A Study of Users' Buffer Variations in Random Access Satellite Channels

A novel approach to modeling the slotted-ALOHA random access channel is presented. The objectives of the model are threefold: (1) to identify the channel's stability boundaries analytically as a function of the system's independent input parameters; (2) to investigate the impact of buffer-control on the channel's behavior; and (3) to facilitate the structuring of individual users' buffer-control as the focus of a dynamic control scheme. The significance of maintaining channel stability via buffer-control lies in its conceptual simplicity and implementability. Simulation runs of a finite-population model have clearly revealed the variation in individual users' buffer sizes and their sensitivity to the retransmission procedure.     

On the use of pressure‐volume‐temperature data of polyethylene liquids for the determination of their solubility and interaction parameters

Specific volumes of high‐density and low‐density polyethylene liquids at several elevated temperatures and pressures were measured. The measured specific volumes were then used to estimate the thermal expansion coefficients $\left( {{\rm \alpha = }\frac{{\rm 1}}{v}\left( {\frac{{\partial v}}{{\partial T}}} \right)_P } \right)$ and isothermal compressibility $\left( {{\rm \beta = } - \frac{{\rm 1}}{v}\left( {\frac{{\partial v}}{{\partial P}}} \right)_T } \right)$ of the polymers. Two different approaches were used in which one was simply to fit the raw data by second order polynomials to obtain (?v/?T)_P and (?v/?P)_T, while the other by the Sanchez‐Lacombe (S‐L) equation of state. It was found that the resultant α and β obtained from the above methods differ significantly, indicating that the S‐L equation of state may not be suitable for determining α and β at elevated temperatures. When these two sets of α and β were used to calculate the corresponding solubility parameters and then the Flory‐Huggins interaction parameters (χ) of the polymers, the results also differ considerably. Nonetheless, χ obtained from the first method agrees well with the results obtained from small angle neutron scattering measurements while the S‐L equation of state method does not. The current results suggest that solubility and interaction parameters obtained from pressure‐volume‐temperature experiments depend critically on the manner by which the data analysis is performed. Polym. Eng. Sci. 44:853–860, 2004. © 2004 Society of Plastics Engineers.     

The elastostatic axisymmetric problem of a sphere containing a penny-shaped crack in a nonequatorial plane

The axisymmetric problem of a sphere containing a penny-shaped crack in a nonequitorial plane is solved with the use of Bousinesq stress functions. Two coordinate systems—oblate spheroidal for representing the crack surface and spherical polars for the spherical surface, translated along the z-axis with respect to each other—are used to satisfy boundary conditions. Integral representations and transformations of harmonic functions are used to relate stress functions in the two coordinate systems. This procedure-leads to a system of algebraic equations which is solved, for axisymmetric tractions on both the surfaces. Graphical results are presented for a specific loading case.     

Need for recovery from work: evaluating short-term effects of working hours,patterns and schedules

In this paper working hours, patterns and work schedules of employees were evaluated in terms of need for recovery from work. Self-administered questionnaire data from employees of the Maastricht Cohort Study on Fatigue at Work (n = 12,095) were used. Poisson regression analyses and multivariate logistic regression analyses revealed that higher working hours a day and working hours a week generally went together with more need for recovery from work. Overtime work was particularly associated with higher need for recovery from work in both genders. Both male and female three-shift or irregular shift workers had higher odds of elevated need for recovery compared to day workers. When additionally controlling for work-related factors, need for recovery levels among shift workers substantially lowered. This study clearly showed that working hours and schedules are associated with need for recovery from work, with different associations for men and women. Especially the associations between work schedules and need for recovery from work were very interrelated with other work-related factors. Future studies could further investigate the possibility that shift work might function as a proxy of other work-related factors that explain the different levels in need for recovery from work, or that job demands are perceived higher among shift workers and may therefore lead to more need for recovery from work.     

Multiaxial fatigue life modelling using hybrid approach of critical plane and genetic algorithm

This paper presents a new hybrid approach for multiaxial fatigue life estimation, based on continuum damage mechanics theory and a genetic algorithm with critical plane model formulation. The hybrid model employs a genetic algorithm based setup for calibration with standard proportional and non‐proportional profiles to predict fatigue life for complex loading profiles. The model is evaluated using experimental fatigue life data for SS304 steel. Calibration using simplified profiles is in agreement with the requirement for cost‐effective experimental fatigue life testing. In‐phase and out‐of‐phase loads are used for calibration, and fatigue life is predicted for more complicated profiles. The results show good agreement between the estimated and experimental fatigue life, and calibration through simple loading histories to predict fatigue life for complex histories appears to be an effective solution using the proposed model. A brief comparison is presented with fatigue life estimation performance of the proposed model with models available in commercial codes. Proposed model found to be more consistent in fatigue life prediction against various loading conditions.     

