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Mathematical Models (mathematical + models)
Kinds of Mathematical Models Selected AbstractsCASE STUDIES AND MATHEMATICAL MODELS OF ECOLOGICAL SPECIATION.EVOLUTION, Issue 10 2009We build a spatial individual-based multilocus model of homoploid hybrid speciation tailored for a tentative case of hybrid origin of Heliconius heurippa from H. melpomene and H. cydno in South America. Our model attempts to account for empirical patterns and data on genetic incompatibility, mating preferences and selection by predation (both based on coloration patterns), habitat preference, and local adaptation for all three Heliconius species. Using this model, we study the likelihood of recombinational speciation and identify the effects of various ecological and genetic parameters on the dynamics, patterns, and consequences of hybrid ecological speciation. Overall, our model supports the possibility of hybrid origin of H. heurippa under certain conditions. The most plausible scenario would include hybridization between H. melpomene and H. cydno in an area geographically isolated from the rest of both parental species with subsequent long-lasting geographic isolation of the new hybrid species, followed by changes in the species ranges, the secondary contact, and disappearance of H. melpomene -type ecomorph in the hybrid species. However, much more work (both empirical and theoretical) is necessary to be able to make more definite conclusions on the importance of homoploid hybrid speciation in animals. [source] EVALUATING THE ECONOMIC COST OF ENVIRONMENTAL MEASURES IN PLANTATION HARVESTING THROUGH THE USE OF MATHEMATICAL MODELSPRODUCTION AND OPERATIONS MANAGEMENT, Issue 3 2003FELIPE CARO An important issue being discussed for Chilean pine plantation policies is the application of environmental protection measures when managing its timber areas. Typical measures, already in place in more developed countries, include imposing riparian strips and protecting fragile soils from the use of heavy machinery. While environmental protection measures have been considered vital for decades, so far there has been almost no attempt to quantify both the benefits and costs of these measures. This paper attempts to measure the costs associated with the main measures which can help both the forestry firms and the government evaluate the cost impact of the new environmental protection regulations being studied. The analysis for different environmental scenarios is carried out by modifying a mixed integer LP, currently used for tactical planning by one forestry firm. [source] A Review of Mathematical Models for Hydrogen and Direct Methanol Polymer Electrolyte Membrane Fuel CellsFUEL CELLS, Issue 1-2 2004K.Z. Yao Abstract This paper presents a review of the mathematical modeling of two types of polymer electrolyte membrane fuel cells: hydrogen fuel cells and direct methanol fuel cells. Models of single cells are described as well as models of entire fuel cell stacks. Methods for obtaining model parameters are briefly summarized, as well as the numerical techniques used to solve the model equations. Effective models have been developed to describe the fundamental electrochemical and transport phenomena occurring in the diffusion layers, catalyst layers, and membrane. More research is required to develop models that are validated using experimental data, and models that can account for complex two-phase flows of liquids and gases. [source] Mathematical Models of Social Evolution: A Guide for the Perplexed by Richard McElreath, Robert BoydINTERNATIONAL STATISTICAL REVIEW, Issue 3 2007Hanna Kokko No abstract is available for this article. [source] Prediction of the Distance from the Skin to the Lumbar Epidural Space in the Greek Population, Using Mathematical ModelsPAIN PRACTICE, Issue 2 2005Emmanouil Stamatakis MD Abstract Background and Objectives:, The skin to lumbar epidural space distance (SLED) is variable, and therefore the ability to clinically predict the SLED may help increase the success of epidural anesthesia/analgesia. The goal of this study was to determine the relationship between the SLED and demographic/anthropometric variables in the Greek population, and develop a mathematical model for its prediction. Methods:, This prospective randomized study enrolled 406 male and female Greek patients who required an epidural block as part of their anesthetic management. With patients placed in the left lateral and knee-chest position, the lumbar epidural space was located by the loss of resistance to normal saline technique. Statistical analysis was used to identify the relationship between SLED, and the following variables were evaluated: age, weight, height, body mass index, body surface area, intervertebral space used, pregnancy, and geographic origin within Greece. Results:, No adverse events or dural punctures occurred. Mean SLED in the general population was 4.98 ± 0.95 cm, with values significantly higher in males (5.37 ± 0.88 cm) compared with females (4.83 ± 0.93 cm). SLED was best associated with weight, body surface area, and body mass index. Mathematical formulae for prediction of SLED in the general population and the female population were derived from linear regression analysis. These formulae were able to predict approximately half of the observed variability in SLED. Conclusions:, While