Showing papers by "Virginia Tech published in 1985"

Forest Litter Decomposition in Relation to Soil Nitrogen Dynamics and Litter Quality

Abstract: Decomposition and changes in nitrogen and organic—chemical content of six types of forest litter were studied for 2 yr in five adjacent Wisconsin forests. The five forests were floristically dissimilar, being dominated respectively by sugar maple (Acer saccharum), white oak (Quercus alba), bigtooth aspen (Populus grandidentata), white pine (Pinus strobus), and hemlock (Tsuga canadensis), Nitrogen mineralization rates in the five stands ranged from 29 to 125 kg°ha—1°yr—1. Decomposition rates of transplanted sugar maple leaves and red maple (Acer rubrum) wood were not correlated with nitrogen mineralization rates in all five stands, indicating that nitrogen mineralization rates do not affect initial decomposition rates. However, mineralization rates were correlated with decomposition rates of the native dominant foliage litter. Nitrogen first accumulated in all litters, but by the end of the 2—yr incubation period nitrogen release had begun in all foliage litters. Nitrogen concentrations increased approximately linearly with cumulative mass loss but eventually declined in some foliage litters. Neither maximum amount of nitrogen accumulated nor amount accumulated per gram of litter mass loss was related to the rate of soil nitrogen mineralization. Chemical composition of litter affected decomposition rates and patterns. Soluble substances and litters relatively rich in solubles disappeared rapidly during early stages of decomposition. Eventually, slowly disappearing acid—soluble and acid—insoluble substances dominated the pattern of mass loss in all litters.

Analysis of laminated composite plates using a higher‐order shear deformation theory

Abstract: A higher-order deformation theory is used to analyse laminated anisotropic composite plates for deflections, stresses, natural frequencies and buckling loads. The theory accounts for parabolic distribution of the transverse shear stresses, and requires no shear correction coefficients. A displacement finite element model of the theory is developed, and applications of the element to bending, vibration and stability of laminated plates are discussed. The present solutions are compared with those obtained using the classical plate theory and the three-dimensional elasticity theory.

Fine root turnover in forest ecosystems in relation to quantity and form of nitrogen availability: a comparison of two methods.

Abstract: Two methods of estimating fine root production and turnover are compared for 13 forest ecosystems exhibiting a wide range in form (NH4+ vs. NO3-) and quantity of available nitrogen. The two methods are by comparison of seasonal maxima and minima in biomess and by nitrogen budgeting. Both methods give similar results for stands with low rates of nitrification. The budgeting method predicts higher fine root turnover and productivity than the max-min method for systems with significant rates of nitrification.

Are uniform crime reports a valid indicator of the index crimes? an affirmative answer with minor qualifications*

Abstract: For more than two decades, the validity and reliability of official statistics on crime have been treated as highly questionable. Recently a number of studies have investigated the construction of official statistics on crime and issues related to their reliability and validity. However, there has been no recent attempt to bring together and evaluate these studies; thus, we do not know what official statistics mean and how we should interpret them. In the present paper it is argued that it is now possible to develop an understanding of what official statistics measure. It appears that both citizens and the police are in general agreement about what a serious crime is: it involves bodily injury (or serious threat of bodily injury), the property stolen is of high value, the act is committed by a stranger, or it involves breaking and entering. The authors argue that the perceived seriousness of the crime, first and primarily as defined by the victim, second as determined by the police, apparently accounts for most of the variance in whether a crime is reported and officially recorded; personal characteristics of the offender and victim have only minor effects. It is concluded that the Uniform Crime Reports (UCR), as measured by the FBI Index of Crime, are valid indicators of serious crimes as defined by the citizenry. The evidence supporting this conclusion is very substantial with regard to motor vehicle theft, robbery, burglary, and homicide, while with larceny, rape, and aggravated assault, the evidence supporting this conclusion is substantial but does require the interpretation of conflicting evidence.

An Examination of Conflicting Findings on the Relationship Between Job Satisfaction and Absenteeism: A Meta-Analysis

Abstract: This study, which applied meta-analytic procedures, found a significant negative relationship between certain facets of job satisfaction and absenteeism. Findings suggest that sampling errors, scal...

