106 research outputs found
Estimation and Identification of Neighbours in Wireless Networks Considering the Capture Effect and Long-delay
No full textThe number of neighbour nodes within the direct communication range for a particular node can vary significantly due to the dynamic nature of the wireless communication networks. For a large number of neighbours, it is feasible to obtain an estimation of the number of neighbours very quickly. Moreover, identification (collecting the identities) of the neighbours gives more accurate information about them. Sometimes, signal propagation between nodes has an impact on these
procedures. For terrestrial communication networks the signals suffer only spreading loss for electromagnetic (EM) waves. In underwater communication networks, signals suffer spreading and absorption loss for both EM and acoustic waves. Due to the difference in signal propagation, the capture effect (the reception of a packet even from the collision) also differs in different networks. Furthermore, there are long propagation delays in underwater acoustic networks (UANs) due to the low speed of propagating signals, and in long-range EM communications, such as space communication networks (SCNs). Current approaches to estimation and identification fail to cope with either long-delay or the capture effect. In this work, we present a procedure for estimation of number of neighbours which takes into account the capture effect and propagation delay into consideration so that it can be used in any type of network. A concurrent procedure for estimating the neighbour s spatial dimensionality, which determines whether the neighbours are oriented in 1D, 2D or 3D, is also given. The time taken for estimation is independent of the number of neighbours, and is suitable for networks of all sizes. By a performance and sensitivity analysis, we show the robustness of the dimensionality estimation procedure.
Finally, we propose a procedure for identifying the neighbours for any longdelay networks (LDNs), such as UANs and SCNs using either un-slotted or slotted protocols, which are insensitive to the propagation delay. If the existing identification
approaches are used in LDNs, a large amount of time is wasted. The procedures developed in this work outperform those from previous work showing improvements of 75% in time and 45% in power consumption. Furthermore, our approach is less sensitive to receive time variation of a packet
Analysis of collision injuries with consideration of selectivity bias in linked police-hospital data
No full textThe Maximum Abbreviated Injury Scale (MAIS), which is utilized in linked police-hospital data, is a better estimation of severity than the KABCO scale. However, the issues of sample selection should be taken into consideration while using the linked police-hospital data that generates MAIS. Past studies have overlooked this issue in the injury severity models involving MAIS. A bivariate sample selection model is the established method for mitigating the selection bias. This study conducted a Monte Carlo simulation to investigate the sample selection issues in police-hospital linked data. Three alternative model specifications for a bivariate ordered probit model were compared with the univariate ordered probit model. The parameters were compared at different censoring levels, and at different correlations between the errors in sample selection and outcome equations. The results show that the univariate model computed biased estimates and the magnitude of bias increased with higher levels of censoring and correlation between the errors. Pedestrian injury severity analysis in Indiana was demonstrated as a case study. Certain important factors, such as pedestrian actions, weather variables, road type, and functional classification, were confirmed in the case study. The injury analysis was also extended to injury by body regions. The results of this study can assist to precisely estimate injury outcome by hospital data; provide a better understanding of factors affecting different body parts; and help comprehend some relevant updating process for the KABCO or MAIS injury scales
RANS computation of propeller tip vortex flow for steady and unsteady cases