mathematical models of SLED can be a useful tool, they should not be exclusively relied on in the clinical setting, but rather should be used as an adjunct to standardized techniques to improve the safety and efficacy of epidural anesthesia/analgesia. [source] From segment to somite: Segmentation to epithelialization analyzed within quantitative frameworksDEVELOPMENTAL DYNAMICS, Issue 6 2007Paul M. Kulesa Abstract One of the most visually striking patterns in the early developing embryo is somite segmentation. Somites form as repeated, periodic structures in pairs along nearly the entire caudal vertebrate axis. The morphological process involves short- and long-range signals that drive cell rearrangements and cell shaping to create discrete, epithelialized segments. Key to developing novel strategies to prevent somite birth defects that involve axial bone and skeletal muscle development is understanding how the molecular choreography is coordinated across multiple spatial scales and in a repeating temporal manner. Mathematical models have emerged as useful tools to integrate spatiotemporal data and simulate model mechanisms to provide unique insights into somite pattern formation. In this short review, we present two quantitative frameworks that address the morphogenesis from segment to somite and discuss recent data of segmentation and epithelialization. Developmental Dynamics 236:1392,1402, 2007. © 2007 Wiley-Liss, Inc. [source] Is Vancouver Canada's supervised injection facility cost-saving?ADDICTION, Issue 8 2010Steven D. Pinkerton ABSTRACT Objective To determine whether Vancouver's Insite supervised injection facility and syringe exchange programs are cost-saving,that is, are the savings due to averted HIV-related medical care costs sufficient to offset Insite's operating costs? Methods The analyses examined the impact of Insite's programs for a single year. Mathematical models were used to calculate the number of additional HIV infections that would be expected if Insite were closed. The life-time HIV-related medical costs associated with these additional infections were compared to the annual operating costs of the Insite facility. Results If Insite were closed, the annual number of incident HIV infections among Vancouver IDU would be expected to increase from 179.3 to 262.8. These 83.5 preventable infections are associated with $17.6 million (Canadian) in life-time HIV-related medical care costs, greatly exceeding Insite's operating costs, which are approximately $3 million per year. Conclusions Insite's safe injection facility and syringe exchange program substantially reduce the incidence of HIV infection within Vancouver's IDU community. The associated savings in averted HIV-related medical care costs are more than sufficient to offset Insite's operating costs. [source] The mid-season crash in aphid populations: why and how does it occur?ECOLOGICAL ENTOMOLOGY, Issue 4 2004A. J. Karley Abstract., 1. Aphid populations on agricultural crops in temperature regions collapse over a few days from peak numbers to local extinction soon after mid-summer (e.g. mid-July in the U.K.). The populations recover 6,8 weeks later. There is anecdotal or incidental evidence of an equivalent mid-season population crash of aphids on grasses and forbs in natural vegetation. 2. The ecological factors causing the mid-season population crash of aphids include a decline in plant nutritional quality and increased natural enemy pressure as the season progresses. Extreme weather events, e.g. severe rainstorms, can precipitate the crash but weather conditions are not a consistent contributory factor. 3. The population processes underlying the crash comprise enhanced emigration, especially by alate (winged) aphids, depressed performance resulting in reduced birth rates, and elevated mortality caused by natural enemies. 4. Mathematical models, previously applied to aphid populations on agricultural crops, have great potential for studies of aphid dynamics in natural vegetation. In particular, they can help identify the contribution of various ecological factors to the timing of the population crash and offer explanations for how slow changes in population processes can result in a rapid collapse of aphid populations. [source] Seasonality and the dynamics of infectious diseasesECOLOGY LETTERS, Issue 4 2006Sonia Altizer Abstract Seasonal variations in temperature, rainfall and resource availability are ubiquitous and can exert strong pressures on population dynamics. Infectious diseases provide some of the best-studied examples of the role of seasonality in shaping population fluctuations. In this paper, we review examples from human and wildlife disease systems to illustrate the challenges inherent in understanding the mechanisms and impacts of seasonal environmental drivers. Empirical evidence points to several biologically distinct mechanisms by which seasonality can impact host,pathogen interactions, including seasonal changes in host social behaviour and contact rates, variation in encounters with infective stages in the environment, annual pulses of host births and deaths and changes in host immune defences. Mathematical models and field observations show that the strength and mechanisms of seasonality can alter the spread and persistence of infectious diseases, and that population-level responses can range from simple annual cycles