Showing papers by "Virginia Tech published in 1987"

Effective Thermal Conductivity of Composites with Interfacial Thermal Barrier Resistance

Abstract: A modification of the original theories of Rayleigh and Maxwell permitted the deriva tion of expressions for the effective thermal conductivity of composites consisting of a continuous matrix phase with dilute concentrations of dispersions with spherical, cylin drical and flat plate geometry with a thermal barrier resistance at the interface between the components.

Epidemiology of Infection by Nontuberculous Mycobacteria

Abstract: Organisms of the Mycobacterium avium, M. intracellular, and M. scrofulaceum (MAIS) group were recovered in significant numbers from 63 soil samples collected from the floodplains of 4 major rivers in the eastern United States: Hudson (NY), James (VA), Savannah (GA, SC), and Tombigbee (AL, MS). The mean numbers of M. avium intracellulars, M. scrofulaceum, and MAIS-intermediate biovars recovered increased significantly from the Hudson to the James to the Tombigbee and Savannah soil samples, agreeing reasonably well with previous findings on MAIS distribution in eastern U.S. waters and with the geographic distribution of naval recruits reacting to PPD-B and PPD-G, which is higher in the southeastern United States. From the Savannah and Tombigbee rivers, soils were collected throughout their lengths. The number of total MAIS and biovars recovered from the different soil sites varied widely and showed no obvious trends. However, statistical analysis revealed a highly significant correlation between high number...

Microbiology of periodontal disease

Abstract: There is a perception of conflicting data and opinions among research laboratories that are studying periodontal microbiology. In response to this perception I was asked to discuss recent literature in this field at the 1986 Annual Meeting of the American Association of Dental Research. This review is based on that discussion which primarily concerned all of the relevant papers that were available to me and that had been published in the last 2 years (1984 and 1985). In order to determine what the data (rather than what our biases) may tell us, accurate comparisons of published reports in this field require an appreciation of the reasons for different results in different laboratories. Therefore, before we compare data, let's consider some of the reasons why results from different laboratories may differ. The periodontal flora is complex. We have found over 325 distinct bacterial species in the human gingival crevice (1). Many of these species have never been formally described, most of them are ignored by some laboratories, identified differently by others, or lumped together in various groups by still others. Some of the predominant species have special growth requirements and are not detected on some media that are widely used. Identification criteria differ among laboratories. Even with rigorous testing, identification of some species is questionable. With minimal testing it is even less reliable.

Masculine gender role stress. Scale development and component factors in the appraisal of stressful situations.

Abstract: It is proposed that masculine gender role socialization affects whether men appraise specific situations as stressful. Behavioral research on stress and coping has remained relatively blind to the possibility of significant gender role differences in appraising events as stressful. Therefore, a new scale was developed to measure masculine gender role stress (MGRS). Data were presented to substantiate hypotheses that MGRS scores (1) significantly distinguish men from women, (2) are unrelated to global measures of sex-typed masculinity, and (3) are significantly associated with at least two measures of self-reported stress (i.e., anger and anxiety). Stressful situations represented on the MGRS scale include cognitive, behavioral, and environmental events associated with the male gender role. Factor analysis demonstrates that these concerns cluster in five particular domains reflecting physical inadequacy, emotional inexpressiveness, subordination to women, intellectual inferiority, and performance failures ...

Biased and Unbiased Cross-Validation in Density Estimation

Abstract: Nonparametric density estimation requires the specification of smoothing parameters. The demands of statistical objectivity make it highly desirable to base the choice on properties of the data set. In this article we introduce some biased cross-validation criteria for selection of smoothing parameters for kernel and histogram density estimators, closely related to one investigated in Scott and Factor (1981). These criteria are obtained by estimating L 2 norms of derivatives of the unknown density and provide slightly biased estimates of the average squared L 2 error or mean integrated squared error. These criteria are roughly the analog of Wahba's (1981) generalized cross-validation procedure for orthogonal series density estimators. We present the relationship of the biased cross-validation procedure to the least squares cross-validation procedure, which provides unbiased estimates of the mean integrated squared error. Both methods are shown to be based on U statistics. We compare the two metho...