Evaluation of mechanical properties using spherical ball indentation and coupled finite element–element-free galerkin approach

In the present work, the mechanical properties of pressure tube material (Zr-Nb2.5) are evaluated using the coupled finite element–element-free Galerkin approach. Penalty approach is used to impose contact constraints and non-penetration condition at the interface. An efficient node-to-segment algorithm is employed to model the contact behavior. An updated Lagrangian approach is used to model the large deformation. Loading and unloading response of the indentation process is analyzed using von-Mises and Gurson-Tvergaard-Needleman (GTN) plasticity models. In multiple indentations, the indentation depth is progressively increased up to a maximum specified limit with partial unloading. Load-indentation depth curves are used to extract the flow properties of the material.     

Optimizing Concentration of Drag Reducing Polymer in Case of One- and Two-Phase Flow in 90° Copper Elbow

Journal of Failure Analysis and Prevention - The rate of diffusion-controlled corrosion of 90° Copper Elbow by acidified dichromate has been investigated in relation to the following...     

Scheduling with an outsourcing option on both manufacturer and subcontractors

This paper addresses a single-stage scheduling problem with outsourcing allowed where each job can be either scheduled for in-house production or outsourced to one of the outside subcontractors available. The manufacturer has an unrelated parallel machine system, and each subcontractor has its own single machine. Subcontractors are capable to process all the jobs. Unlike most of past research, our study considers the joint scheduling of both in-house and outsourced jobs simultaneously. The objective is to minimize sum of the total weighted completion time and total outsourcing cost. An integer programming formulation is presented and then improved through an optimality property on job orders. A heuristic algorithm is also introduced to decompose the problem into smaller and easier subproblems and solve them to optimality.     

Ground-based method for measuring thermal infrared effective emissivities: Implications and perspectives on the measurement of land surface temperature from satellite data

Thermal infrared emissivity is an important parameter for surface characterization and for determining surface temperature. The field-based measurements for ground and vegetation emissivities in 8-14 w m waveband were performed with an emissivity box. A theoretical analysis was carried out using the box and a correcting factor has been determined. The average value for thermal band emissivity of the exposed bare soil was found to be around 0.909; the average value measured for most of the varieties of vegetation present were in the range of 0.980-0.985. A theoretical model is used for obtaining effective emissivity in the 8-14 w m region from Advanced Very High Resolution Radiometer (AVHRR) data considering the proportion of vegetation cover in a pixel and the field-measured emissivity values. The error of the methodology is found to be within 1.5%. Narrow band emissivities for AVHRR channels 4 and 5 have been derived from the emissivity values in the 8-14 w m waveband. The surface temperature has been derived from AVHRR data using a split-window algorithm as a function of emissivities derived in narrow bands. The split-window algorithm accounted for absorption effects of the atmosphere by incorporating the water vapour concentration measured in the campaign. A good agreement was obtained between the satellite-derived surface temperature and the in situ observations. The result suggest that the methodology allows us to derive land surface temperature with an accuracy better than 1.5° C provided the surface emissivity is known. The paper describes the field-based emissivity measurement and approach for deriving surface temperature over land surface.     

Biomass burning and related trace gas emissions from tropical dry deciduous forests of India: A study using DMSP-OLS data and ground-based measurements

Biomass burning is one of the major sources of trace gas emissions in the atmosphere. In India the major sources of biomass burning include deforestation, shifting cultivation, accidental fires, controlled burning, fire wood burning, burning from agricultural residues and burning due to fire lines. Studies on biomass burning practices gain importance due to increasing anthropogenic activities and increasing rates of deforestation. Satellite data have been widely used over the globe to monitor the rates of deforestation and also with respect to biomass burning studies. But, much of the polar orbiting satellites, due to their repetitive cycle, have limitations in observing such events and in the tropics, due to cloud cover, getting a cloud-free image during the daytime is difficult. In this study we used Defence Meteorological Satellite Program Operational Line Scanner (DMSP-OLS) night-time data to study the biomass burning events over a period of 10 years from 1987 to 1998 for the Eastern Ghats region, covering the northern part of Andhra Pradesh, India. Two ground-based experiments were carried out to quantify the emissions from biomass burning practices. The results of the study with respect to trace gases suggested emission ratios for CO, CH4, NOx and N20 during the burning to be about 12.3%, 1.29%, 0.29% and 0.07% at the first site and 12.5%, 1.59%, 0.29% and 0.05% at the second site, suggesting low inter-fire variability between the sites. The variation has been attributed to the fuel load, vegetation characteristics, site conditions and local meteorological parameters affecting the relative amounts of combustion. Using the DMSP OLS derived areal estimates of active fires, the trace gas emissions released from the biomass burning were quantified. The results suggested the emissions of 8.2 2 10 10 g CO 2, 1.8 2 10 8 g CO, 6.0 2 10 6 g N 2 O, 3.0 2 10 6 g NO x and 1.2 2 10 8 8 g CH 4 during March 1987. The emissions increased to 1.0 2 10 11 g CO 2, 2.3 2 10 g CO, 7.8 2 10 6 g N 2 O, 3.9 2 10 7 g NO x and 1.6 2 10 8 g CH 4, over a period of 10 years. The results of the analysis suggest the possible use of monitoring biomass burning events from DMSP-OLS night-time data.