to more complex multiyear fluctuations. From an applied perspective, understanding the timing and causes of seasonality offers important insights into how parasite,host systems operate, how and when parasite control measures should be applied, and how disease risks will respond to anthropogenic climate change and altered patterns of seasonality. Finally, by focusing on well-studied examples of infectious diseases, we hope to highlight general insights that are relevant to other ecological interactions. [source] TK/TD dose,response modeling of toxicityENVIRONMETRICS, Issue 5 2007Munni Begum Abstract In environmental cancer risk assessment of a toxic chemical, the main focus is in understanding induced target organ toxicity that may in turn lead to carcinogenicity. Mathematical models based on systems of ordinary differential equations with biologically relevant parameters are tenable methods for describing the disposition of chemicals in target organs. In evaluation of a toxic chemical, dose,response assessment often addresses only toxicodynamics (TD) of the chemical, while its toxicokinetics (TK) do not enter into consideration. The primary objective of this research is to integrate both TK and TD in evaluation of toxic chemicals while performing dose,response assessment. Population models, with hierarchical setup and nonlinear predictors, for TK concentration and TD effect measures are considered. A one-compartment model with biologically relevant parameters, such as organ volume, uptake rate and excretion rate, or clearance, is used to derive the TK predictor while a two parameter Emax model is used as a predictor for TD measures. Inference of the model parameters with nonnegative and assay's Limit of Detection (LOD) constraints was carried out by Bayesian approaches using Markov Chain Monte Carlo (MCMC) techniques. Copyright © 2006 John Wiley & Sons, Ltd. [source] ADAPTIVE CHANGE IN THE RESOURCE-EXPLOITATION TRAITS OF A GENERALIST CONSUMER: THE CEOLUTION AND COEXISTENCE OF GENERALISTS AND SPECIALISTSEVOLUTION, Issue 3 2006Peter A. Abrams Abstract Mathematical models of consumer-resource systems are used to explore the evolution of traits related to resource acquisition in a generalist consumer species that is capable of exploiting two resources. The analysis focuses on whether evolution of traits determining the capture rates of two resources by a consumer species produce one generalist, two specialists, or all three types, when all types are characterized by a common fitness function. In systems with a stable equilibrium, evolution produces one generalist or two specialists, depending on the second derivative of the trade-off relationship. When there are sustained population fluctuations, the nature of the trade-off between the consumer's capture rates of the two resources still plays a key role in determining the evolutionary outcome. If the trade-off is described by a choice variable between zero and one that is raised to a power n, polymorphic states are possible when n > 1, which implies a positive second derivative of the curve. These states are either dimorphism, with two relatively specialized consumer types, or trimorphism, with a single generalist type and two specialists. Both endogenously driven consumer-resource cycles, and fluctuations driven by an environmental variable affecting resource growth are considered. Trimorphic evolutionary outcomes are relatively common in the case of endogenous cycles. In contrast to a previous study, these trimorphisms can often evolve even when new lineages are constrained to have phenotypes very similar to existing lineages. Exogenous cycles driven by environmental variation in resource growth rates appear to be much less likely to produce a mixture of generalists and specialists than are endogenous consumer-resource cycles. [source] A steady-state modeling approach to validate an in vivo mechanism of the GAL regulatory network in Saccharomyces cerevisiaeFEBS JOURNAL, Issue 20 2004Malkhey Verma Cellular regulation is a result of complex interactions arising from DNA,protein and protein,protein binding, autoregulation, and compartmentalization and shuttling of regulatory proteins. Experiments in molecular biology have identified these mechanisms recruited by a regulatory network. Mathematical models may be used to complement the knowledge-base provided by in vitro experimental methods. Interactions identified by in vitro experiments can lead to the hypothesis of multiple candidate models explaining the in vivo mechanism. The equilibrium dissociation constants for the various interactions and the total component concentration constitute constraints on the candidate models. In this work, we identify the most plausible in vivo network by comparing the output response to the experimental data. We demonstrate the methodology using the GAL system of Saccharomyces cerevisiae for which the steady-state analysis reveals that Gal3p neither dimerizes nor shuttles between the cytoplasm and the nucleus. [source] Extrapolation of metabolic pathways as an aid to modelling completely sequenced nonSaccharomyces yeastsFEMS YEAST RESEARCH, Issue 1 2008Florian Iragne Abstract Mathematical models of biological processes for the model yeast Saccharomyces cerevisiae are the subject of intensive effort and are available in increasing numbers. An open question is whether such models are