1,532 research outputs found
Is nonhelical hydromagnetic turbulence peaked at small scales?
Nonhelical hydromagnetic turbulence without an imposed magnetic field is
considered in the case where the magnetic Prandtl number is unity. The magnetic
field is entirely due to dynamo action. The magnetic energy spectrum peaks at a
wavenumber of about 5 times the minimum wavenumber in the domain, and not at
the resistive scale, as has previously been argued. Throughout the inertial
range the spectral magnetic energy exceeds the kinetic energy by a factor of
about 2.5, and both spectra are approximately parallel. At first glance, the
total energy spectrum seems to be close to k^{-3/2}, but there is a strong
bottleneck effect and it is suggested that the asymptotic spectrum is k^{-5/3}.
This is supported by the value of the second order structure function exponent
that is found to be \zeta_2=0.70, suggesting a k^{-1.70} spectrum.Comment: 6 pages, 6 figure
Hydrodynamic and hydromagnetic energy spectra from large eddy simulations
Direct and large eddy simulations of hydrodynamic and hydromagnetic
turbulence have been performed in an attempt to isolate artifacts from real and
possibly asymptotic features in the energy spectra. It is shown that in a
hydrodynamic turbulence simulation with a Smagorinsky subgrid scale model using
512^3 meshpoints two important features of the 4096^3 simulation on the Earth
simulator (Kaneda et al. 2003, Phys. Fluids 15, L21) are reproduced: a k^{-0.1}
correction to the inertial range with a k^{-5/3} Kolmogorov slope and the form
of the bottleneck just before the dissipative subrange. Furthermore, it is
shown that, while a Smagorinsky-type model for the induction equation causes an
artificial and unacceptable reduction in the dynamo efficiency,
hyper-resistivity yields good agreement with direct simulations. In the
large-scale part of the inertial range, an excess of the spectral magnetic
energy over the spectral kinetic energy is confirmed. However, a trend towards
spectral equipartition at smaller scales in the inertial range can be
identified. With magnetic fields, no explicit bottleneck effect is seen.Comment: 8 pages, 9 figs, Phys. Fluids (in press
Identification of Colletotrichum species associated with anthracnose disease of coffee in Vietnam
Colletotrichum gloeosporioides, C. acutatum, C. capsici and C. boninense associated with anthracnose disease on coffee (Coffea spp.) in Vietnam were identified based on morphology and DNA analysis. Phylogenetic analysis of DNA sequences from the internal transcribed spacer region of nuclear rDNA and a portion of mitochondrial small subunit rRNA were concordant and allowed good separation of the taxa. We found several Colletotrichum isolates of unknown species and their taxonomic position remains unresolved. The majority of Vietnamese isolates belonged to C. gloeosporioides and they grouped together with the coffee berry disease (CBD) fungus, C. kahawae. However, C. kahawae could be distinguished from the Vietnamese C. gloeosporioides isolates based on ammonium tartrate utilization, growth rate and pathogenictity. C. gloeosporioides isolates were more pathogenic on detached green berries than isolates of the other species, i.e. C. acutatum, C capsici and C. boninense. Some of the C. gloeosporioides isolates produced slightly sunken lesion on green berries resembling CBD symptoms but it did not destroy the bean. We did not find any evidence of the presence of C. kahawae in Vietnam
Binding of biexcitons in GaAs/AlxGa1-xAs superlattices
Binding of the heavy-hole excitons and biexcitons in GaAs/Al0.3Ga0.7As superlattices is studied using linear and nonlinear optical techniques. High biexciton binding energies characteristic of quasi two-dimensional biexcitons are observed in superlattices with considerable miniband dispersion
Well-width dependence of exciton-phonon scattering in InxGa1 - xAs/GaAs single quantum wells
The temperature and density dependencies of the exciton dephasing time in In0.18Ga0.82As/GaAs single quantum wells with different thicknesses have been measured by degenerate four-wave mixing. The exciton-phonon scattering contribution to the dephasing is isolated by extrapolating the dephasing rate to zero-exciton density. From the temperature dependence of this rate we have deduced the linewidth broadening coefficients for acoustic and optical phonons. We find acoustic-phonon coefficients that increase from 1.6 to 3 μeV/K when increasing the well width from 1 to 4 nm. This is in quantitative agreement with theoretical predictions when the spatial extension of the exciton wave function, strongly penetrating into the GaAs barrier in thin InxGa1-xAs quantum wells, is taken into account. The optical-phonon coefficient does not show a systematic dependence on well thickness, and is comparable with the value for bulk GaAs
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