10,107 research outputs found
The spatio-temporal spectrum of turbulent flows
Identification and extraction of vortical structures and of waves in a
disorganised flow is a mayor challenge in the study of turbulence. We present a
study of the spatio-temporal behavior of turbulent flows in the presence of
different restitutive forces. We show how to compute and analyse the
spatio-temporal spectrum from data stemming from numerical simulations and from
laboratory experiments. Four cases are considered: homogeneous and isotropic
turbulence, rotating turbulence, stratified turbulence, and water wave
turbulence. For homogeneous and isotropic turbulence, the spectrum allows
identification of sweeping by the large scale flow. For rotating and for
stratified turbulence, the spectrum allows identification of the waves, precise
quantification of the energy in the waves and in the turbulent eddies, and
identification of physical mechanisms such as Doppler shift and wave absorption
in critical layers. Finally, in water wave turbulence the spectrum shows a
transition from gravity-capillary waves to bound waves as the amplitude of the
forcing is increased.Comment: Added new references and analysi
The spatio-temporal spectrum of turbulent flows
Identification and extraction of vortical structures and of waves in a disorganised flow is a mayor challenge in the study of turbulence. We present a study of the spatio-temporal behavior of turbulent flows in the presence of different restitutive forces. We show how to compute and analyse the spatio-temporal spectrum from data stemming from numerical simulations and from laboratory experiments. Four cases are considered: homogeneous and isotropic turbulence, rotating turbulence, stratified turbulence, and water wave turbulence. For homogeneous and isotropic turbulence, the spectrum allows identification of sweeping by the large-scale flow. For rotating and for stratified turbulence, the spectrum allows identification of the waves, precise quantification of the energy in the waves and in the turbulent eddies, and identification of physical mechanisms such as Doppler shift and wave absorption in critical layers. Finally, in water wave turbulence the spectrum shows a transition from gravity-capillary waves to bound waves as the amplitude of the forcing is increased.Fil: Clark Di Leoni, Patricio. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Física de Buenos Aires. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Física de Buenos Aires; ArgentinaFil: Cobelli, Pablo Javier. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Física de Buenos Aires. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Física de Buenos Aires; ArgentinaFil: Mininni, Pablo Daniel. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Física de Buenos Aires. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Física de Buenos Aires; Argentin
Perspectives on Astrophysics Based on Atomic, Molecular, and Optical (AMO) Techniques
About two generations ago, a large part of AMO science was dominated by
experimental high energy collision studies and perturbative theoretical
methods. Since then, AMO science has undergone a transition and is now
dominated by quantum, ultracold, and ultrafast studies. But in the process, the
field has passed over the complexity that lies between these two extremes. Most
of the Universe resides in this intermediate region. We put forward that the
next frontier for AMO science is to explore the AMO complexity that describes
most of the Cosmos.Comment: White paper submission to the Decadal Assessment and Outlook Report
on Atomic, Molecular, and Optical (AMO) Science (AMO 2020
Cascade and Damping of Alfv\'{e}n-Cyclotron Fluctuations: Application to Solar Wind Turbulence Spectrum
With the diffusion approximation, we study the cascade and damping of
Alfv\'{e}n-cyclotron fluctuations in solar plasmas numerically. Motivated by
wave-wave couplings and nonlinear effects, we test several forms of the
diffusion tensor. For a general locally anisotropic and inhomogeneous diffusion
tensor in the wave vector space, the turbulence spectrum in the inertial range
can be fitted with power-laws with the power-law index varying with the wave
propagation direction. For several locally isotropic but inhomogeneous
diffusion coefficients, the steady-state turbulence spectra are nearly
isotropic in the absence of damping and can be fitted by a single power-law
function. However, the energy flux is strongly polarized due to the
inhomogeneity that leads to an anisotropic cascade. Including the anisotropic
thermal damping, the turbulence spectrum cuts off at the wave numbers, where
the damping rates become comparable to the cascade rates. The combined
anisotropic effects of cascade and damping make this cutoff wave number
dependent on the wave propagation direction, and the propagation direction
integrated turbulence spectrum resembles a broken power-law, which cuts off at
the maximum of the cutoff wave numbers or the He cyclotron frequency.
Taking into account the Doppler effects, the model can naturally reproduce the
broken power-law wave spectra observed in the solar wind and predicts that a
higher break frequency is aways accompanied with a greater spectral index
change that may be caused by the increase of the Alfv\'{e}n Mach number, the
reciprocal of the plasma beta, and/or the angle between the solar wind velocity
and the mean magnetic field. These predictions can be tested by future
observations
Noise characterization for LISA
We consider the general problem of estimating the inflight LISA noise power
spectra and cross-spectra, which are needed for detecting and estimating the
gravitational wave signals present in the LISA data. For the LISA baseline
design and in the long wavelength limit, we bound the error on all spectrum
estimators that rely on the use of the fully symmetric Sagnac combination
(). This procedure avoids biases in the estimation that would otherwise
be introduced by the presence of a strong galactic background in the LISA data.
We specialize our discussion to the detection and study of the galactic white
dwarf-white dwarf binary stochastic signal.Comment: 9 figure
Narrow Line Photoassociation in an Optical Lattice
With ultracold Sr in a 1D magic wavelength optical lattice, we
performed narrow line photoassociation spectroscopy near the SP intercombination transition. Nine least-bound vibrational molecular
levels associated with the long-range and potential energy surfaces
were measured and identified. A simple theoretical model accurately describes
the level positions and treats the effects of the lattice confinement on the
line shapes. The measured resonance strengths show that optical tuning of the
ground state scattering length should be possible without significant atom
loss.Comment: 4 pages, 4 figure
Resonant Absorption of Transverse Oscillations and Associated Heating in a Solar Prominence. I- Observational aspects
Transverse magnetohydrodynamic (MHD) waves have been shown to be ubiquitous
in the solar atmosphere and can in principle carry sufficient energy to
generate and maintain the Sun's million-degree outer atmosphere or corona.
However, direct evidence of the dissipation process of these waves and
subsequent heating has not yet been directly observed. Here we report on high
spatial, temporal, and spectral resolution observations of a solar prominence
that show a compelling signature of so-called resonant absorption, a long
hypothesized mechanism to efficiently convert and dissipate transverse wave
energy into heat. Aside from coherence in the transverse direction, our
observations show telltale phase differences around 180 degrees between
transverse motions in the plane-of-sky and line-of-sight velocities of the
oscillating fine structures or threads, and also suggest significant heating
from chromospheric to higher temperatures. Comparison with advanced numerical
simulations support a scenario in which transverse oscillations trigger a
Kelvin-Helmholtz instability (KHI) at the boundaries of oscillating threads via
resonant absorption. This instability leads to numerous thin current sheets in
which wave energy is dissipated and plasma is heated. Our results provide
direct evidence for wave-related heating in action, one of the candidate
coronal heating mechanisms.Comment: 28 pages, 9 figures, accepted for publication in ApJ. Part II by
Patrick Antolin et al. will appear soo
- …