informative for related yeasts of biotechnological and medical interest that will not themselves benefit from an equivalent effort. In this study, we assess a method for extrapolating reference models to other completely sequenced yeasts, using a combination of graph-theoretic analysis and reliable identification of homologous genes using Génolevures protein families. In this first assessment, we focus on subtractive modeling, identified through the correlated loss of input and output ports in metabolic pathways. We confirm that the major, highly connected, pathways of central metabolism are conserved and might be universal. In 60,80% of our results, further analysis is not required to determine whether the pathway is lost or conserved, so that our method can be systematically applied as a first step in developing species-specific models. [source] A semimechanistic model predicting the growth and production of the bioenergy crop Miscanthus×giganteus: description, parameterization and validationGCB BIOENERGY, Issue 4 2009FERNANDO E. MIGUEZ Abstract Biomass based bioenergy is promoted as a major sustainable energy source which can simultaneously decrease net greenhouse gas emissions. Miscanthus×giganteus (M.×giganteus), a C4 perennial grass with high nitrogen, water, and light use efficiencies, is regarded as a promising energy crop for biomass production. Mathematical models which can accurately predict M.×giganteus biomass production potential under different conditions are critical to evaluate the feasibility of its production in different environments. Although previous models based on light-conversion efficiency have been shown to provide good predictions of yield, they cannot easily be used in assessing the value of physiological trait improvement or ecosystem processes. Here, we described in detail the physical and physiological processes of a previously published generic mechanistic eco-physiological model, WIMOVAC, adapted and parameterized for M.×giganteus. Parameterized for one location in England, the model was able to realistically predict daily field diurnal photosynthesis and seasonal biomass at a range of other sites from European studies. The model provides a framework that will allow incorporation of further mechanistic information as it is developed for this new crop. [source] Evaluation of soluble benzene migration in the Uinta BasinGEOFLUIDS (ELECTRONIC), Issue 2 2005Y. ZHANG Abstract Field sampling and mathematical modeling are used to study the long-distance transport and attenuation of petroleum-derived benzene in the Uinta Basin, Utah. Benzene concentration was measured from oil and oil field formation waters of the Altamont-Bluebell and Pariette Bench oil fields in the basin. It was also measured from springs located in the regional groundwater discharge areas, hydraulically down-gradient from the oil fields sampled. The average benzene concentration in oils and co-produced waters is 1946 and 4.9 ppm at the Altamont-Bluebell field and 1533 and 0.6 ppm at the Pariette Bench field, respectively. Benzene concentration is below the detection limit in all springs sampled. Mathematical models are constructed along a north,south trending transect across the basin through both fields. The models represent groundwater flow, heat transfer and advective/dispersive benzene transport in the basin, as well as benzene diffusion within the oil reservoirs. The coupled groundwater flow and heat transfer model is calibrated using available thermal and hydrologic data. We were able to reproduce the observed excess fluid pressure within the lower Green River Formation and the observed convective temperature anomalies across the northern basin. Using the computed best-fit flow and temperature, the coupled transport model simulates water washing of benzene from the oil reservoirs. Without the effect of benzene attenuation, dissolved benzene reaches the regional groundwater discharge areas in measurable concentration (>0.01 ppm); with attenuation, benzene concentration diminishes to below the detection limit within 1,4 km from the reservoirs. Attenuation also controls the amount of water washing over time. In general, models that represent benzene attenuation in the basin produce results more consistent with field observations. [source] A discontinuous Galerkin method for elliptic interface problems with application to electroporationINTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN BIOMEDICAL ENGINEERING, Issue 10 2009Grégory Guyomarc'h Abstract We solve elliptic interface problems using a discontinuous Galerkin (DG) method, for which discontinuities in the solution and in its normal derivatives are prescribed on an interface inside the domain. Standard ways to solve interface problems with finite element methods consist in enforcing the prescribed discontinuity of the solution in the finite element space. Here, we show that the DG method provides a natural framework to enforce both discontinuities weakly in the DG formulation, provided the triangulation of the domain is fitted to the interface. The resulting discretization leads to a symmetric system that can be efficiently solved with standard algorithms. The method is shown to be optimally convergent in the L2 -norm. We apply our method to the numerical study of electroporation, a widely used medical technique with applications to gene therapy and cancer