Showing papers by "Worcester Polytechnic Institute published in 1987"
TL;DR: In this paper, a fixed-duration, fuel-optimal rendezvous problem for the constant-mass case through the solution for the primer vector is investigated and the original differential equations that describe the spacecraft motion during unpowered flight.
Abstract: Based on the linearized equations of motion of a spacecraft near a satellite in general Keplerian orbit and the assumptions of a bounded thrust magnitude and constant exhaust velocity, a fixed-duration, fuel-optimal rendezvous problem is formulated and is investigated for the constant-mass case through the solution for the primer vector, which is shown to satisfy the original differential equations that describe the spacecraft motion during unpowered flight. It is shown that there are no singular solutions to this rendezvous problem for noncircular Keplerian orbits and that consequently all optimal solutions for orbit eccentricities greater than zero are constructed from a finite number of intervals of full thrust and coast where the switches are determined from the primer vector.
187 citations
TL;DR: In this paper, the mechanism of phosphine-modified rhodium-catalyzed hydrofonnylation of 1-hexene was studied by in situ infrared spectroscopy using high pressure autoclaves equipped with embedded cylindrical internal reflectance crystals (CIR-FTIR).
144 citations
TL;DR: This paper analyzes a direct-sequence, spread-spectrum, multiple-access SSMA communication system which assigns a set of M orthogonal sequences to each user, and obtains approximations for the multiuser probability of error by using a Gaussian approximation for the multiple- access interference.
Abstract: This paper analyzes a direct-sequence, spread-spectrum, multiple-access (SSMA) communication system which assigns a set of M orthogonal sequences to each user. With all direct sequence SSMA systems, K users share a channel by phase modulating their transmissions with signature sequences. However, the users of our system transmit log_{2}M bits of information/sequence. This contrasts classical SSMA schemes which use a pair of antipodal sequences and transmit 1 bit/sequence. In this paper, we assume that the channel noise is a combination of additive white Gaussian noise (AWGN) and multiple-access interference. We employ the optimum (single-user) demodulator for orthogonal signals in Gaussian noise. The multiple-user performance of this receiver is analyzed. We obtain approximations for the multiuser probability of error by using a Gaussian approximation for the multiple-access interference. We also obtain an upper bound on the exact probability by using characteristic functions. Our SSMA system is Well suited for application at the lower radio frequencies. Therefore, a companion paper describes a realistic model for low-frequency radio noise, modifies the receiver to include a zero-memory nonlinearity, and studies the performance of the nonlinear receiver.
133 citations
TL;DR: In this paper, the conversion of infra-red to visible light has been studied in a series of fluoride glasses doped with YbF3 and ErF3, and the frequency up-conversion efficiency is four orders of magnitude greater than that observed in doped oxide glasses, and compares very favourably with results obtained for Yb3+/Er3+-containing crystals.
Abstract: The conversion of infra-red to visible light has been studied in a series of fluoride glasses doped with YbF3 and ErF3. The frequency up-conversion efficiency of these bulk materials is four orders of magnitude greater than that observed in doped oxide glasses, and compares very favourably with results obtained for Yb3+/Er3+-containing crystals.
79 citations
TL;DR: In this paper, a low-angle laser light scattering detector (LALLS) used with size exclusion chromatography was applied for the determination of molecular weight, molecular weight distribution (MWD), and degree of branching of polysaccharides in 0.5N NaOH aqueous solution.
Abstract: A low-angle laser light scattering detector (LALLS) used with size exclusion chromatography (SEC/LALLS) has been applied for the determination of molecular weight, molecular weight distribution (MWD), and degree of branching of polysaccharides in 0.5N NaOH aqueous solution. Data from both detectors [differential refractive index (DRI) and LALLS] are used to calculate the absolute molecular weight at each point in a sample chromatogram. The correct average molecular weight and MWD can be obtained without calibration methods used in conventional SEC. As a consequence of this technique, Mark—Houwink coefficients can be predicted from a single broad-distribution, homopolymer without recourse to time-consuming fractionation methods. Moreover, the hydrodynamic volume separation mechanism of SEC can be exploited with the SEC/LALLS method to gain information about polymer branching. In the studies described in this paper, SEC/LALLS has been employed to obtain data about the branching parameters gv and gM for samples of amylose, amylopectin, starch, and glycogen. For three homopolymers (amylose, amylopectin, and glycogen), branching frequency (as measured by chemical means), and the branching parameters (gv and gM) are inversely related. This trend is consistent with theoretical predictions. For starch, a nonhomogeneous branching distribution is observed as a function of molecular weight.
