27 research outputs found
Effective complex permittivity tensor of a periodic array of cylinders
We determine the effective complex permittivity of a two-dimensional
composite, consisting of an arbitrary doubly periodic array of identical
circular cylinders in a homogeneous matrix, and whose dielectric properties are
complex-valued. Efficient formulas are provided to determine the effective
complex permittivity tensor which are in excellent agreement with numerical
calculations. We also show that in contrast to the real-valued case, the real
and imaginary parts of the effective complex-valued tensor can exhibit
non-monotonic behavior as functions of volume fraction of cylinders, and can be
either greater or less than that of the constituents.Comment: 13 pages, 14 figure
On the determination of the boundary impedance from the far field pattern
We consider the Helmholtz equation in the half space and suggest two methods
for determining the boundary impedance from knowledge of the far field pattern
of the time-harmonic incident wave. We introduce a potential for which the far
field patterns in specially selected directions represent its Fourier
coefficients. The boundary impedance is then calculated from the potential by
an explicit formula or from the WKB approximation. Numerical examples are given
to demonstrate efficiency of the approaches. We also discuss the validity of
the WKB approximation in determining the impedance of an obstacle.Comment: 10 pages, 4 figure
The effect of disorder on the wave propagation in one-dimensional periodic optical systems
The influence of disorder on the transmission through periodic waveguides is
studied. Using a canonical form of the transfer matrix we investigate
dependence of the Lyapunov exponent on the frequency and
magnitude of the disorder . It is shown that in the bulk of the bands
, while near the band edges it has the order . This dependence is illustrated by numerical simulations.Comment: 15 pages, 4 figure
Clusters of Bloch waves in three-dimensional periodic media
We consider acoustic wave propagation through a periodic array of the
inclusions of arbitrary shape. The inclusion size is much smaller than the
array period while the wavelength is fixed. We derive and rigorously justify
the dispersion relation for general frequencies and show that there are
exceptional frequencies for which the solution is a cluster of waves
propagating in different directions with different frequencies so that the
dispersion relation cannot be defined uniquely. Examples are provided for the
spherical inclusions.Comment: 28 pages, 1 figur
The effective conductivity of a periodic lattice of circular inclusions
We determine the effective conductivity of a two-dimensional composite
consisting of a doubly periodic array of identical circular cylinders within a
homogeneous matrix. We obtain an exact analytic expression for the effective
conductivity tensor as well as its expansion in terms of volume fraction of the
cylinders. Results are illustrated by examples.Comment: 21 pages, 5 figure
Dietary intake based on physical activity level in Korean elementary school students
Prevalence of childhood obesity is increasing significantly worldwide due to energy imbalance perhaps stemming from undesirable dietary behavior and physical activity level. The objective of the study was to examine the effects of physical activity level on nutritional status in elementary school students. The subjects were comprised of 287 elementary school students between 4th and 6th grades in Seoul, Korea. The level of physical activity was scored with a modified Godin leisure-time exercise questionnaire and was categorized as active, moderately active, and sedentary. Dietary intakes were obtained using a 24-hour food recall method. An analysis of variance (ANOVA) was conducted to test for global significant differences of nutrient intakes by physical activity level. Boys were more active than girls. Daily intakes of energy in moderately active boys were significantly higher than in the sedentary group, but intakes of calcium and iron in moderately active boys were lower than active boys. For girls, physical activity level did not affect nutrient density at all. Intakes of calcium, vitamin C, and folate for both boys and girls were below 50% of recommended intake. Physical activity did not affect nutrient density and our participants were exposed to nutritional imbalance. Therefore, the results suggest that nutrition education regarding balanced diet and optimum physical activity is required for children's health and growth