treatment. Mathematical models of electroporation involve elliptic problems with dynamic interface conditions. We discretize such problems into a sequence of elliptic interface problems that can be solved by our method. We obtain numerical results that agree with known exact solutions. Copyright © 2008 John Wiley & Sons, Ltd. [source] Optimisation of the medium composition for production of protease and soybean peptides by Bacillus subtilis SHZ using response surface methodologyINTERNATIONAL JOURNAL OF FOOD SCIENCE & TECHNOLOGY, Issue 7 2008Bo Yu Summary Responses surface methodology was employed to enhance the production of protease and soybean peptides by Bacillus subtilis SHZ. For screening of medium composition significantly influencing protease and soybean peptides yield, the two-level Plackett,Burman design was used. Among thirteen variables tested; KH2PO4, glucose and defatted soybean flour (DSF) were selected based on their high significant effect on both protease activity and soybean peptides yield. Then, a three-level Box,Behnken design was employed to optimise the medium composition for the production of the protease and soybean peptides in submerged fermentation. Mathematical models were then developed to show the effect of each medium composition and their interactions on the production of protease and soybean peptides. The model estimated that, the maximal protease activity (320 ± 1 U mL,1) could be obtained when the concentrations of glucose, KH2PO4, DSF were set at 8,9 g L,1, 2,3 g L,1, 55,65 g L,1, respectively; while a maximal yield of soybean peptides (8.5 ± 0.1 g L,1) could be achieved when the concentrations of glucose, KH2PO4, DSF were set at 7,9 g L,1, 3,4 g L,1 and 55,58 g L,1, respectively. These predicted values were also verified by validation experiments. [source] Effects of climate on the growth of exotic and indigenous trees in central ZambiaJOURNAL OF BIOGEOGRAPHY, Issue 1 2005E. N. Chidumayo Abstract Aim, Climate change has far-reaching effects on species and ecosystems. The aims of this study were to determine how climate factors affect the growth pattern of indigenous and exotic trees in Zambia and to predict tree growth responses to a warmer climate with the use of mathematical models. Location, Two savanna sites in central Zambia. Methods, Diameter at breast height (1.3 m above ground, d.b.h.) of 91 permanently marked trees belonging to three indigenous and four exotic species was measured fortnightly for periods of 1,2 years from 1998 to 2003. Correlation and regression analysis was used to determine the effect of climate factors (minimum, maximum and average temperature and rainfall) on monthly daily d.b.h. increment of each species. Regression models were used to predict the growth behaviour of trees under a 0.5 °C warmer climate. Results, Interactions between temperature and rainfall explained 60,98% of the variation in d.b.h. increment in all the tree species, except the exotic Eucalyptus grandis. For deciduous species, stem expansion was delayed by 2,12 weeks following leaf-flush and d.b.h. increment peaked during the rainy season. Evergreen and deciduous species could not be separated on the basis of annual d.b.h. increment because the higher growth rates of deciduous species compensated for the shorter growing period. Mathematical models predicted slight changes in d.b.h. growth pattern under a 0.5 °C warmer climate in five of the seven species. Significant changes in d.b.h. growth patterns were predicted in the indigenous Bridelia micrantha and exotic Gmelina arborea under a warmer climate. However, models failed to adequately represent potential soil water stress that might result from changes in tree growth patterns and a warmer climate. Main conclusions, Climate factors explained a large proportion of the variation in diameter growth of both indigenous and exotic trees, rendering it possible to model tree growth patterns from climate data. Tree growth models suggest that a rise in temperature of 0.5 °C is unlikely to induce significant changes in the growth behaviour of the majority of the studied species. However, because the growth behaviour of some species may be substantially affected by climate change, it is recommended that strategies for the future production of such climate-sensitive trees should incorporate aspects of climate change. [source] In Silico Modeling and Simulation of Bone Biology: A ProposalJOURNAL OF BONE AND MINERAL RESEARCH, Issue 7 2005Nadine A Defranoux Abstract Contemporary, computer-based mathematical modeling techniques make it possible to represent complex biological mechanisms in a manner that permits hypothesis testing in silico. This perspective shows how such approaches might be applied to bone remodeling and therapeutic research. Currently, the dominant conceptual model applied in bone research involves the dynamic balance between the continual build-up and breakdown of bone matrix by two cell types, the osteoblasts and osteoclasts, acting together as a coordinated, remodeling unit. This conceptualization has served extraordinarily well as a focal point for understanding how mutations, chemical mediators, and mechanical force, as well as external influences (e.g., drugs, diet) affect bone structure and function. However, the need remains to better understand and predict the consequences of manipulating any single factor, or combination