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Difference Model (difference + model)

Distribution by Scientific Domains
Engineering66%

Kinds of Difference Model

  • finite difference model
    		•

    Selected Abstracts

    The possible hydrologic effects of the proposed lignite open-cast mining in Drama lignite field, Greece

    HYDROLOGICAL PROCESSES, Issue 11 2008
    Sotiris Panilas
    Abstract The present study investigates the possible hydrologic effects of the proposed lignite open-cast mining in Drama lignite field (north Greece). Recent years have seen a rapid increase in surface mining. This activity has generated a growing concern for the potential environmental impacts associated with large scale surface mining. In order to achieve a safe mine operation and allow extraction of lignite to considerable depths, extensive dewatering by pumping will be necessary, while at the same time it is desirable to avoid presence of overpumping conditions in the broader area. Based on stratigrafic, hydrologic and hydrogeologic data, a three-dimensional finite difference model was developed in order to simulate the dewatering process of the western part of the lignite open-cast mine in Drama and to predict both spatially and temporally the decline of ground water level down to the lignite surface. The dewatering of the part of the aquifer which underlies the mine area will influence the hydrological conditions of the broader region. The most important anticipated effects will be the abandonment of shallow wells as well as the decrease of ground water pumping rates of deep wells. Aquifer discharge towards the ditches of the study area will cease and there will be an inversion of ground water flow from the ditches towards the underlying aquifer. Dewatering activities will probably result in minor subsidence of the nearby peat deposits of Drama Philippi marshes. Moreover, sand pumping as well as the presence of gasses is likely to cause local subsidence phenomena, mainly in the pit slopes. Copyright © 2007 John Wiley & Sons, Ltd. [source]

    Numerical analysis of Rayleigh,Plesset equation for cavitating water jets

    INTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN ENGINEERING, Issue 7 2007
    H. Alehossein
    Abstract High-pressure water jets are used to cut and drill into rocks by generating cavitating water bubbles in the jet which collapse on the surface of the rock target material. The dynamics of submerged bubbles depends strongly on the surrounding pressure, temperature and liquid surface tension. The Rayleigh,Plesset (RF) equation governs the dynamic growth and collapse of a bubble under various pressure and temperature conditions. A numerical finite difference model is established for simulating the process of growth, collapse and rebound of a cavitation bubble travelling along the flow through a nozzle producing a cavitating water jet. A variable time-step technique is applied to solve the highly non-linear second-order differential equation. This technique, which emerged after testing four finite difference schemes (Euler, central, modified Euler and Runge,Kutta,Fehlberg (RKF)), successfully solves the Rayleigh,Plesset (RP) equation for wide ranges of pressure variation and bubble initial sizes and saves considerable computing time. Inputs for this model are the pressure and velocity data obtained from a CFD (computational fluid dynamics) analysis of the jet. Copyright © 2007 John Wiley & Sons, Ltd. [source]

    A particle finite element method applied to long wave run-up

    INTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN FLUIDS, Issue 3 2006
    J. Birknes
    Abstract This paper presents a Lagrangian,Eulerian finite element formulation for solving fluid dynamics problems with moving boundaries and employs the method to long wave run-up. The method is based on a set of Lagrangian particles which serve as moving nodes for the finite element mesh. Nodes at the moving shoreline are identified by the alpha shape concept which utilizes the distance from neighbouring nodes in different directions. An efficient triangulation technique is then used for the mesh generation at each time step. In order to validate the numerical method the code has been compared with analytical solutions and a preexisting finite difference model. The main focus of our investigation is to assess the numerical method through simulations of three-dimensional dam break and long wave run-up on curved beaches. Particularly the method is put to test for cases where different shoreline segments connect and produce a computational domain surrounding dry regions. Copyright © 2006 John Wiley & Sons, Ltd. [source]

    Computational energy analysis of an innovative isothermal chamber for testing of the special equipment used in the transport of perishable products

