55 research outputs found
Water-like anomalies for core-softened models of fluids: One dimension
We use a one-dimensional (1d) core-softened potential to develop a physical
picture for some of the anomalies present in liquid water. The core-softened
potential mimics the effect of hydrogen bonding. The interest in the 1d system
stems from the facts that closed-form results are possible and that the
qualitative behavior in 1d is reproduced in the liquid phase for higher
dimensions. We discuss the relation between the shape of the potential and the
density anomaly, and we study the entropy anomaly resulting from the density
anomaly. We find that certain forms of the two-step square well potential lead
to the existence at T=0 of a low-density phase favored at low pressures and of
a high-density phase favored at high pressures, and to the appearance of a
point at a positive pressure, which is the analog of the T=0 ``critical
point'' in the Ising model. The existence of point leads to anomalous
behavior of the isothermal compressibility and the isobaric specific heat
.Comment: 22 pages, 7 figure
Sediment delivery on rill and interrill areas
Equations which relate sediment delivery to a power function of flow rate and slope gradient were
evaluated in this study. The data used to parameterize the equations were obtained from sites where crop
residues had been removed, and moldboard plowing and disking had occurred. Measurements of sediment
delivery resulting from simulated rainfall were obtained from preformed rills and interrill areas. The
equations provided reliable sediment delivery estimates for selected soils located throughout the United
States. To use the sediment delivery equations, soil-related parameter values must be identified. Multiple
regression analyses were performed to relate parameter values used in the equations to selected soil
properties. Equations were also developed for estimating rill sediment delivery under rainfall conditions
from rill soil loss and discharge data collected without the addition of rainfall. The equations identified in
this study, and appropriate soils information, can be used to predict sediment delivery on both rill and
interrill areas
Theoretical description of phase coexistence in model C60
We have investigated the phase diagram of the Girifalco model of C60
fullerene in the framework provided by the MHNC and the SCOZA liquid state
theories, and by a Perturbation Theory (PT), for the free energy of the solid
phase. We present an extended assessment of such theories as set against a
recent Monte Carlo study of the same model [D. Costa et al, J. Chem. Phys.
118:304 (2003)]. We have compared the theoretical predictions with the
corresponding simulation results for several thermodynamic properties. Then we
have determined the phase diagram of the model, by using either the SCOZA, or
the MHNC, or the PT predictions for one of the coexisting phases, and the
simulation data for the other phase, in order to separately ascertain the
accuracy of each theory. It turns out that the overall appearance of the phase
portrait is reproduced fairly well by all theories, with remarkable accuracy as
for the melting line and the solid-vapor equilibrium. The MHNC and SCOZA
results for the liquid-vapor coexistence, as well as for the corresponding
critical points, are quite accurate. All results are discussed in terms of the
basic assumptions underlying each theory. We have selected the MHNC for the
fluid and the first-order PT for the solid phase, as the most accurate tools to
investigate the phase behavior of the model in terms of purely theoretical
approaches. The overall results appear as a robust benchmark for further
theoretical investigations on higher order C(n>60) fullerenes, as well as on
other fullerene-related materials, whose description can be based on a
modelization similar to that adopted in this work.Comment: RevTeX4, 15 pages, 7 figures; submitted to Phys. Rev.
Recommended from our members
Calculation of the mutual diffusion coefficient by equilibrium and nonequilibrium molecular dynamics
A nonequilibrium molecular dynamics method for the calculation of the mutual diffusion coefficient for a mixture of hard spheres is described. The method is applied to a 50-50 mixture of equidiameter particles having a mass ratio of 0.1 for the two species, at a volume of three times close-packing. By extrapolating the results to the limit of vanishing concentration gradient and infinite system size, we obtain a value in statistical agreement with the result obtained using a Green-Kubo molecular dynamics procedure which is also described. The nonequilibrium calculation yields a mutual diffusion coefficient which decreases slightly with increasing concentration gradient. The Green-Kubo time correlation function for mutual diffusion displays a slow decay with time, qualitatively similar to the long-time tail which has been predicted by the hydrodynamic theory of Pomeau
Sediment delivery on rill and interrill areas
Equations which relate sediment delivery to a power function of flow rate and slope gradient were
evaluated in this study. The data used to parameterize the equations were obtained from sites where crop
residues had been removed, and moldboard plowing and disking had occurred. Measurements of sediment
delivery resulting from simulated rainfall were obtained from preformed rills and interrill areas. The
equations provided reliable sediment delivery estimates for selected soils located throughout the United
States. To use the sediment delivery equations, soil-related parameter values must be identified. Multiple
regression analyses were performed to relate parameter values used in the equations to selected soil
properties. Equations were also developed for estimating rill sediment delivery under rainfall conditions
from rill soil loss and discharge data collected without the addition of rainfall. The equations identified in
this study, and appropriate soils information, can be used to predict sediment delivery on both rill and
interrill areas
Simulação das perdas de água por evaporação e arraste, no aspersor NY-7 (4,6 mm x 4,0 mm), em sistemas de aspersão convencional Simulation of evaporation and wind drift losses, in the NY-7 sprinkler (4.6 mm x 4.0 mm), in stationary sprinler irrigation systems
As perdas de água por evaporação e arraste em sistemas de irrigação por aspersão podem assumir valores consideráveis, reduzindo a eficiência do sistema. Os objetivos do presente trabalho foram avaliar a capacidade preditiva de cinco modelos empíricos para estimar perdas de água por evaporação e arraste em aspersores modelo NY-7 (bocais de 4,6 mm x 4,0 mm), trabalhando sob diferentes condições operacionais e ambientais, e ajustar modelos específicos para o aspersor NY-7. Comparando os resultados medidos em ensaios de campo, com os resultados simulados, foi possível concluir que os cinco modelos empíricos considerados apresentaram pouca ou nenhuma adequação, tanto para os ensaios com um único aspersor (quadrado do erro-médio de 5,27; 20,70; 5,07; 6,95 e 7,06% para os modelos empíricos 1; 2; 3; 4 e 5, respectivamente) quanto para os ensaios com linhas laterais contendo aspersores (quadrado do erro-médio de 7,41; 24,43; 6,72; 3,16 e 2,9% para os modelos empíricos 1; 2; 3; 4 e 5, respectivamente). Comparados aos cinco modelos empíricos considerados, os novos modelos ajustados apresentaram menores erros, indicando que a aplicação de modelos empíricos deve ser limitada às condições de operação (diâmetro de bocal, pressão de operação, etc.) similares àquelas em que os modelos foram desenvolvidos.<br>Evaporation and wind drift losses during sprinkler irrigation may reach significant values, cutting system efficiency down. The present work aims: (a) to evaluate the ability of five empirical models in predicting losses of a NY-7 model sprinkler (nozzle of 4.6 mm x 4.0 mm), working under different operational and climatic conditions; and (b) to adjust specific models to the NY-7 sprinkler. By comparing measured values - obtained on field trials - with simulated ones, it was possible to conclude that, in general, the five considered empirical models presented little or no adjustment for the single-sprinkler outdoor tests (root mean square error of 5.27; 20.70; 5.07; 6.95 and 7.06% for empirical models 1; 2; 3; 4 and 5, respectively) as well as for the block irrigation outdoor tests (root mean square error of 7.41; 24.43; 6.72; 3.16 and 2.90% for empirical models 1; 2; 3; 4 and 5, respectively). When compared to the five considered empirical models, the new adjusted models showed lower errors, indicating that the application of empirical models must be limitated to working conditions (nozzle size, operational pressure, etc.) similar to the ones in which they were developed
- …