Quasi Two-dimensional Model Research Articles

Transient bottom topography changes in alluvial streams

The development of a quasi two-dimensional computational model for simulating the transient variations of bed topography profiles in alluvial river channels is reported. The formulation of the model is based on combining the longitudinal flow momentum with the continuity principle of the sediment bed load. The Engelund-Hansen formula is employed in estimating the total sediment bed load along the reach of a river channel. The lateral bed load contribution from the total load is calculated in the same way as in calculating the lateral secondary currents from the main flow velocities. The numerical scheme and the computational procedure used in the study are described in detail. The simulated bed level profiles are verified through comparisons with experimental and field measurements taken from case studies in the literature for different flow conditions, channel characteristics, and sediment properties. The correlation between flow discharge, bed load, boundary friction, and channel slope is discussed. On the basis of the reasonably good comparisons with field data, it may be deduced that the model can be used for predicting the bottom topography variations in river channels.Key words: meandering rivers, bottom topography, sediment transport, bed load, boundary roughness, field measurements, experimental data, computational modelling, finite difference method.

Calculation of transverse energy regime in curved channels

The development of a quasi two-dimensional model for simulating the transverse and longitudinal energy gradient effects on the streamwise flow regime in meandering river channels is reported. The mathematical formulation of the curved flow is based on the principle of minimum stream power, where the total flow energy rate was divided into two components; longitudinal and transverse energy components. The mechanisms by which the energy is dissipated has been formulated on the basis of boundary resistance (longitudinal and transverse), internal turbulent friction and secondary circulation. The resulting increase of the lateral energy dissipation rate in the flow is emphasised. A direct relationship between the lateral energy dissipation rate and the ensuing excess boundary resistance to flow is proposed. The procedure used in the numerical calculations is described in detail. The model has been applied successfully to channels with rigid boundaries and found to give accurate predictions for the velocity and shear stress distributions. Finally, the model has been applied to two different case studies representing quasi river channel situations; a water supply canal and a curved channel with mobile bed. On the basis of the good comparisons obtained with measurements, it can be concluded that the model can be applied successfully to flow situations in river systems.

Field Emission Structure with Shottky-Barrier Electrode

ABSTRACTWe analyze a non-traditional version of semiconductor field emission structure, based on silicon cones arrays and destined for application in low-voltage vacuum microelectronics devices. In proposed construction, the gate electrode is used as Shottky-barrier contact on silicon tips. Due to Shottkybarrier contact we have obtained depletion layer in tip body under the gate electrode. Therefore variation of the gate electric potential provides the emitter current modulation. Experimental structures were fabricated with about 28000 silicon cones per 1 mm2 by reactive ion etching through a silica mask. Using a quasi two-dimensional model, we have computed these emitter structures. The model takes into account non-uniformity of silicon cone cross-section. Here, we study the influence of emitter tips parameters on the structure performance. Initial results prove the possibility of cathode current electric control with the gate electrode potential. Additionally we have obtained some other electric parameters of the emitter with the Shottky-barrier contact. The results of numerical analysis and experimental study provide a guide for design of proposed field emitter structure.

Calculation of Neel temperature in quasi-two dimensional antiferromagnetic solids: Application to La 2CuO 4

The role of interlayer coupling on Neel temperature has been studied by using the quasi-two dimensional Heisenberg model and Zubarev double time temperature Greens function. We have used the Callen random phase approximation method to decouple the higher order Greens functions. We have compared our theory with the experimental results La 2CuO 4. A good agreement between theory and experiment is found.

Quasi–Two-Dimensional Simulation of Scour and Deposition in Alluvial Channels

Most existing sediment routing models are one-dimensional models and hence they cannot be used to simulate lateral channel bed deformations. Conversely, a two-dimensional model is computationally time consuming, and is not suitable for long-term simulation. Using the stream tube concept, a one-dimensional model is extended to a quasi–two-dimensional model in this study. It is capable of simulating lateral variations of channel cross section through the adjustments of stream tube boundaries. The model has an option to treat the suspended load and bed load separately and hence is able to simulate the deposition behavior of the suspended sediment under a nonequilibrium process. The model also provides options to solve either de Saint Venant equation or energy equation and hence can be applied in both steady and unsteady flow conditions. An assessment of this model's performance has been conducted through a comparison to an analytical solution and a set of experimental data. The application of this model to the Keelung River and the Shiemen Reservoir in Taiwan also gave convincing results.