77 citations
TL;DR: In this article, two cloud water collector designs are presented, over others reported in the literature, is their ability to greatly reduce the entry of horizontally blown drizzle and rain droplets (> 200 μm) into the collector.
67 citations
TL;DR: In this paper, a finite difference computer program was developed to model the microstructural changes that occur in overlay coatings during simultaneous oxidation and interdiffusion with the base alloy, and the degradation of the coating was predicted by modelling the aluminum concentrations in the coating and calculating the phase distribution from the Co-Cr-Al phase diagram.
Abstract: A finite difference computer program has been developed to model the microstructural changes that occur in overlay coatings during simultaneous oxidation and interdiffusion with the base alloy. The degradation of the coating is predicted by modelling the aluminum concentrations in the coating and calculating the phase distribution from the Co-Cr-Al phase diagram. The results of the model are compared with experimental results for the isothermal oxidation of a Co-Cr-Al-Y coating on MM509 and a Co-24Cr alloy. Excellent agreement is found for cases where the diffusion coefficients are known.
65 citations
TL;DR: In this article, the authors used triethanolamine (TEA) to suspend zeolite NaA nuclei in solution, which resulted in hindered settling and reduced the shear normally associated with settling.
58 citations
21 Jun 1987
TL;DR: In this article, the authors present the results of a detailed theoretical and experimental analysis of an elementary switched power conditioning system with unity power factor, where the line current stays sinusoidal and in phase with the bus voltage while its amplitude can be varied within a wide range.
Abstract: This work presents the results of a detailed theoretical and experimental analysis of an elementary switched power conditioning system with unity power factor. The line current stays sinusoidal and in phase with the bus voltage while its amplitude can be varied within a wide range. The introduction of a Penalty Factor (cents/kWh) enabled to define the most cost effective operation in the context of a given resistive load and maximum allowable amplitude of the ripples in the line current.
52 citations
TL;DR: In this paper, it was found that the improved barrier performance of the films was due to the loss of the β-W phase and the formation of TiN and W 2 N phases as well as the TiO 2 and WO 3 phases.
51 citations
TL;DR: An auxotrophic mutant of Saccharomyces cerevisiae, containing a recombinant 2‐μ‐based plasmid, was grown in selective media in continuous culture and retained the ability to synthesize acid phosphatase as product, which was deleted from the host.
Abstract: An auxotrophic mutant of Saccharomyces cerevisiae, containing a recombinant 2-micro-based plasmid, was grown in selective media in continuous culture. The plasmid retained the ability to synthesize acid phosphatase as product, which was deleted from the host. Plasmid loss was followed at various dilution rates, and the level of plasmid expression was controlled by changing the beta-glycero/inorganic phosphate ratio.Some interesting trends were observed. As the level of plasmid expression was raised, the stability dropped markedly. Since acid phosphatase expression is regulated at the level of transcription, it is possible that increased transcription interfered with plasmid replication, hindered segregation, or overburdened the cell's DNA repair capability. It was also observed that plasmid stability was substantially increased at high growth rates. At dilution rates of 0.3 and 0.37 h(-1), feeding only inorganic phosphate, the plasmid was completely stable.
TL;DR: Although theppc− mutation stimulates phenylalanine production, it also stimulates the production of unwanted by-products such as acetate and pyruvate, both of which are found in isogenicppc+/Phec strains.
Abstract: Isogenic strains ofEscherichia coli were grown aerobically in minimal medium in a 2-liter airlift fermentor to determine whether appc (phosphoenolpyruvate carboxylase) mutation had the effect of directing glucose carbon into phenylalanine synthesis. Two host strains, YMC9 (ppc+) and KB285 (ppc−) were used, either with (Phec) or without (Phe0) a plasmid which determines constitutive phenylalanine production. Carbon consumption and metabolic products were monitored. Phenylalanine production occurred only in strains carrying the Phec plasmid.ppc− strains produced less cell mass and more acetate, pyruvate, and phenylalanine (in the Phec strains) than did isogenicppc+ strains. Lactate and ethanol production were not detected in any of the strains. Phec strains produced less acetate and pyruvate than their Phe0 homologs. Importantly,ppc−/Phec produced at least six times as much phenylalanine (0.32 g phenylalanine/g dry weight cells) asppc+/Phec. Even in this case, however, phenylalanine was produced at ten-fold lower levels than acetate. Thus, although theppc− mutation stimulates phenylalanine production, it also stimulates the production of unwanted by-products such as acetate and pyruvate.
TL;DR: In this paper, a doubly curved quadrilateral Love-Kirchhoff shell finite element is used to study the dynamic weakening of shell structures under a general class of time-dependent loadings.