    INTERNATIONAL JOURNAL OF ENERGY RESEARCH, Issue 10 2004
    S. K. Chatzidakis
    Abstract This paper describes an improved numerical simulation study of an isothermal chamber recently constructed at Zografou Campus of the National Technical University of Athens (NTUA) for the testing of special equipment used for transporting perishable foodstuffs in accordance with the United Nations ATP agreement. Using a transient finite difference model, a simulation is developed for a modern ATP test chamber and a typical specimen refrigerated vehicle to be tested. The simulation results are compared to experimental measurements taken under real conditions by a data acquisition system and a refrigerated semi-trailer as specimen. Proportional,integral control is employed for the regulation of the cooling and heating system. The impact of various parameters on the time required to reach the set-point temperature (tset) is investigated and the energy consumption is simulated for a period of 22 h. In particular, the impact of specimen insulation thickness and the thickness of the chamber insulation floor are considered in detail. The total energy consumption increases by approximately 16% when the concrete floor layer thickness is increased from 8 to 16 cm for typical initial conditions and desired chamber and specimen temperatures of 32.5 and 7.5°C, respectively. Using a floor insulation of 6 cm extruded heavy strain-resistant polystyrene reduces the energy consumption by at least 13%. Specimen insulation thickness increase from U -value of 0.35 W m,2 K to 0.75 W m,2 K result to an increase in energy consumption by a percentage of 28%. Thermal capacity, temperature of car body and specimen dimensions are also treated as variables that affect the total duration of an ATP test and its total energy consumption. Copyright © 2004 John Wiley & Sons, Ltd. [source]

    Keyhole Dixon method for faster, perceptually equivalent fat suppression

    JOURNAL OF MAGNETIC RESONANCE IMAGING, Issue 1 2003
    Chris A. Flask MS
    Abstract Purpose To reduce the acquisition time associated with the two-point Dixon fat suppression technique by combining a keyhole in-phase (Water + Fat) k-space data set with a full out-of-phase (Water , Fat) k-space data set and optimizing the keyhole size with a perceptual difference model. Materials and Methods A set of keyhole Dixon images was created by varying the number of lines in the keyhole data set. Off-resonance correction was incorporated into the image reconstruction process to improve the homogeneity of the fat suppression. A perceptual difference model (PDM) was validated with human observer experiments and used to compare the keyhole images to images from a full two-point Dixon acquisition. The PDM was used to determine the smallest keyhole width required to obtain perceptual equivalence to images obtained from the full two-point Dixon method. Results In experimental phantom studies, the keyhole Dixon image reconstructed from 96 of 192 Water + Fat k-space lines and 192 Water , Fat k-space lines was perceptually equivalent to the full (192 + 192) two-point Dixon images, resulting in a 25% reduction in scan time. Clinical images of a volunteer's knee, orbits, and abdomen created from the smallest, perceptually equivalent keyhole width resulted in a 27%,38% reduction in total scan time. Conclusion This method improves the temporal efficiency of the conventional two-point Dixon technique and may prove especially useful for high-field systems where specific absorption rate (SAR) limits will constrain radiofrequency (RF)-based fat suppression techniques. J. Magn. Reson. Imaging 2003;18:103,112. © 2003 Wiley-Liss, Inc. [source]

    Breastfeeding as obesity prevention in the United States: A sibling difference model

    AMERICAN JOURNAL OF HUMAN BIOLOGY, Issue 3 2010
    Molly W. Metzger
    In light of the growing prevalence of obesity in the United States, and the health risks associated with childhood obesity in particular, it is critical to identify avenues for obesity prevention. This study tests the hypothesis that breastfeeding serves as one protective factor against children's subsequent development of obesity. We used linear-, logistic-, and sibling fixed-effects regression models to evaluate the association between infant feeding history and body mass index (BMI) in late childhood or adolescence (9,19 years, mean = 14 years). Complete data were available for 976 participants (488 sibling pairs) in the 2002 Child Development Supplement of the Panel Study of Income Dynamics, a nationally representative survey of families in the United States. In sibling pairs in which only one sibling was breastfed, the breastfed sibling had an adolescent BMI that was 0.39 standard deviations lower than his or her sibling, controlling for child-specific factors that may have influenced parents' feeding decisions. This effect is equivalent to a difference of more than 13 pounds for a 14-year-old child of average height. Furthermore, fixed-effects logistic regressions predicting overweight and obese status showed that breastfed siblings were less likely to reach those BMI thresholds. We therefore conclude that breastfeeding in infancy may be an important protective factor against the development of obesity in the United States. The application of a sibling fixed-effects model provides stronger evidence of a causal relationship than prior research reporting similar patterns of association. Am. J. Hum. Biol. 2010. © 2009 Wiley-Liss, Inc. [source]