of factors, within the context of this complex system's multiple interacting pathways. Mathematical models are a natural extension of conceptual models, providing dynamic, quantitative descriptions of the relationships among interacting components. This formalization creates the ability to simulate the natural behavior of a system, as well as its modulation by therapeutic or dietetic interventions. A number of mathematical models have been developed to study complex bone functions, but most include only a limited set of biological components needed to address a few specific questions. However, it is possible to develop larger, multiscale models that capture the dynamic interactions of many biological components and relate them to important physiological or pathological outcomes that allow broader study. Examples of such models include Entelos' PhysioLab platforms. These models simulate the dynamic, quantitative interactions among a biological system's biochemicals, cells, tissues, and organs and how they give rise to key physiologic and pathophysiologic outcomes. We propose that a similar predictive, dynamical, multiscale mathematical model of bone remodeling and metabolism would provide a better understanding of the mechanisms governing these phenomena as well as serve as an in silico platform for testing pharmaceutical and clinical interventions on metabolic bone disease. [source] Modelling cellular senescence as a result of telomere stateAGING CELL, Issue 3 2003Carole J. Proctor Summary Telomeres in mammalian cells end in large duplex T loops. These loops protect the single-strand overhangs from degradation and/or interactions with signalling proteins. This protection is sometimes referred to as capping. At each cell division, telomeres shorten and there is a general consensus that telomere shortening triggers cell cycle exit. However, the exact mechanism by which telomere shortening causes cell cycle arrest is not known. Mathematical models of telomere shortening have been developed to help us understand the processes involved. Until now most models have assumed that the trigger for cell cycle arrest is the first telomere or a group of telomeres reaching a critically short length. However, there is evidence that cells stop cycling over a wide range of telomere lengths. This suggests that telomere length per se may not in fact be the trigger for cellular senescence. In this paper we develop a model which examines the hypothesis that uncapping of a telomere is the main trigger. By letting the probability of uncapping depend upon telomere length, we show that the hypothesized model provides a good fit to experimental data. [source] Thermal Inactivation of Salmonella Senftenberg and Listeria innocua in Beef/Turkey Blended Patties Cooked via Fryer and/or Air Convection OvenJOURNAL OF FOOD SCIENCE, Issue 5 2002R.Y. Murphy ABSTRACT: Beef/turkey blended patties, containing 107 to 108 cfu/g of Salmonella Senftenberg or Listeria innocua, were battered and breaded. The effect of frying (177 °C) and air convection cooking (288 °C) on thermal inactivation of S. Senftenberg and L. innocua was evaluated. A model was obtained to correlate product internal temperature with frying and oven cooking time. Cooking method significantly affected thermal history and subsequently the thermal inactivation of S. Senftenberg and L. innocua. The effect of frying time interacted with oven cooking time. Increasing frying time reduced the oven cooking time. Mathematical models were developed to correlate the survival rate of S. Senftenberg or L. innocua with frying and oven cooking time. [source] One-dimensional model of vacuum filtration of compressible flocculated suspensionsAICHE JOURNAL, Issue 10 2010Anthony D. Stickland Abstract This work details the one-dimensional modeling of the different processes that may occur during the vacuum filtration of compressible flocculated suspensions. Depending on the operating conditions of the applied pressure and the initial solids concentration relative to the material properties of the compressive yield stress and the effective capillary pressure at the air,liquid interface, the dewatering process undergoes a combination of cake formation, consolidation, and/or desaturation. Mathematical models for these processes based on the compressional rheology approach are presented and appropriate solution methods outlined. Results using customary material properties are given for different operating conditions to illustrate the three dewatering processes. This approach lays the theoretical basis for further work understanding two- and three-dimensional effects during desaturation, such as cracking and wall detachment. © 2010 American Institute of Chemical Engineers AIChE J, 2010 [source] Alternative concept for SOFC with direct internal reforming operation: Benefits from inserting catalyst rodAICHE JOURNAL, Issue 6 2010Pannipha Dokamaingam Abstract Mathematical models of direct internal reforming solid oxide fuel cell (DIR-SOFC) fueled by methane are developed using COMSOL® software. The benefits of inserting Ni-catalyst rod in the middle of tubular-SOFC are simulated and compared to conventional DIR-SOFC. It reveals that DIR-SOFC with inserted catalyst provides smoother temperature gradient along the system and gains higher power density and electrochemical efficiency with less carbon deposition. Sensitivity