A path analytic model of store loyalty involving self-concept, store image, geographic loyalty, and socioeconomic status

Abstract: A model is introduced which posits that store loyalty is primarily determined by store-image evaluation and shopping-complex loyalty. Store image-evaluation, in turn, is hypothesized to be determined, to a major extent, by self-image/store-image congruity, and shopping-complex loyalty by area loyalty and socioeconomic status. The model was tested using a path analytic procedure and the results were consistent with the model. A replication study was attempted and the results reinforced the validity of the proposed model.

Resonant switches - Topologies and characteristics

Abstract: In designing conventional switching converters, the effort to increase operating frequency in order to reduce weight, size and cost of magnetic and filter elements is constantly hampered by higher switching stresses and switching losses. To overcome these obstacles, the concept of resonant switch was proposed. By incorporating additional Land C elements to shape device current and voltage waveforms, the desired zero-current switching property can be realized which enables converters to operate in the megahertz range.

A Model for the Thermal Response of Polymer Composite Materials with Experimental Verification

Abstract: The thermal response of a polymer composite material undergoing decomposition has been modeled. A one-dimensional transient thermal model with an nth order ap proximation for the rate of decomposition was applied. The model was tested by ex perimentally measuring the temperature profiles during decomposition for a glass- filled phenol-formaldehyde polymer composite. Additionally, the specific heat and thermal conductivity of the virgin and char components, heat of decomposition, and the kinetic parameters were experimentally determined and used as input to the model. The predicted temperature profiles are in good agreement with experimental temperatures obtained using a radiant heat flux apparatus.

Dry bean tannins: A review of nutritional implications

Abstract: Tannins are one of several antinutritional factors present in dry beans and are located mainly in the seed coat or testa. The tannin content of dry beans ranges from 0.0 to 2.0% depending on the bean species and color of the seed coat. Many high tannin bean varieties are of lower nutritional quality than low tannin varieties of beans. Naturally occurring food legume tannins are reported to interact with proteins (both enzyme and nonenzyme proteins) to form tannin-protein complexes resulting in inactivation of digestive enzymes and protein insolubility. Both in vitro and in vivo studies indicate that bean tannins decrease protein digestibility, either by inactivating digestive enzymes or by reducing the susceptibility of the substrate proteins after forming complexes with tannins and absorbed ionizable iron. Other deleterious effects of tannins include a lowered feed efficiency and growth depression in experimental animals. The antinutritional activity of bean tannins can be reduced by processing (1 or a combination of 2 or more methods), for example dehulling, soaking, cooking and germination. Genetic selection also may help in breeding varieties low in tannins. Potential chemical treatments such as use of tannin complexing agents are discussed.

Ceramers: Hybrid materials incorporating polymeric/oligomeric species with inorganic glasses by a sol-gel process

Abstract: A sol-gel process has been successfully utilized to produce hybrid materials incorporating polymeric/oligomeric components of polydimethyl siloxane(PDMS) with silicon glasses. All the samples made were transparent and flexible. Dynamic mechanical studies indicate that a portion of the siloxane species is phase-separated while the remainder is well dispersed. The effect of acid content were proven to be significant on the dispersion of the siloxane components and on the structure and properties of final products.

Chemical Modification of Matrix Resin Networks with Engineering Thermoplastics. 1. Phenolic Hydroxyl Terminated Poly(Aryl Ether Sulfone)-Epoxy Systems,

Abstract: Functionally terminated bisphenol-A polysulfone oligomers were used in the modification of Epon Resin 828/4,4′-diamino-diphenylsulfone (DDS) network system. Phenolic hydroxyl terminated PSF oligomers were first capped with a large excess of bisphenol-A diglycidyl ether or Epon Resin 828 at both ends and then the resulting system was cured with DDS, in a two-step process. During these studies molecular weight and the amount of PSF oligomers incorporated into the network were varied and their effect on the overall properties of the resulting systems were investigated. The capping and curing reactions were followed by using FT-IR and NMR spectroscopy, GPC, HPLC and DSC techniques. As a function of the oligomer molecular weight, SEM studies showed the formation of two-phase structures with ductile PSF particles dispersed in the continuous epoxy matrix. Mechanical characterization and fracture toughness measurements showed a remarkable increase in KIC or gIC values of the modified networks over that of control, without significant loss in the modulus. This work would appear to be one of the first studies where well bonded ductile glassy modifiers have significantly improved the fracture toughness of highly crosslinked networks.