Virginia farmers' soil conservation decisions: an application of tobit analysis

Abstract: tergenerational considerations, and income Using data from a survey of farm operators (Ervin and Ervin; Nowak and Korsching; Lee in two Virginia counties, the authors analyze and Stewart; Forster and Stem). farmers' soil conservation decisions. Results Several issues have not been adequately indicate that financial factors, including in- treated in previous studies. The first is the apcome and debt, are the most important in- propriate model of farmers' conservation befluences on the sample farmers' use of conser- havior. In their conceptual model of the convation practices. Additional factors such as servation decision-making process, Ervin and perception of erosion, education level, off-farm Ervin recognize three components: the peremployment, and tenancy also influence con- ception of an erosion problem, the decision to servation expenditures. Factors influencing adopt conservation practices, and the amount conservation tillage acreage differ from those of soil conservation effort. Previous researchinfluencing expenditures for other conserva- ers have analyzed only one of the three comtion practices. In particular, age and race of ponents or have analyzed the components the operator and on-farm erosion potential are separately. significantly related to the use of conservation A second issue is the consideration of contillage but not other practices. These results servation tillage versus other conservation are discussed in terms of their implications for practices. Previous studies have combined conservation programs.

Algorithm 652: HOMPACK: a suite of codes for globally convergent homotopy algorithms

Abstract: There are algorithms for finding zeros or fixed points of nonlinear systems of equations that are globally convergent for almost all starting points, i.e., with probability one. The essence of all such algorithms is the construction of an appropriate homotopy map and then tracking some smooth curve in the zero set of this homotopy map. HOMPACK provides three qualitatively different algorithms for tracking the homotopy zero curve: ordinary differential equation-based, normal flow, and augmented Jacobian matrix. Separate routines are also provided for dense and sparse Jacobian matrices. A high-level driver is included for the special case of polynomial systems.

Boundary control of the Timoshenko beam

Abstract: It is shown that the Timoshenko beam can be uniformly stabilized by means of a boundary control. A numerical study on the spectrum is also presented.

Independence of clones as a criterion for voting rules

Abstract: “Independence of clones” is a generalization of the condition of not being subject to the perverse consequences of vote splitting that arise under plurality voting. A new voting rule that is at least “almost always” independent of clones is obtained by the following algorithm: Require the collective ranking of the candidates to be consistent with the paired comparisons decided by the largest and second largest margins, and then, if possible, with the paired comparison decided by the third largest margin, and so on. The advantages of this “ranked pairs” rule over previously proposed voting rules that are independent of clones is that it possesses Condorcet consistency, non-negative responsiveness, and “resolvability” (the property that every tie be within one vote of being broken).

Total least squares approach for frequency estimation using linear prediction

Abstract: The resolution of the estimated closely spaced frequencies of the multiple sinusoids degrades as the signal-to-noise ratio (SNR) of the received signal becomes low. This resolution can be improved by using the total least squares (TLS) method in solving the linear prediction (LP) equation. This approach makes use of the singular value decomposition (SVD) of the augmented matrix for low rank approximation to reduce the noise effect from both the observation vector and the LP data matrix simultaneously. Comparison is made to the principle eigenvector (PE) method of Tufts and Kumaresan, both on theoretical and experimental grounds. The TLS algorithm exhibits superior performance over the PE method where low rank approximation is applied to the data matrix only.

Bacteriology of Human Gingivitis

Abstract: The subgingival bacterial floras of naturally occurring gingivitis in adults and children were characterized and compared with the floras of other periodontal conditions previously studied. The composition of the gingivitis floras was found to be distinct from that of floras associated with health or with moderate, severe, or juvenile periodontitis. There were no major differences between the floras of naturally-occurring gingivitis and the floras of the human experimental gingivitis model. Data indicated that the flora of healthy sites within a mouth is influenced by the number of inflamed sites, which argues against independence of sites bacteriologically. Proportions of ten bacterial species increased in both gingivitis and periodontitis, as compared with health, in both adults and children. These species were found in both affected and unaffected sites of people with gingivitis. The numbers of five other cultivable species and the "large treponeme", which was not cultivated, increased in gingivitis and periodontitis of adults only. Significant differences in non-spirochetal floras between children and adults were not found, although they were in the experimental gingivitis model studied previously. Cultivable spirochetes did differ between children and adults. Children had fewer samples positive for spirochetes, and children's positive samples contained greater proportions of T. socranskii subsp. paredis. Some species that predominate in periodontitis, but which are absent from healthy gingivae, were found as a small percentage of the flora in gingivitis. This suggests that increased serum and blood in the gingival crevice encourage species that relate to periodontitis.

Quasi-Resonant Converters-Topologies and Characteristics

Abstract: In designing switching dc-dc converters, the effort to increase operating frequency 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 the "resonant switch" is proposed. By incorporating additional inductor and capacitor elements to shape the semiconductor switch's current waveform, a "zero-current switching" property can be realized. Based on the resonant switch technique, a new host of quasi-resonant converter circuits have been derived, which can be operated in the megahertz range.