DRAINMOD-N, A NITROGEN MODEL FOR ARTIFICIALLY DRAINED SOIL

DRAINMOD-N, a quasi two-dimensional model that simulates the movement and fate of nitrogen in shallow water table soils with artificial drainage, is described. Results of sensitivity analyses are presented and model predictions are compared with results from VS2DNT, a more complex, two-dimensional model. The nitrogen transport component is based on an explicit solution to the advective-dispersive-reactive (ADR) equation. Nitrate-nitrogen is the main N pool considered. Functional relationships are used to quantify rainfall deposition, fertilizer dissolution, net mineralization, denitrification, plant uptake, and surface runoff and subsurface drainage losses. A sensitivity analysis showed DRAINMOD-N predictions are most sensitive to the standard rate coefficients for denitrification and mineralization and nitrogen content in rainfall. Simulated daily water table depths were within 0.121 m, cumulative subsurface drainage rates were within 0.016 m, and cumulative surface runoff rates were within 0.003 m, of those predicted by VS2DNT for a 250-day period. DRAINMOD-N predictions for NO3-N losses in subsurface drainage water only differed from VS2DNT predictions by less than 2.6 kg ha–1. DRAINMOD-N predictions for denitrification were within 8%, for plant uptake were within 15%, and for net mineralization were within 26%, of those simulated by VS2DNT.

Generalized low-temperature NMOSFET substrate current mechanism

Based on the quasi two-dimensional model proposed for low-temperature NMOSFET operation, the substrate current mechanism in the NMOSFET structure in the temperature range 77–295K is investigated. It is found that the electron mean-free path is temperature-independent, with a value of about 7.6nm. Although the maximum channel electron energy at low temperatures is a little larger than at room temperature, the reduced impact ionization results in an increase of the substrate current of less than one order of magnitude. With this mechanism, the simulation could reach fairly good agreement with the experiment.

Development of numerical simulation methods and analysis of extrusion processes of particle-filled plastic materials subject to slip at the wall

Many particle-filled materials, such as powder/binder mixtures, are known to show slip phenomena at a solid wall. It is thus of great importance to characterize the slip phenomena for rheological data of particle-filled plastic materials and ultimately to introduce an appropriate slip modeling into the numerical simulations of the extrusion process. In order to simulate screw extrusion processes of such particle-filled materials, numerical analysis programs were developed via a finite element method and a finite difference method for quasi three-dimensional and two-dimensional flow models of extrusion processes, respectively, with the slip phenomena taken into account in terms of a slip velocity. The present paper presents detailed numerical methods to incorporate the slip velocity as a function of wall shear stress and discusses essential slip effects in the screw extrusion process using the slip-corrected and uncorrected viscosity data for both the two-dimensional flow and the quasi three-dimensional flow model. It was confirmed that the quasi three-dimensional result at the center line of the channel from the finite element analysis was in good agreement with the two-dimensional one from the finite difference analysis. Slip effects were found to be very significant on the overall flow behavior during the extrusion process from the numerical results based on viscosity data without the slip correction. These deviated from those based on the slip-corrected viscosity data, especially in analyzing the screw characteristics.

High-altitude explosions and their magnetohydrodynamic description

The paper provide, approaches for formulating general ideas of a gas dynamic stage for an explosion in the upper atmosphere in which the geomagnetic field is of importance. Various versions concerned with magnetic pressure influence, particle collisions, and the latitudinal atmosphere nonuniformity are discussed. The following magnetogasdynamic models are constructed: 1) a quasitwo-dimensional model which treats the spherically symmetric flows with the magnetic force averaged over angles, 2) a two-dimensional model, and 3) a three-dimensional model. The latter model is applied to a general case of an arbitrary directed magnetic field. The computational results obtained for particular flows according to the magnetohydrodynamic (MHD) models are given. In the next paper we substantiate the MHD-theory application even for very rarefied magnetified plasma

Magnetic correlation length in oxide superconductors

We have used a self-consistent theory based on the quasi-two dimensional Heisenberg model to explain the magnetic correlation length for both undoped and doped samples of La 2 CuO 4− χ . We have used the self-consistent Green's function approach to find an analytical expression for the magnetic correlation length. Our analytical expression gives an exponential decay of correlation length with temperature and it includes the effect of doping through it's dependence over the interlayer coupling. We have compared our numerical calculations for magnetic correlation length with experiments. A good agreement between our theory and experimental results is achieved. We have also compared our numerical results for the undoped material with other theoretical results.