Abstract: Structural buckling under dynamic loads may occur at load levels that are less than the corresponding static loads. Presence of local geometric imperfections may induce an early buckling for both static and dynamic loadings. The phenomenon of "dynamic weakening" is studied for a general class of shell structures under a general class of time-dependent loadings. A doubly curved quadrilateral Love-Kirchhoff shell finite element is used. Geometric deviations of the shell middle surface are included within the element formulation by suitably modifying the strain-displacement relations. This is accomplished by retaining additional terms that are quadratic in spatial derivatives of imperfections and displacement components. The nonlinear equations of motion are written in the Lagrangian system and are solved by using an incremental algorithm based on Newmark's generalized operator. The dynamic responses up to buckling are obtained for a perfect spherical cap and an imperfect spherical cap both under external pressure, as well as a complete imperfect sphere under external pressure. Numerical results include the effects of amplitude of imperfection and thickness of shell on the dynamic buckling loads. The formulation is general and can be applied to obtain the dynamic buckling responses of a wide variety of shell structures.
TL;DR: Various iterative solution techniques are explored in conjunction with a self-adaptive h -refinement technique to solve the two-dimensional Laplace equations by the boundary element method on a micro-computer.
TL;DR: A distributed model describing the growth of Saccharomyces cerevisiae containing a recombinant plasmid in selective media was developed and indicated that despite selective pressure, plasmids as recombinant vectors growth was significant.
Abstract: A major problem in the use of plasmids as recombinant vectors is the problem of plasmid-free cell generation from plasmid shedding and subsequent growth. A common technique for controlling the population of plasmidfree cells is the use of selective media against these cells using an auxotrophic host and a plasmid that has the ability to produced the essential metabolite. A distributed model describing the growth of Saccharomyces cerevisiae containing a recombinant plasmid in selective media was developed. The model allows for growth and production of a metabolite by the plasmid-carrying strain and growth of the plasmid-free cells on resulting metabolite concentrations. Through a determination of system constants and numerical solution to the equations, experimental batch and continuous culture results for cell concentration transients could be simulated by the model. The results indicated that despite selective pressure, plasmid-free cell growth was significant.
TL;DR: In this paper, the metastable transformation of molecular sieve zeolite NaA to hydroxysodalite (HS) in hydrothermal systems was studied in order to understand these phenomena better.
01 Jan 1987
TL;DR: In this article, the effect of time variation of system impedances and voltage harmonics on the value of optimum capacitor for power factor correction at busses with nonsinusoidal voltage was determined.
Abstract: The goal of this paper is to determine the effect of time variation of system impedances and voltage harmonics on the value of optimum capacitor for power factor correction at busses with nonsinusoidal voltage. Two types of 24 hours time-variation of voltage harmonices and Thevenin impedance are assumed. The equivalent load impedance is also considered time variable and assumed to contain a large proportion of induction motors. The daily energy losses are computed and graphed in function of the shunt capacitance used for power factor correction. The results of this study indicate that in order to avoid resonances and to find the optimum capacitor the time variation of harmonics and system impedances must be known as precise as possible.
TL;DR: In this article, the spatial stability of disturbances in the parallel and non-parallel Blasius boundary layers is considered, where Chebyshev polynomials are used for discretization.
Abstract: In this paper numerical calculation of the spatial stability of disturbances in the parallel and nonparallel Blasius boundary layers is considered. Chebyshev polynomials are used for discretization. The problem with the boundary condition at infinity is overcome, and the resulting nonlinear matrix eigenvalue problem is attacked directly. The secondary eigenvalue problem for three‐dimensional disturbances is shown to be uniformly stable, and particular solutions of this problem generated by the Orr–Sommerfeld equation are shown. A numerical solution of the nonparallel problem is considered using Chebyshev polynomials. The matrix equations are analyzed directly and the problem of uniqueness of the nonparallel correction is settled by careful application of the Fredholm alternative. Nonparallel corrections to the streamwise eigenfunction are shown.
TL;DR: In this article, the authors investigated the natural frequencies and forced responses of thin laminated composite shells of general form using a high-order curved shell finite element and the effect of geo metric nonlinearity is included in the determination of the forced responses which are ob tained using Newmark's numerical integration scheme.
Abstract: Natural frequencies and forced responses of thin laminated composite shells of general form are investigated using a high-order curved shell finite element. The effect of geo metric nonlinearity is included in the determination of the forced responses which are ob tained using Newmark's numerical integration scheme. A class of shell problems which are general with respect to geometry; boundary conditions; number, stacking sequence, and angle of orientation of the lamina; and load function can be solved using the present formulations. Numerical results and their comparisons with existing solutions are pre sented for cylindrical shells, shells of positive and negative Gaussian curvature, spherical caps, and shells of translation with different boundary conditions.