analyses are performed. By increasing inlet fuel flow rate, the temperature gradient and power density improve, but less electrical efficiency with higher carbon deposition is predicted. The feed with low inlet steam/carbon ratio enhances good system performances but also results in high potential for carbon formation; this gains great benefit of DIR-SOFC with inserted catalyst because the rate of carbon deposition is remarkably low. Compared between counter- and co-flow patterns, the latter provides smoother temperature distribution with higher efficiency; thus, it is the better option for practical applications. © 2009 American Institute of Chemical Engineers AIChE J, 2010 [source] Modeling and inferential control of the batch acetylation of celluloseAICHE JOURNAL, Issue 6 2006Anshul Dubey Abstract The batch acetylation of cellulose involves two main stages, acetylation followed by hydrolysis. Mathematical models were constructed for these processes using reaction parameters obtained from earlier publications. The two models were then combined, along with a model predicting the cellulose feedstock moisture content. The overall temperature profiles as well as the final degree of acetylation and polymerization were shown to match the data obtained from a commercial process. From the results of the model and the observations made in a typical process, it has been established that this process is highly sensitive to disturbances in the input streams that can result in unacceptable final properties of the batch. Because the exact disturbance occurring in a batch cannot be identified and measured, inferential control is used to formulate an intrabatch control strategy. Support vector regression is used to make predictions on the final properties of the batch. These predictions, along with the model of the hydrolysis stage, are used to estimate the inputs that, when applied during hydrolysis, can counter the disturbances caused during the acetylation stage. Simulations show that this hydrolysis control strategy can improve the average batch properties of cellulose acetate required for subsequent processing. © 2006 American Institute of Chemical Engineers AIChE J, 2006 [source] 3-D DYNAMIC OPTICAL TECHNIQUES TO MODEL FOOD MECHANICAL DEFORMATIONJOURNAL OF TEXTURE STUDIES, Issue 4 2010MURIEL JACQUOT ABSTRACT Finding new instrumental rheological parameters that better describe sensory textures can improve correlation between rheological and sensory measurements of food. Two optical three-dimensional (3-D) techniques commonly used in mechanical engineering field were studied. These techniques have never been used in food science. Digital image correlation and Breuckmann scanning systems were successful to distinguish gelatin gels and soft cheeses varying in firmness and viscoelastic properties. These two systems were coupled with a universal testing machine to provide information regarding 3-D displacements and surface deformation of sample. Mathematical models were developed to determine surface displacement profiles of samples from their firmness and viscoelastic properties. Three parameters were obtained to describe surface displacement profiles linked to samples textural properties. These parameters may be useful to develop models predicting accurately food sensory texture from instrumental measurements. PRACTICAL APPLICATIONS Finding new instrumental rheological parameters that better describe sensory textures to improve correlation between rheological and sensory measurements of food. [source] Mathematical models for coinfection by two sexually transmitted agents: the human immunodeficiency virus and herpes simplex virus type 2 caseJOURNAL OF THE ROYAL STATISTICAL SOCIETY: SERIES C (APPLIED STATISTICS), Issue 4 2010S. Guy Mahiane Summary., To study the interactions between two sexually transmitted diseases without remission of the infections, we propose to use Markovian models. One model allows the estimation of the per-partnership female-to-male transmission probabilities for each infection, and the other the per-sex-act transmission probabilities. These models take into account the essential factors for the propagation of both infections, including the variability according to age of the rates of prevalence in the population of female partners for the male individuals constituting our sample. We estimate transmission probabilities and relative risks (for circumcision, usage of condoms and the effect of one infection on the infectivity of the other) by using the maximum likelihood method. Bootstrap procedures are used to provide confidence intervals for the parameters. We illustrate the new procedures with the study of the interactions between herpes simplex virus type 2 and human immunodeficiency virus by using data from the male circumcision trial that was conducted in Orange Farm (South Africa). The study shows that the probability that a susceptible male individual acquires one of the viruses is significantly higher when he is already infected with the other. Using the Akaike information criterion, we show that the per-partnership model fits the data better than the per-sex-act model. [source] Host specificity and reproductive success of yucca moths (Tegeticula spp.MOLECULAR ECOLOGY, Issue 24 2009Lepidoptera: Prodoxidae) mirror patterns of gene flow between host plant varieties of the Joshua tree (Yucca