CO2 cycling in the coastal ocean. I – A numerical analysis of the southeastern Bering Sea with applications to the Chukchi Sea and the northern Gulf of Mexico

A quasi-two dimensional model of the carbon and nitrogen cycling above the 70m isobath of the southeastern Bering Sea at 57°N replicates the observed seasonal cycles of nitrate, ammonium, ΣCO2, pCO2, light penetration, chlorophyll, phytoplankton growth rate, and primary production, as constrained by changes in wind, incident radiation, temperature, ice cover, vertical and lateral mixing, grazing stress, benthic processing of phytodetritus and zooplankton fecal pellets, and the pelagic microbial loop of DOC, bacteria, and their predators. About half of the seasonal resupply of nitrate stocks to their initial winter conditions is derived from in situ nitrification, with the rest obtained from deep-sea influxes. Under the present conditions of atmospheric forcing, shelf-break exchange, and food web structure, this shelf ecosystem serves as a sink for atmospheric CO2, with storage in the forms of exported DOC, DIC, and unutilized POC (phytoplankton, bacteria, and fecal pellets).As a consequence of just the rising levels of atmospheric pCO2 since the the Industrial Revolution, however, the biophysical CO2 status of the Southeastern Bering Sea shelf may have switched over the last 250 years, from a prior source to the present sink, since this relatively pristine ecosystem has unergone little eutrophication. Such fluctuations of CO2 status may thus be reversed by the physical processes of : (1) reduction of atmospheric pCO2, (2) increased on welling of deep-sea ΣCO2, and (3) warming of shelf waters. Based on our application of this model to the Chukchi Sea and the Gulf of Mexico, about 1.0–1.2 gigatons C y-1 of atmospheric CO2 may now be sequestered by temperate and polar shelf ecosystems. When tropical systems are included, however, a positive net sink of only 0.6–0.8. × 1015g C y−1 may prevail over all shelves.

Numerical analysis of InP-JFET by use of a quasi 2D-model

The electrical performance of InP JFET in dependence on transport properties and technological parameters were investigated in detail by use of a quasi two-dimensional model including nonstationary transport effects. The analysis is carried out for gate lengths between 4μm and 0.4μm. The results were compared with experimental data and with those of GaAs and InGaAs FET.

Simulation of surface effects in planar GaAs MESFET structures by use of a quasi‐2D model

AbstractSurface effects in planar submicron GaAs MESFET are investigated in detail by use of a quasi twodimensional model taking into account the nonstationary transport effects in the channel which are very important for simulation of submicron structures. The model explains the transconductance compression in submicron MESFET due to the influence of the free surface in the parasitic gate‐source and gate‐drain region. The dependence of device performance on the surface potential is shown.

Superconductivities of a quasi two-dimensional Peierls-Hubbard model

As a model for Cu O type high-Tc ceramics, we study a quasi two-dimensional Peierls-Hubbard model. The aim of this study is twofold. The first is to clarify whether the high-Tc's such as 100K can really be possible when the electron-phonon(e-p) coupling alone exists. The second is to clarify the effects of charge and spin fluctuations. For this sake, we have derived a new theory which can cover the whole region of the e-p coupling strength, from the weak (BCS) limit to the strong (bipolaronic) limit. Tc is found to become maximum at the intermediate region between these two limits, and the breathing mode of the ceramics is shown to be able to give a high-Tc of about 100K. In the next, the Hubbard-type correlation is taken into account, and the double excitation of magnons as well as the charge transfer excitations are shown to enhance this phonon-mediated s-wave pairing.

THEORY OF A CW SUPERSONIC NITROGEN RECOMBINATION LASER

THEORY OF A CW SUPERSONIC NITROGEN RECOMBINATION LASER