The morphology and rheology of polymer blends containing a liquid crystalline copolyester

Abstract: The morphology of blends of polycarbonate and nylon 6,6 with a copolyester of 60 mole percent p-hydroxybenzoic acid/40 mole percent poly(ethylene terephthalate) was characterized under different processing conditions. In particular, single-screw extrusion, steady simple shear flow, and flow through a capillary were studied to determine what conditions were necessary for the development of a fibrillar morphology of the liquid crystalline polymer (LCP). Results indicate that some extensional flow is required for the coalescence and extension of the particulate LCP phase. The viscosity of the blends was determined both in a cone-and-plate geometry of a Rheometrics Mechanical Spectrometer at low shear rates and in the Instron Capillary Rheometer at higher rates. In general, only a small (10 or 30 percent) weight fraction of LCP was required to reduce the viscosity of the thermoplastics to that of the polymeric liquid crystal. An attempt was made to correlate the structure of the blends seen under the scanning electron microscope with the observed rheology. Not all aspects of the morphology were possible to explain in terms of the viscous properties of the blends.

Equal or proportional division of a surplus, and other methods

Abstract: A cooperative venture yields a nonnegative surplus. Agents are differentiated by their opportunity costs only. Two surplus sharing methods (equal sharing, proportional sharing) are characterized with the help of four axioms. Separability and No Advantageous Reallocation deal with coalitional changes in the opportunity costs. Additivity and Path Independence take into account variations in the surplus level.Any triple of these axioms characterizes equalor proportional sharing. Any pair of axioms characterize a distinct, infinite family of methods, compromising between equal and proportional sharing.

Mathematical analysis of viscoelastic flows

Abstract: Preface 1. Phenomena in Non-Newtonian Flows 2. Mathematical Formulation 3. Behavior in Simple Flows 4. Existence Theory 5. Numerical Methods 6. High Weissenberg Number Asymptotics 7. Reentrant Corner Singularities 8. Instabilities 9. Change of Type 10. Jet Breakup Bibliography Index.

Extreme scattering events caused by compact structures in the interstellar medium

Abstract: Daily flux density measurements of 36 extragalactic radio sources over a seven year period reveal several unusual minima in the light curves that do not follow typical source variations. The observations were performed using the Green Bank interferometer operating at 2.7 and 8.1 GHz1. The most significant departure from typical source variability occurred at both frequencies in the quasar 0954 + 658 between 1980.95 and 1981.3. It is unlikely that this event can be explained by intrinsic variability. The most satisfactory explanation appears to involve refractive focusing by small-scale inhomogeneities in an ionized structure in the interstellar medium.

Attention Deficit Disorder With and Without Hyperactivity: Comparison of Behavioral Characteristics of Clinic-referred Children

Abstract: Using a reliable assessment procedure, 41 clinic outpatients, 6–13 years of age, were given a diagnosis of Attention Deficit Disorder with Hyperactivity (ADD/H), and 22 outpatients were diagnosed as having Attention Deficit Disorder without Hyperactivity (ADD/WO). Comparisons of measures involved in the diagnostic process showed that the ADD/H children were rated by teachers as being more impulsive. More importantly, comparisons of measures not used in the diagnoses showed that children with ADD/H were more likely to display a severe pattern of aggressive conduct disorder in spite of being younger at the time of assessment. In contrast, the ADD/WO group displayed a more sluggish cognitive tempo, was more likely to receive a codiagnosis of an anxiety or affective disorder, and tended to be rated as more anxious. The results were found not to be explainable by differences in age, intelligence, or socioeconomic status. These findings demonstrate that ADD/WO can be reliably diagnosed in a clinic sample and suggest that ADD/H and ADD/WO differ in important ways.

An approach to structure/control simultaneous optimization for large flexible spacecraft

Abstract: This paper presents an approach to the simultaneous optimal design of a structure and control system for large flexible spacecrafts based on realistic objective function and constraints The weight or total cost of structure and control system is minimized subject to constraints on the magnitude of response to a given disturbance involving both rigid-body and elastic modes A nested optimization technique is developed to solve the combined problem As an example, simple beam-like spacecraft under a steady-state white-noise disturbance force is investigated and some results of optimization are presented In the numerical examples, the stiffness distribution, location of controller, and control gains are optimized Direct feedback control and linear quadratic optimal controls laws are used with both inertial and noninertial disturbing force It is shown that the total cost is sensitive to the overall structural stiffness, so that simultaneous optimization of the structure and control system is indeed useful

Family Relations and the Self-esteem of Adolescents and their Parents

Abstract: Previous research, especially that of Gecas and Schwalbe (1986), relating adolescent self- esteem to parental support, control, and participation is extended in the present investigation, which examines the effects of parent-adolescent communication and investigates dimensions of parent-adolescent interaction that predict parents' self-esteem. The findings suggest that (a) adolescents and their parents have similar but distinct perceptions of their relationships; and (b) self-perceptions of these relationships, especially self-judgments of communication, are important in predicting levels of self-esteem for both adolescents and their parents. Our discussion focuses on gender differences in the relationships reported, the central role of communication within the family unit, and the reciprocal character of parent-adolescent socialization.