No full textCFD simulations were conducted for different marine propellers at steady and unsteady flow conditions using a commercial RANS solver ANSYS ® CFX ®. For steady simulation, a spiral-like domain aligned with the vortex core was generated with structured grids. The simulation was validated with the David Taylor Model Basin DTMB 5168 propeller model at open-water condition. -- Various eddy viscosity turbulence models and Reynolds-stress models were employed in the computations. The effect of the turbulence modeling on the solution was investigated. The blade surface pressure and the propeller performance were also computed. The simulation data were compared with the experimental data. -- The unsteady simulation was conducted for propeller at inclined flow condition. A single domain was generated with structured grids. A simulation technique for inclined flow condition was presented. The simulation was validated with the model test data of DTMB 4718 at design condition. A fully implicit coupled solver was used. A segregated solver with Incomplete Lower Upper (ILU) factorisation technique was employed in the simulation. Algebraic Multi-grid (MG) model was used to accelerate the convergence. Advection terms were descretised by high resolution scheme while the viscous terms were treated by employing central difference scheme. The transient terms were descretised with Second Order Backward Euler scheme. The Shear Stress Turbulence model was employed in the computation. -- The effect of grid sensitivity and domain size were investigated. The periodic loadings on the pressure and suction sides of the blades were compared with the experimental data. Reasonable agreement with the computed amplitude of the pressure variations was found. The predictions of the phase of the pressure variations were less favourable. -- This work is the first attempt of the CFD simulation for unsteady propeller flow investigation using a spiral like computational grid. Further improvements and extensions of this work can be made. Suggestions are made regarding future work on the use of CFD simulations of propeller flow.Bibiography: l. 93-99
Theoretical and numerical methods for predicting ship-wave impact generated sea spray
No full textSpray generated by ships traveling in cold oceans often leads to topside ice accretion, which can be dangerous to vessels. To develop a full methodology of goal based design for ice accretion there are two critical knowledge gaps, both of which are complex to close, and require new methods and techniques. One is a comparison of ice accretion rates for different structures in the same icing conditions. The second knowledge gap is validation data that compares predicted ice growth rates for all types of ship and offshore structures against observed values.
Estimation of the spray flux is a first step in predicting icing accumulation. The amount of spray water, the duration of exposure to the spray, and the frequency at which the spray is generated are all important parameters in estimating the spray flux. Most existing spray flux formulae are based on field observations from small fishing vessels. They consider meteorological and oceanographic parameters but neglect the vessel behavior. Ship heave and pitch motions, together with ship speed and heading relative to the waves, determine the frequency of spray events. Thus the existing formulae are not generally applicable to different sizes and types of vessels. The current study develops simple methods to quantify spray properties in terms that can be applied to vessels of any size or type, which consequently addresses the first knowledge gap. Formulae to estimate water content and spray duration are derived based on principles of energy conservation and dimensional analysis.
To estimate spray frequency considering ship motions, a theoretical model is proposed. The model inputs are restricted to ship’s principal particulars, operating conditions, and environmental conditions. Wave-induced motions are estimated using semi-empirical analytical
expressions. A novel spray threshold is developed to separate deck wetness frequency from spray frequency. Spray flux estimates are validated against full-scale field measurements available in the open literature and reasonable agreement was obtained.
The complex interaction between the structure and a multi-phase fluid, including spray are not fully understood. Limitations of field measurements and model experiments encourage the use of numerical simulation to understand the formation of such spray. In this study, full-scale simulation models of wave-generated sea spray are also developed by implementing a smooth particle hydrodynamics (SPH) method. A three-dimensional (3D) numerical wave tank equipped with a flap-type wave maker and a wave absorber is created to produce regular waves of various heights and steepness. A full-scale medium-size fishing vessel (MFV) is modeled to impact waves in head sea conditions at various forward speeds. Moving ship dynamics with three degree-of-freedom (3-DOF) in waves are resolved instead of mimicking a relative ship speed. The resultant spray water amount is measured using a numerical collection box and compared against field measurements and the theoretical model, where a reasonable agreement is found. The model is able to distinguish between green water and spray water. A multi-phase two-dimensional (2D) simulation is also performed that demonstrates the role of wind in the fragmentation of water sheets into droplets and their distributions over the deck. The simulation results indicate energy released from a surging ship significantly contributes to the generation of spray.
