45,611 research outputs found
Numerical simulations of strong incompressible magnetohydrodynamic turbulence
Magnetised plasma turbulence pervades the universe and is likely to play an
important role in a variety of astrophysical settings. Magnetohydrodynamics
(MHD) provides the simplest theoretical framework in which phenomenological
models for the turbulent dynamics can be built. Numerical simulations of MHD
turbulence are widely used to guide and test the theoretical predictions;
however, simulating MHD turbulence and accurately measuring its scaling
properties is far from straightforward. Computational power limits the
calculations to moderate Reynolds numbers and often simplifying assumptions are
made in order that a wider range of scales can be accessed. After describing
the theoretical predictions and the numerical approaches that are often
employed in studying strong incompressible MHD turbulence, we present the
findings of a series of high-resolution direct numerical simulations. We
discuss the effects that insufficiencies in the computational approach can have
on the solution and its physical interpretation
Strong and weak thermalization of infinite non-integrable quantum systems
When a non-integrable system evolves out of equilibrium for a long time,
local observables are expected to attain stationary expectation values,
independent of the details of the initial state. However, intriguing
experimental results with ultracold gases have shown no thermalization in
non-integrable settings, triggering an intense theoretical effort to decide the
question. Here we show that the phenomenology of thermalization in a quantum
system is much richer than its classical counterpart. Using a new numerical
technique, we identify two distinct thermalization regimes, strong and weak,
occurring for different initial states. Strong thermalization, intrinsically
quantum, happens when instantaneous local expectation values converge to the
thermal ones. Weak thermalization, well-known in classical systems, happens
when local expectation values converge to the thermal ones only after time
averaging. Remarkably, we find a third group of states showing no
thermalization, neither strong nor weak, to the time scales one can reliably
simulate.Comment: 12 pages, 21 figures, including additional materia
Strong magnetohydrodynamic turbulence with cross helicity
Magnetohydrodynamics (MHD) provides the simplest description of magnetic
plasma turbulence in a variety of astrophysical and laboratory systems. MHD
turbulence with nonzero cross helicity is often called imbalanced, as it
implies that the energies of Alfv\'en fluctuations propagating parallel and
anti-parallel the background field are not equal. Recent analytical and
numerical studies have revealed that at every scale, MHD turbulence consists of
regions of positive and negative cross helicity, indicating that such
turbulence is inherently locally imbalanced. In this paper, results from high
resolution numerical simulations of steady-state incompressible MHD turbulence,
with and without cross helicity are presented. It is argued that the inertial
range scaling of the energy spectra (E^+ and E^-) of fluctuations moving in
opposite directions is independent of the amount of cross-helicity. When cross
helicity is nonzero, E^+ and E^- maintain the same scaling, but have differing
amplitudes depending on the amount of cross-helicity.Comment: To appear in Physics of Plasma
Use of Lagrangian simulations to hindcast the geographical position of propagule release zones in a Mediterranean coastal fish
The study of organism dispersal is fundamental for elucidating patterns of connectivity between populations, thus crucial for the design of effective protection and management strategies. This is especially challenging in the case of coastal fish, for which information on egg release zones (i.e. spawning grounds) is often lacking. Here we assessed the putative location of egg release zones of the saddled sea bream (Oblada melanura) along the south-eastern coast of Spain in 2013. To this aim, we hindcasted propagule (egg and larva) dispersal using Lagrangian simulations, fed with species-specific information on early life history traits (ELTs), with two approaches: 1) back-tracking and 2) comparing settler distribution obtained from simulations to the analogous distribution resulting from otolith chemical analysis. Simulations were also used to assess which factors contributed the most to dispersal distances. Back-tracking simulations indicated that both the northern sector of the Murcia region and some traits of the North-African coast were hydrodynamically suitable to generate and drive the supply of larvae recorded along the coast of Murcia in 2013. With the second approach, based on the correlation between simulation outputs and field results (otolith chemical analysis), we found that the oceanographic characteristics of the study area could have determined the pattern of settler distribution recorded with otolith analysis in 2013 and inferred the geographical position of main O. melanura spawning grounds along the coast. Dispersal distance was found to be significantly affected by the geographical position of propagule release zones. The combination of methods used was the first attempt to assess the geographical position of propagule release zones in the Mediterranean Sea for O. melanura, and can represent a valuable approach for elucidating dispersal and connectivity patterns in other coastal species
Numerical Simulation of Nano Scanning in Intermittent-Contact Mode AFM under Q control
We investigate nano scanning in tapping mode atomic force microscopy (AFM)
under quality (Q) control via numerical simulations performed in SIMULINK. We
focus on the simulation of whole scan process rather than the simulation of
cantilever dynamics and the force interactions between the probe tip and the
surface alone, as in most of the earlier numerical studies. This enables us to
quantify the scan performance under Q control for different scan settings.
Using the numerical simulations, we first investigate the effect of elastic
modulus of sample (relative to the substrate surface) and probe stiffness on
the scan results. Our numerical simulations show that scanning in attractive
regime using soft cantilevers with high Qeff results in a better image quality.
We, then demonstrate the trade-off in setting the effective Q factor (Qeff) of
the probe in Q control: low values of Qeff cause an increase in tapping forces
while higher ones limit the maximum achievable scan speed due to the slow
response of the cantilever to the rapid changes in surface profile. Finally, we
show that it is possible to achieve higher scan speeds without causing an
increase in the tapping forces using adaptive Q control (AQC), in which the Q
factor of the probe is changed instantaneously depending on the magnitude of
the error signal in oscillation amplitude. The scan performance of AQC is
quantitatively compared to that of standard Q control using iso-error curves
obtained from numerical simulations first and then the results are validated
through scan experiments performed using a physical set-up
Role of cross helicity in magnetohydrodynamic turbulence
Strong incompressible three-dimensional magnetohydrodynamic turbulence is
investigated by means of high resolution direct numerical simulations. The
simulations show that the configuration space is characterized by regions of
positive and negative cross-helicity, corresponding to highly aligned or
anti-aligned velocity and magnetic field fluctuations, even when the average
cross-helicity is zero. To elucidate the role of cross-helicity, the spectra
and structure of turbulence are obtained in imbalanced regions where
cross-helicity is non-zero. When averaged over regions of positive and negative
cross-helicity, the result is consistent with the simulations of balanced
turbulence. An analytical explanation for the obtained results is proposed.Comment: 4 pages, 4 figure
Regge calculus from a new angle
In Regge calculus space time is usually approximated by a triangulation with
flat simplices. We present a formulation using simplices with constant
sectional curvature adjusted to the presence of a cosmological constant. As we
will show such a formulation allows to replace the length variables by 3d or 4d
dihedral angles as basic variables. Moreover we will introduce a first order
formulation, which in contrast to using flat simplices, does not require any
constraints. These considerations could be useful for the construction of
quantum gravity models with a cosmological constant.Comment: 8 page
Lavoura de arroz integrada à pecuária de corte: resultados do primeiro ano de trabalhos na Embrapa Pecuária Sul.
bitstream/item/55819/1/DT92.pd
- …