Resonant Power Processors, Part I---State Plane Analysis

Abstract: State-plane techniques in conjunction with piecewise-linear analysis is employed to study the steady-state and transient characteristics of a series resonant converter. With the direct viewing of the resonant tank energy and the device switching instant, the state portrayal provides unique insights into the complex behavior of the converter. Operation of the converter under both continuous and discontinuous current modes and at frequencies both below and above resonant frequency are discussed.

Simultaneous analysis and design

Abstract: Optimization techniques are increasingly being used for performing nonlinear structural analysis. The development of element by element (EBE) preconditioned conjugate gradient (CG) techniques is expected to extend this trend to linear analysis. Under these circumstances the structural design problem can be viewed as a nested optimization problem. There are computational benefits to treating this nested problem as a large single optimization problem. The response variables (such as displacements) and the structural parameters are all treated as design variables in a unified formulation which performs simultaneously the design and analysis. Two examples are used for demonstration. A seventy-two bar truss is optimized subject to linear stress constraints and a wing box structure is optimized subject to nonlinear collapse constraints. Both examples show substantial computational savings with the unified approach as compared to the traditional nested approach.

Cryoconite Holes on Glaciers

Abstract: Cryoconite holes are water-filled depressions on the surface of glaciers. They contain microbial communities and may contribute to glacial wastage and biological colonization of ice-free areas.

The implementation of modal filters for control of structures

Abstract: The most common technique for the control of structures is modal control. In modal control, the differential equations in terms of actual coordinates are replaced by a set of ordinary differential equations in terms of the modal coordinates known as modal equations. In designing feedback controls in conjunction with the modal equations, one must know the modal states for the modes targeted for control. The sensors measure actual states, however. The modal states can be estimated by means of a Luenberger observer or modal filters. The modal filters produce estimates of the modal states from distributed measurements of the states. If distributed measurements are not available, then they can be reconstructed from measurements at discrete points via interpolation. This paper examines various questions associated with the implementation of modal filters, such as the effect of choice of interpolation functions and sensors locations, as well as of measurement errors, on the state estimation process. The method is demonstrated by means of two numerical examples.

The tetrahedral framework in glasses and melts — inferences from molecular orbital calculations and implications for structure, thermodynamics, and physical properties

Abstract: Results of ab initio molecular orbital (MO) calculations provide a basis for the interpretation of structural and thermodynamic properties of crystals, glasses, and melts containing tetrahedrally coordinated Si, Al, and B. Calculated and experimental tetrahedral atom-oxygen (TO) bond lengths are in good agreement and the observed average SiO and AlO bond lengths remain relatively constant in crystalline, glassy, and molten materials. The TOT framework geometry, which determines the major structural features, is governed largely by the local constraints of the strong TO bonds and its major features are modeled well by ab initio calculations on small clusters. Observed bond lengths for non-framework cations are not always in agreement with calculated values, and reasons for this are discussed in the text. The flexibility of SiOSi, SiOAl, and AlOAl angles is in accord with easy glass formation in silicates and aluminosilicates. The stronger constraints on tetrahedral BOB and BOSi angles, as evidenced by much deeper and steeper calculated potential energy versus angle curves, suggest much greater difficulty in substituting tetrahedral B than Al for Si. This is supported by the pattern of immiscibility in borosilicate glasses, although the occurrence of boron in trigonal coordination is an added complication. The limitations on glass formation in oxysulfide and oxynitride systems may be related to the angular requirements of SiSSi and Si(NH)Si groups. Although the SiO and AlO bonds are the strongest ones in silicates and aluminosilicates, they are perturbed by other cations. Increasing perturbation and weakening of the framework occurs with increasing ability of the other atom to compete with Si or Al for bonding to oxygen, that is, with increasing cation field strength. The perturbation of TOT groups, as evidenced by TO bond lengthening predicted by MO calculations and observed in ordered crystalline aluminosilicates, increases in the series Ca, Mg and K, Na, Li. This perturbation correlates strongly with thermochemical mixing properties of glasses in the systems SiO2-M 1 /+ AlO2 and SiO2-M n+O n/2 (M=Li, Na, K, Rb, Cs, and Mg, Ca, Sr, Ba, Pb), with tendencies toward immiscibility in these systems, and with systematics in vibrational spectra. Trends in physical properties, including viscosity at atmospheric and high pressure, can also be correlated.