TL;DR: In this paper, the shape of the phase and group-velocity surfaces for each of the three modes and thus the phonon focusing properties of the crystal were determined, and the energy flow was highly enhanced along such strongly focused directions.
Abstract: Phonon focusing in dielectric crystals depends upon the second-order elastic constants [1]. These constants determine the shape of the phase and group-velocity surfaces for each of the three modes and thus the phonon focusing properties of the crystal. For certain ratios between these constants, the transversely polarized group-velocity surfaces can exhibit cuspidal edges [1, 2]. It is along directions coincident with these cuspidal edges that the most intense phonon focusing can be observed [3]. Energy flow is highly enhanced along such strongly focused directions.
TL;DR: In this article, the deactivation mechanism for 1-hexene was studied by cylindrical internal reflectance infrared spectroscopy (CIR-FTIR) under autogenous conditions, and it was shown that when the first two complexes were observable in solution hydroformylation was facile, and when they were replaced by the phosphido complex all activity stopped.
TL;DR: The numerical interpretation of the concentration dependence of the micelle mutual diffusion coefficient D in terms of intermicellar forces is discussed in this article, where the authors use the correct form for a, rather than the forms previously used to analyze experimental data, leads to modestly different predictions for a as a function of inter-cell potential.
TL;DR: In this article, a review of various modeling approaches to the problem of shear localization is presented, including flow softening, deformation heating, textural softening and void nucleation softening.
TL;DR: In this paper, the non-Gaussian closure technique is applied to a nonlinear single degree of freedom oscillator subjected to a stationary white noise excitation, and the nonlinear restoring force in this system is a softening spring with a hyperbolic tangent behavior.
TL;DR: In this article, a gradientless reactor was used to study the xylene isomerization reaction over a series of cation-exchanged (H+, Li+, Na+, and K+) ultrastable faujasite and on ZSM5 and silicalite to observe the effects of site modification on the reaction.
Abstract: A gradientless reactor was used to study the xylene isomerization reaction over a series of cation-exchanged (H+, Li+, Na+, and K+) ultrastable faujasite and on ZSM5 and silicalite to observe the effects of site modification on the reaction. Adsorption, diffusion, and kinetic parameters were determined in the presence of the reaction; the results generally agreed with the available literature data. The gradientless reactor and associated modeling techniques are shown to be effective tools in investigating catalyst behavior. Precise analytical techniques are required to directly measure the diffusion coefficient in the presence of reaction.
TL;DR: Effects on ethanol production by increases in gene dosage independent of heterosis in yeast are compared for an isogenic ploidy series ranging from haploid to tetraploid, finding that the efficiency of ethanol production per unit cell mass is greater in cells of higher ploids.
Abstract: Effects on ethanol production by increases in gene dosage independent of heterosis in yeast are compared for an isogenic ploidy series ranging from haploid to tetraploid. The per-cell rate of ethanol accumulation in parallel batch cultures increases with cell ploidy, and is attributable to intrinsic, ploidy-associated increases in cell mass-adjusted ethanol production rates. This increase in per-cell ethanol accumulation in the tetraploid strain is as high as 6.9 times the level of accumulation in the haploid. That is, the efficiency of ethanol production per unit cell mass is greater in cells of higher ploidy.
24 Mar 1987
TL;DR: In this article, the authors have shown that two distinct instability modes are encountered as the external velocity is increased, and these modes correspond to different instability modes of the shear layer.
Abstract: Flow past deep wall cavities exhibits strong interaction with the standing wave system in the cavity. The shear layer separating the exterior flow from the cavi ty flow supports i nstabil iti es which excite the normal cavity modes. In turn, the cavity wave system reirlforces the instabil i ties. Experimental studies have shown that two distinct instability modes are encountered as the external velocity is increased. The pressure coeffi ci ent versus Strouha1 number behavi or re veals two maxima which correspond to different instability modes of the shear layer. Flow vis ualization studies have shown that these modes correspond to di st i nct shear 1ayer vortex struc tures. At high velocities, single large-scale vortices are formed on the interface. At lower velocities, two vortices are present, simultan eous ly.
TL;DR: It is proved that every interval of a line-closed geometry is line-closure, and this implies that every supersolvable geometry isLine-closed, though not true in general.
TL;DR: In this article, a method for computing an approximate solution for an initial value problem in general dynamics without reference to the theory of differential equations associated therewith is presented. But this method does not satisfy the initial conditions beforehand.