18,159 research outputs found
Shear-Improved Smagorinsky Model for Large-Eddy Simulation of Wall-Bounded Turbulent Flows
A shear-improved Smagorinsky model is introduced based on recent results
concerning shear effects in wall-bounded turbulence by Toschi et al. (2000).
The Smagorinsky eddy-viscosity is modified subtracting the magnitude of the
mean shear from the magnitude of the instantaneous resolved strain-rate tensor.
This subgrid-scale model is tested in large-eddy simulations of plane-channel
flows at two different Reynolds numbers. First comparisons with the dynamic
Smagorinsky model and direct numerical simulations, including mean velocity,
turbulent kinetic energy and Reynolds stress profiles, are shown to be
extremely satisfactory. The proposed model, in addition of being physically
sound, has a low computational cost and possesses a high potentiality of
generalization to more complex non-homogeneous turbulent flows.Comment: 10 pages, 6 figures, added some reference
Diffraction microstrain in nanocrystalline solids under load - heterogeneous medium approach
This is an account of the computation of X-ray microstrain in a polycrystal
with anisotropic elasticity under uniaxial external load. The results have been
published in the article "Microstrain in nanocrystalline solids under load by
virtual diffraction", at Europhysics Letters 89, 66002 (2010). The present
information was submitted to Europhysics Letters as part of the manuscript
package, and was available to the reviewers who recommended the paper for
publication.Comment: Supporting online material for J. Markmann, D. Bachurin, L.-H. Shao,
P. Gumbsch, J. Weissm\"uller, Microstrain in nanocrystalline solids under
load by virtual diffraction, Europhys. Lett. 89, 66002 (2010
Spectral imbalance and the normalized dissipation rate of turbulence
The normalized turbulent dissipation rate is studied in decaying
and forced turbulence by direct numerical simulations, large-eddy simulations,
and closure calculations. A large difference in the values of is
observed for the two types of turbulence. This difference is found at moderate
Reynolds number, and it is shown that it persists at high Reynolds number,
where the value of becomes independent of the Reynolds number, but
is still not unique. This difference can be explained by the influence of the
nonlinear cascade time that introduces a spectral disequilibrium for
statistically nonstationary turbulence. Phenomenological analysis yields simple
analytical models that satisfactorily reproduce the numerical results. These
simple spectral models also reproduce and explain the increase of
at low Reynolds number that is observed in the simulations
Decay of scalar variance in isotropic turbulence in a bounded domain
The decay of scalar variance in isotropic turbulence in a bounded domain is
investigated. Extending the study of Touil, Bertoglio and Shao (2002; Journal
of Turbulence, 03, 49) to the case of a passive scalar, the effect of the
finite size of the domain on the lengthscales of turbulent eddies and scalar
structures is studied by truncating the infrared range of the wavenumber
spectra. Analytical arguments based on a simple model for the spectral
distributions show that the decay exponent for the variance of scalar
fluctuations is proportional to the ratio of the Kolmogorov constant to the
Corrsin-Obukhov constant. This result is verified by closure calculations in
which the Corrsin-Obukhov constant is artificially varied. Large-eddy
simulations provide support to the results and give an estimation of the value
of the decay exponent and of the scalar to velocity time scale ratio
Behavior of X-Ray Dust Scattering and Implications for X-Ray Afterglows of Gamma-Ray Bursts
The afterglows of gamma-ray bursts (GRBs) have commonly been assumed to be
due to shocks sweeping up the circum-stellar medium. However, most GRBs have
been found in dense star-forming regions where a significant fraction of the
prompt X-ray emission can be scattered by dust grains. Here we revisit the
behavior of dust scattering of X-rays in GRBs. We find that the features of
some X-ray afterglows from minutes to days after the gamma-ray triggers are
consistent with the scattering of prompt X-ray emission from GRBs off host dust
grains. This implies that some of the observed X-ray afterglows (especially
those without sharp rising and decaying flares) could be understood with a
dust-scattering--driven emission model.Comment: ApJ, in pres
Microwave Slow-Wave Structure and Phase-Compensation Technique for Microwave Power Divider
In this paper, T-shaped electromagnetic bandgap is loaded on a coupled transmission line itself and its electric performance is studied. Results show that microwave slow-wave effect can be enhanced and therefore, size reduction of a transmission-line-based circuit is possible. However, the transmission-line-based circuits characterize varied phase responses against frequency, which becomes a disadvantage where constant phase response is required. Consequently, a phase-compensation technique is further presented and studied. For demonstration purpose, an 8-way coupled-line power divider with 22.5 degree phase shifts between adjacent output ports, based on the studied slow-wave structure and phase-compensation technique, is developed. Results show both compact circuit architecture and improved phase imbalance are realized, confirming the investigated circuit structures and analyzing methodologies
Feasibility studies for quarkonium production at a fixed-target experiment using the LHC proton and lead beams (AFTER@LHC)
Used in the fixed-target mode, the multi-TeV LHC proton and lead beams allow
for studies of heavy-flavour hadroproduction with unprecedented precision at
backward rapidities - far negative Feyman-x - using conventional detection
techniques. At the nominal LHC energies, quarkonia can be studies in detail in
p+p, p+d and p+A collisions at sqrt(s_NN) ~ 115 GeV as well as in Pb+p and Pb+A
collisions at sqrt(s_NN) ~ 72 GeV with luminosities roughly equivalent to that
of the collider mode, i.e. up to 20 fb-1 yr-1 in p+p and p+d collisions, up to
0.6 fb-1 yr-1 in p+A collisions and up to 10 nb-1 yr-1 in Pb+A collisions. In
this paper, we assess the feasibility of such studies by performing fast
simulations using the performance of a LHCb-like detector.Comment: 12 pages, 14 figure
Octet Quark Contents from SU(3) Flavor Symmetry
With the parametrization of parton distribution functions (PDFs) of the
proton by Soffer \textit{et al.}, we extend the valence quark contents to other
octet baryons by utilizing SU(3) flavor symmetry. We find the method
practically useful. Fragmentation functions (FFs) are further obtained through
the phenomenological Gribov-Lipatov relation at the region. Our
results are compared with different models, and these different predictions can
be discriminated by upcoming experiments.Comment: 6 pages, 5 figures, final version for journal publicatio
Note on a new fundamental length scale instead of the Newtonian constant
The newly proposed entropic gravity suggests gravity as an emergent force
rather than a fundamental one. In this approach, the Newtonian constant
does not play a fundamental role any more, and a new fundamental constant is
required to replace its position. This request also arises from some
philosophical considerations to contemplate the physical foundations for the
unification of theories. We here consider the suggestion to derive from
more fundamental quantities in the presence of a new fundamental length scale
, which is suspected to originate from the structure of quantum space-time,
and can be measured directly from Lorentz-violating observations. Our results
are relevant to the fundamental understanding of physics, and more practically,
of natural units, as well as explanations of experimental constraints in
searching for Lorentz violation.Comment: 10 latex pages, final version for journal publicatio
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