An investigation was also performed to explore means to speed up the computationally intensive SPH simulations. A comparison with a traditional CPU (central processing unit) clusters with GPU
(graphics processing unit) was performed where GPUs demonstrated faster executions. All the SPH simulations were run on GPUs.Includes bibliographical references (pages 151-173)
Agriculture, Markets, and Poverty: A Comparative Analysis of Lao PDR and Cambodia
No full textLao PDR and Cambodia have been transitioning to a market-oriented policy regime. Both are agrarian economies with agriculture contributing about one-third of the gross domestic product (GDP). This study assessed their prospects of achieving the first Millennium Development Goal (MDG 1, extreme
poverty eradication) and the centrality of agricultural growth in achieving this goal. As these are macro relationships, richer insights on determinants of poverty were obtained by detailed analyses of recent household surveys in these countries. Some of these insights relate to access to markets, returns to crops, education, land size, non-farm activities, ethnic affiliation, and rural infrastructure, with
unavoidable variation due to differences in the coverage of the household surveys used. Another major theme studied for Cambodia is integration of farmers (especially smallholders) into markets, focusing
on barriers between large and smallholders (e.g., transaction costs). An accelerated transition to a more market-oriented policy regime may promote not just a more efficient agriculture but also a more equitable outcome
Pedestrian injury analysis with consideration of the selectivity bias in linked police-hospital data
No full textA bioinspired strategy for immobilizing silver nanoparticles towards the synthesis of antimicrobial paper
No full textMeasuring Households' Vulnerability to Idiosyncratic and Covariate Shocks – the case of Bangladesh
No full textThe paper examines the level and sources of vulnerability in rural Bangladesh using a household survey. We use a simple two-level random intercept model to estimate expected mean and variance in consumption as well as to decompose the variance into idiosyncratic and covariate components. Our results indicate that both idiosyncratic and covariate shocks have considerable impact on household's vulnerability and idiosyncratic shocks seem to have greater impact on household's consumption vulnerability than the covariate shocks. Furthermore, idiosyncratic shocks appear to have a relatively higher impact on relatively well endowed (i.e. in terms of human capital, land holdings, activity status etc.), well off households and covariate shocks seem to have a relatively higher impact on poorer, less educated, household's vulnerability. Our results also reveal that rural vulnerability in Bangladesh is mainly poverty induced rather than risk induced. Around 78 per cent all who are vulnerable is accounted for by low expected mean consumption and only 22 per cent of them are due to high consumption volatility. Overall vulnerability in rural areas is estimated to be 50 per cent. The categorization of poverty into transient and chronic poverty is even more insightful. The study finds that those without education or agricultural households are likely to be the most vulnerable. The geographical diversity of vulnerability is considerable. It is suggested that ex ante measures to prevent households from becoming poor as well as ex post measures to alleviate those already in poverty should be combined.Poverty, Vulnerability, Risks, Poverty dynamics, Bangladesh
Selective Probing of Thin-Film Interfaces Using Internal Reflection Sum-Frequency Spectroscopy
No full textThe study of interfacial properties of thin films such as polymers is an important area of surface science. The application of visible–infrared sum-frequency generation spectroscopy to such systems requires a careful interpretation of the results, as the electric field magnitude and phase at each interface must be determined in a manner that takes thin-film interference effects into account. Several schemes have been proposed for handling the local field corrections, and these methods all have their origins in linear optics. We first provide an extensive discussion of the cases in which the film is sufficiently thick that multiple beam interference can be ignored or sufficiently thin in which the relevant expressions collapse to simple forms. Then, we illustrate a straightforward method that has a concise analytic solution in the case of a single thin film that exhibits interference effects. We demonstrate a visualization technique that allows the experimental geometry to be tuned to select the interface of interest and rapidly switch between the interfaces when the film thickness is chosen to accommodate this.This work was supported by a Discovery Grant from the Natural Sciences and Engineering Research Council of Canada (NSERC), and an NSERC Collaborative Research and Discovery Grant in partnership with ASASoft (Canada) Inc. Equipment was purchased with support from the Canadian Foundation for Innovation and the British Columbia Knowledge Development Fund. Raman and profilometry measurements were performed at the University of Victoria Centre for Advanced Materials and Related Technologies (CAMTEC). Dr. Stanislav Konorov (UVic Chemistry) provided advice on the application of Raman spectroscopy to the thickness measurements. The authors thank Prof. Aaron Massari (University of Minnesota) and Prof. Sean Roberts (University of Texas at Austin) for stimulating discussions on the topic of multiple beam interference in SFG spectroscopy. The authors thank Prof. Akihiro Morita (Tohoku University) for valuable discussion on the topic of electric fields at interfaces.FacultyReviewe
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