The Use of Software Complexity Metrics in Software Maintenance

Abstract: This paper reports on a modest study which relates seven different software complexity metrics to the experience of maintenance activities performed on a medium size software system. Three different versions of the system that evolved over a period of three years were analyzed in this study. A major revision of the system, while still in its design phase, was also analyzed.

Study of Parameter Sensitivity in High-Performance Inverter-Fed Induction Motor Drive Systems

Abstract: Vector control schemes are used in inverter-fed induction motor drives to obtain high performance. Crucial to the success of the vector control scheme is the knowledge of the instantaneous position of the rotor flux. The position of the rotor flux is measured in the direct scheme and estimated in the indirect schemes. Since the estimation of the flux position requires a priori knowledge of the induction motor parameters, the indirect schemes are all machine-parameter dependent. Changes in temperature and saturation levels of the machine vary the machine parameters and, hence, indirectly influence both the steady state and the dynamic operation of the drive system. Analytic expressions are derived to evaluate the effects due to parameter sensitivity. The simulation is experimentally verified on a drive system.

Assaying and isolating individual differences in searching a hierarchical file system

Abstract: Individual differences among users of a hierarchical file system were investigated. Results indicate that psychometric tests of vocabulary and spatial visualization are the best predictors of task ...

The ROS planning system: Evolution, basic concepts, and research needed

Abstract: This paper describes the evolution of the concept of the Recreation Opportunity Spectrum (ROS)—a largely resource‐based approach to providing recreational diversity. It explains the needs of the USDA Forest Service and USDI Bureau of Land Management for a recreation resource planning system and relates those needs to the development of the ROS system to guide recreation planning of large areas. The basic concepts and tenets of the ROS system are explained and needed research is outlined.

Ecological considerations in the development and application of instream flow-habitat models

Abstract: Methodologies for recommending instream flows for protecting lotic ecosystems are evolving amid constructive criticism. During this period of change it is important that all concerned parties are aware of the ecological factors that control stream ecosystems and fish populations. Ecological factors relating to stream ecology, population dynamics, energetics, predation, and competition are reviewed to explain why indices of microhabitat availability are not expected to be consistent predictors of fish population density. Implications of these concepts for development and application of instream flow-habitat models for recommending instream flow regimes are discussed. Current ecological theory and empirical studies support the hypothesis that microhabitat availability may limit fish populations but not continuously. Therefore, assessments must consider the limiting habitat events as well as temperature and water quality constraints. Also, invertebrates and non-game fishes must be considered in instream flow assessments because of their importance in stream ecosystems.

Review of the entry flow problem: Experimental and numerical

Abstract: This paper is concerned with a review of both experimental and numerical studies of axisymmetric and planar entry flows which have been considered as test problems for the numerical simulation of viscoelastic fluids. The test of the method is usually based upon whether the numerical model predicts vortices in the entry corners. However, it is not clear as to whether one should observe vortices for all viscoelastic fluids. Polyacrylamide solutions and Boger fluids exhibit vortices in axisymmetric flow and the size of the vortex does increase with fluid elasticity. However, the vortex is nearly suppressed in planar entry flow. On the other hand, not all polymer melts are found to exhibit vortices in either axisymmetric or planar entry flow. It is our belief that the origin of vortices is not related to the elasticity based on shear flow propertes but to the behavior of the transient extensional viscosity. Certain polymer melts such as low density polyethylene exhibit vortices in both planar and axisymmetric flow along with unbounded stress growth at the start up of extensional flow. It is believed that the constitutive equations used in the numerical simulation must reflect this extensional behavior if vortices are to be predicted. A review of the numerical simulations concerned with entry flow shows that there is considerable doubt about the accuracy of the predictions for most of the studies. Even for those where the numerical solution is thought to be accurate, the magnitude of the stream function associated with the vortices is usually very low. None of the differential models used to date predicts strain hardening extensional viscosity, but those which are thought to predict vortices do rise more rapidly to the steady-state extensional viscosity values with time. It is recommended that the search of test fluids be widened beyond polymer solutions as there may already exist a number of polymer melts which behave similarly to the predictions of existing constitutive equations.