O(n^{2.376})$ is the time needed to multiply two $n$ by $n$ matrices with 0,1 entries over the integers. We show that the algorithm can be parallelized and implemented in $NC^5$.
01 Jul 1996
TL;DR: In this article, a plume characterization of a PULSE 8/9 flight PPT was performed using planar and cylindrical Langmuir probes and collimated quartz contamination sensors.
Abstract: Pulsed Plasma Thrusters (PPT's) are currently baselined for the Air Force Mightysat II.1 flight in 1999 and are under consideration for a number of other missions for primary propulsion, precision positioning, and attitude control functions. In this work, PPT plumes were characterized to assess their contamination characteristics. Diagnostics included planar and cylindrical Langmuir probes and a large number of collimated quartz contamination sensors. Measurements were made using a LES 8/9 flight PPT at 0.24, 0.39, 0.55, and 1.2 m from the thruster, as well as in the backflow region behind the thruster. Plasma measurements revealed a peak centerline ion density and velocity of approx. 6 x 10(exp 12) cm(exp -3) and 42,000 m/s, respectively. Optical transmittance measurements of the quartz sensors after 2 x 10(exp 5) pulses showed a rapid decrease in plume contamination with increasing angle from the plume axis, with a barely measurable transmittance decrease in the ultraviolet at 90 deg. No change in optical properties was detected for sensors in the backflow region.
TL;DR: In this article, a two-dimensional axisymmetric model was developed to investigate the plasma environment induced by an ion thruster and to assess plume backflow contamination, and it was verified that previous scaling relationships for the backflowing contamination, for a given geometry configuration, can be used to scale backflow currents as a function of thruster operating conditions.
Abstract: A two-dimensional axisymmetric model was developed to investigate the plasma environment induced by an ion thruster and to assess plume backflow contamination. Predictions of the propellant charge-exchange plasma and sputtered molybdenum-grid metal effluents from the NASA 30-cm xenon ion thruster over a wide range of operating conditions during space operation are presented and discussed. It is verified that previous scaling relationships for the backflowing contamination, for a given geometry configuration, can be used to scale backflowing currents as a function of thruster operating conditions. The deposition of sputtered molybdenum-grid ions is computed and is found to be less than previous estimates. It is shown that the electron temperature plays an important role in the plume, with the backflow currents increasing with electron temperature. In addition, the effectiveness of a plume shield in reducing backflow contamination is assessed and found to be effective.
TL;DR: In this article, a finite element method applied to the three-dimensional incompressible Navier-Stokes equations and the energy equation is used to solve the problem of flow and heat transfer in a fixed bed of tube-to-particle-diameter ratio 2.14.
Abstract: Fluid flow and heat transfer in a fixed bed of tube-to-particle-diameter ratio 2.14 is modeled by an arrangement of three staggered spheres in a tube. The problem is solved numerically by a finite element method applied to the three-dimensional incompressible Navier-Stokes equations and the energy equation. Results are obtained for Reynolds numbers in the range 25-101, with a Prandtl number of 0.72 and particle conductivities in the range 0.25-48.0 W/ (m K) Watt heat transfer coefficients obtained from the calculated temperature profiles are in good agreement with values predicted by a model-matching theory using correlations based on experimental measurements,
TL;DR: A framework for studying organizational computing relates technological infrastructure to the purposes for which organizational infrastructure exists and the activities that it enables and derives a taxonomy for computer-based support of organizations.
TL;DR: The role of dissolution of silicic acid powders in alkaline aluminosilicate synthesis mixtures in the crystallization of mordenite was investigated in this paper.
TL;DR: In this article, the performance of a commercial Y-type zeolite (Promoted Octacat) was studied under subcritical and supercritical hydrocarbon conditions, and it was shown that the activity of the catalyst was substantially higher during supercritical cracking.
TL;DR: Porous Vycor membrane tubes were used in shell-and-tube type membrane reactors to study the effect on the oxidative coupling of methane of metering the oxygen into the catalyst bed as mentioned in this paper.
TL;DR: In this article, an in situ cylindrical internal reflection infrared technique (CIR-FTIR) was used in order to understand the behavior of subcritical and supercritical hydrocarbons processing over a commercial Y-type zeolite (P-Octacat) at 475°C and pressures up to 41.6 bar.
TL;DR: In this article, a theoretical analysis of surface-incidence and waveguide-mode second harmonic generation with detuned inter-subband transitions in GaAs-AlGaAs, InGaAs-InAlAs and GaSb was presented.