Real Time Voltage-Phasor Measurements for Static State Estimation

Abstract: Recent introduction of microcomputers in substations to assume all substation tasks including protections makes possible accurate real-time measurements of positive sequence voltage phasors. Since static state estimators use measurements of line flows and injections to estimate the positive sequence voltage phasors it is clear that direct measurement of the state vector may be used to enhance state estimation. In order to obtain simultaneous measurements of different buses it is necessary to synchronize sampling clocks at different locations. After reviewing the techniques for obtaining positive sequence phasors from samples. techniques for clock synchronization are reviewed. Data volume reductions possible with direct measurement of state are examined and static state estimators which incorporate direct measurements are presented. Simulation results for the IEEE 118 bus system are presented.

Upwind relaxation algorithms for the Navier-Stokes equations

Abstract: The development of upwind relaxation algorithms for obtaining efficient steady-state solutions to the compressible Navier-Stokes equations is described. The method is second-order accurate spatially and naturally disipative, using third-order flux splitting of the pressure and convective terms and second-order central differencing for shear and heat flux terms. A line Gauss-Seidel relaxation approach, shown to be unconditionally stable for model convection and diffusion equations, is used. The algorithm is demonstrated for several flows using the thin-layer form of the equations, including the problem of shock-induced separation over a flat plate.

Minimum threshold for hydrogen metabolism in methanogenic bacteria.

Abstract: Methanogenic isolates did not consume hydrogen below partial pressures of 6.5 Pa. Thus, in contrast to a previous report, results from pure-culture studies do not invalidate the threshold model for methane production from hydrogen in sediments.

A Reward/Measurement Model of Organizational Buying Behavior:

Abstract: A new model of the organizational buying process is presented. The ontological framework of the model is based on the assumption that organizational buying behavior is essentially a form of work be...

A molecular orbital study of bond length and angle variations in framework structures

Abstract: Molecular orbital calculations on a variety of silicate and aluminosilicate molecules have been used to explore the bonding forces that govern tetrahedral bond length variations, r(TO), in framework silicates and aluminosilicates Not only do the calculations provide insight into the variety of structural types and the substitution limits of one tetrahedral atom for another, but they also provide an understanding of the interrelationships among r(TO) and linkage factors, bond strength sum, coordination number, and angles within and between tetrahedra A study of these interrelationships for a theoretical data set shows that r(SiO) and r(AlO) are linearly correlated with (1) p o, the bond strength sum to a bridging oxygen, (2) f s(O), the fractional s-character of a bridging oxygen, and (3) f s (T), the fractional s-character of the T atom In a multiple linear regression analysis of the data, 92% of the variation of r(SiO) and 99% of the variation of r(AlO) can be explained in terms of a linear dependence on p o, f s (O), and f s (T) Analogous regression analyses completed for observed r(Al, SiO) bond length data from a number of silica polymorphs and ordered aluminosilicates account for more than 75% of the bond length variation The lower percentage of bond length variation explained is ascribed in part to the random and systematic errors in the experimental data which have a negligible effect on the theoretical data The modeling of more than 75% of the variation of r(Al, SiO) in the framework silicates using the same model used for silicate and aluminosilicate molecules strengthens the viewpoint that the bonding forces that govern the shapes of such molecules are quite similar to the forces that govern the shapes of chemically similar groups in solids The different regression coefficients calculated for f s (T) indicate that SiO and AlO bond length variations in framework structures should not be treated as a single population in estimating the average Al, Si content of a tetrahedral site

Choice Functions over a Finite Set: A Summary

Abstract: A choice function picks some outcome(s) from every issue (subset of a fixed set A of outcomes). When is this function derived from one preference relation on A (the choice set being then made up of the best preferred outcomes within the issue), or from several preference relations (the choice set being then the Pareto optimal outcome within the issue, or the union of the best preferred outcomes for each preference relation)? A complete and unified treatment of these problems is given based on three functional properties of the choice function. None of the main results is original.