brevifolia: Agavaceae) Abstract Coevolution between flowering plants and their pollinators is thought to have generated much of the diversity of life on Earth, but the population processes that may have produced these macroevolutionary patterns remain unclear. Mathematical models of coevolution in obligate pollination mutualisms suggest that phenotype matching between plants and their pollinators can generate reproductive isolation. Here, we test this hypothesis using a natural experiment that examines the role of natural selection on phenotype matching between yuccas and yucca moths (Tegeticula spp.) in mediating reproductive isolation between two varieties of Joshua tree (Yucca brevifolia var. brevifolia and Y. brevifolia var. jaegeriana). Using passive monitoring techniques, DNA barcoding, microsatellite DNA genotyping, and sibship reconstruction, we track host specificity and the fitness consequences of host choice in a zone of sympatry. We show that the two moth species differ in their degree of host specificity and that oviposition on a foreign host plant results in the production of fewer offspring. This difference in host specificity between the two moth species mirrors patterns of chloroplast introgression from west to east between host varieties, suggesting that natural selection acting on pollinator phenotypes mediates gene flow and reproductive isolation between Joshua-tree varieties. [source] Predicting the effect of muscle length on fatigue during electrical stimulationMUSCLE AND NERVE, Issue 4 2009M. Susan Marion PhD Abstract Mathematical models have been developed to predict fatigue during functional electrical stimulation, but the predictive accuracy at different muscle lengths is unknown. The objectives of our study were to: (1) experimentally determine the relationship between knee extension angle (20°, 40°, 65°, and 90°) and fatigue of the quadriceps muscles, and (2) predict that relationship using a mathematical model. A computer-controlled stimulator sent trains of pulses to surface electrodes on the thighs of five subjects while forces were measured at the ankle. A two-component mathematical model was developed. One component accounted for force, and the other accounted for fatigue. The model was fit to the data, and parameters were identified at 90°. The fitted subject-averaged r2 value was 0.89. The model was used to predict fatigue at the remaining angles, and the subject-averaged r2 values were >0.75. Therefore, at least 75% of the variability in the measurements was explained by the model. The force model is explicitly dependent on angle, and the fatigue model is explicitly dependent on force; therefore, the dependence of fatigue on knee angle was implicit. Muscle Nerve, 2009 [source] Mathematical models to reconstruct phylogenetic trees under the minimum evolution criterionNETWORKS: AN INTERNATIONAL JOURNAL, Issue 2 2009Daniele Catanzaro Abstract A basic problem in molecular biology is to rebuild phylogenetic trees (PT) from a set of DNA or protein sequences. Among different criteria used for this purpose, the minimum evolution criterion is an optimality based criterion aiming to rebuild PT characterized by a minimal length. This problem is known to be ,,,,-hard. We introduce in this article some mixed integer programming models, and we also study possible cuts and lower bounds for the optimal value. So far, the number of sequences that can be involved in optimal phylogenetic reconstruction is still limited to 10. © 2008 Wiley Periodicals, Inc. NETWORKS, 2009 [source] vCJD , predicting the future?NEUROPATHOLOGY & APPLIED NEUROBIOLOGY, Issue 5 2000D. A. Hilton The recent emergence of variant Creutzfeldt,Jakob disease (vCJD) in the UK, and demonstration that vCJD is caused by the same prion strain that causes bovine spongiform encephalopathy, have led to concerns about the possibility of a human epidemic. Although only 79 cases of vCJD have occurred to date, it is likely that hundreds of thousands of infected cattle entered the human food chain in the late 1980s and early 1990s, and the average incubation period of vCJD is unknown. Mathematical models have not yet been able to give useful predictions of future numbers of cases, and in the absence of a blood test for vCJD, current attempts to reduce uncertainties about future numbers of cases are based on the accumulation of PrPSc in lymphoreticular tissues. Extensive lymphoreticular PrPSc accumulation has been seen in all cases of symptomatic vCJD so far examined, and in one case 8 months prior to the onset of symptoms. Animal models of prion disease suggest that lymphoreticular involvement occurs early in the incubation period and reliably predicts future neurological disease. Based on these data, large scale anonymous studies looking for PrP accumulation in surgically removed tonsillectomy and appendicectomy specimens are underway. Examination of the first 3000 specimens has not revealed any positive samples, but at the moment the significance of negative findings is uncertain. It is anticipated that by the time these studies are complete more data will be available on how early PrP can be demonstrated in lymphoreticular tissue in vCJD, which together with the results from examination of further samples, will allow some comment as to the likelihood of a large human vCJD epidemic. [source] | ||||||||||||||||||||||||||||||||||||||||||||||||||||