Abstract: We present a theoretical analysis of surface-incidence and waveguide-mode second harmonic generation with detuned intersubband transitions in GaAs-AlGaAs, InGaAs-InAlAs and GaSb-InGaSb-AlGaSb asymmetric double quantum wells The analysis includes the effects of absorption, saturation, pump depletion, optical carrier heating, mode confinement and competition, and the loss of phase coherence due to waveguide, bulk and resonant intersubband contributions to the refractive index mismatch Optimal structures have been determined for each material system in both surface-incidence and waveguide-mode geometries A scheme for maintaining phase matching by incorporation of a separate region with an intersubband transition tuned midway between the first and second harmonic frequencies is analyzed At 106 /spl mu/m, the maximum conversion efficiency for the optimized InGaAs-InAlAs waveguide-mode device is /spl ap/16% at a pump-beam intensity of 40 MW/cm/sup 2/ Furthermore, the same device can be modulated to vanishing second harmonic output power when an electric field of -32 kV/cm is applied
TL;DR: In this article, the WPI cavitational materials process for the production of phase-pure, nanostructured materials was developed and applied to the synthesis of several nanophase oxides.
Abstract: We have developed the WPI cavitational materials process for the production of phase-pure, nanostructured materials. Hydrodynamic cavitation is generated mechanically using a high-pressure fluid system. The method begins with a precipitant stream that experiences a large pressure drop (21 000 psi ambient) across a patented geometry interaction chamber. Due to the large pressure drop, bubbles are formed and collapse, causing localized heating of the solvent. In addition, the precipitate undergoes a high degree of mechanical shear in the interaction chamber. This paper deals with the application of this novel technique to the synthesis of several nanostructured oxides. The nanophase oxides are characterized using transmission electron microscopy, X-ray diffraction, thermal gravimetric analysis, differential scanning calorimetry, and Fourier transform infrared spectroscopy. Select area diffraction patterns of the dried (110 °C) oxide samples in this study show distinct rings indicating nanosized particles. T...
TL;DR: It is demonstrated that wild-type C. elegans can be induced to display the M37 antigen on a later larval stage by altering the growth conditions, and the mechanisms of nematode surface antigen switching can now be investigated directly.
Abstract: Nematodes can alter their surface coat protein compositions at the molts between developmental stages or in response to environmental changes; such surface alterations may enable parasitic nematodes to evade host immune defenses during the course of infection. Surface antigen switching mechanisms are presently unknown. In a genetic study of surface antigen switching, we have used a monoclonal antibody, M37, that recognizes a surface antigen on the first larval stage of the free-living nematode Caenorhabditis elegans. We demonstrate that wild-type C. elegans can be induced to display the M37 antigen on a later larval stage by altering the growth conditions. Mutations that result in nonconditional display of this antigen on all four larval stages fall into two classes. One class defines the new gene srf-6 II. The other mutations are in previously identified dauer-constitutive genes involved in transducing environmental signals that modulate formation of the dauer larva, a developmentally arrested dispersal stage. Although surface antigen switching is affected by some of the genes that control dauer formation, these two process can be blocked separately by specific mutations or induced separately by environmental factors. Based on these results, the mechanisms of nematode surface antigen switching can now be investigated directly.
TL;DR: In this paper, the authors examined the effect of stable crack extension on fracture toughness test results using single-edge precracked beam specimens and concluded that the stiffness of the test system played a major role in crack growth stability.
Abstract: The effect of stable crack extension on fracture toughness test results was determined using single-edge precracked beam specimens. Crack growth stability was examined theoretically for bars loaded in three-point bending under displacement control. The calculations took into account the stiffness of both the specimen and the loading system. The results indicated that the stiffness of the testing system played a major role in crack growth stability. Accordingly, a test system and specimen dimensions were selected which would result in unstable or stable crack extension during the fracture toughness test, depending on the exact test conditions. Hot-pressed silicon nitride bend bars (NC132) were prepared with precracks of different lengths, resulting in specimens with different stiffnesses. The specimens with the shorter precracks and thus higher stiffness broke without stable crack extension, while those with longer cracks, and lower stiffness, broke after some stable crack extension. The fracture toughness values from the unstable tests were 10% higher than those from the stable tests. This difference, albeit small, is systematic and is not considered to be due to material or specimen-to-specimen variation. It is concluded that instability due to the stiffness of test system and specimen must be minimized to ensure some stable crack extension in a fracture toughness test of brittle materials in order to avoid inflated fracture toughness values.
TL;DR: A model of con flicts and negotiations in the SiFA framework is presented and a hierarchy of possible conflicts is proposed and the steps of the negotiation process are discussed.
Abstract: A basic ingredient of Concurrent Engineering (CE) is that all aspects of the product's life cycle should be considered as early as possi ble during design A key idea that has been adopted for the implementation of CE is the use of a team, in which each member is an expert on some aspect of the product in a phase of its life cycle in our work, we use a team of software agents Our goal is to study in detail the interactions, conflicts, and conflict resolution that is possible with such a multiagent system The approach is to use Single Function Agents (SiFAs), each of which can perform a very specialized task, from a single point of view The ability to represent expertise with different points of view is very impor tant in design These different points of view usually cause conflicts among agents, and these conflicts need to be resolved in order for the design process to be successful Therefore, agents need to be capable of detecting and resolving these conflicts This paper presents a model of con flicts and...