2,340 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
Inertial range scaling in numerical turbulence with hyperviscosity
Numerical turbulence with hyperviscosity is studied and compared with direct
simulations using ordinary viscosity and data from wind tunnel experiments. It
is shown that the inertial range scaling is similar in all three cases.
Furthermore, the bottleneck effect is approximately equally broad (about one
order of magnitude) in these cases and only its height is increased in the
hyperviscous case--presumably as a consequence of the steeper decent of the
spectrum in the hyperviscous subrange. The mean normalized dissipation rate is
found to be in agreement with both wind tunnel experiments and direct
simulations. The structure function exponents agree with the She-Leveque model.
Decaying turbulence with hyperviscosity still gives the usual t^{-1.25} decay
law for the kinetic energy, and also the bottleneck effect is still present and
about equally strong.Comment: Final version (7 pages
The "zeroth law" of turbulence: Isotropic turbulence simulations revisited
The dimensionless kinetic energy dissipation rate C_epsilon is estimated from
numerical simulations of statistically stationary isotropic box turbulence that
is slightly compressible. The Taylor microscale Reynolds number Re_lambda range
is 20 < Re_lambda < 220 and the statistical stationarity is achieved with a
random phase forcing method. The strong Re_lambda dependence of C_epsilon
abates when Re_lambda approx. 100 after which C_epsilon slowly approaches
approx 0.5 a value slightly different to previously reported simulations but in
good agreement with experimental results. If C_epsilon is estimated at a
specific time step from the time series of the quantities involved it is
necessary to account for the time lag between energy injection and energy
dissipation. Also, the resulting value can differ from the ensemble averaged
value by up to +-30%. This may explain the spread in results from previously
published estimates of C_epsilon.Comment: 7 pages, 7 figures. Submitted to Phys. Rev.
SPECIFICATION AND PROCUREMENT OF CP-5 FUEL TUBES.
Contains the specifications of CP-5 fuel tubes
Beta lives - some statistical perspectives on the capital asset pricing model
This note summarizes some technical issues relevant to the use of the idea of excess return in empirical modelling. We cover the case where the aim is to construct a measure of expected return on an asset and a model of the CAPM type is used. We review some of the problems and show examples where the basic CAPM may be used to develop other results which relate the expected returns on assets both to the expected return on the market and other factors
Spin Coupling Effect on Geometry-Dependent X-ray Absorption of Diradicals
We theoretically investigate the influence of diradical electron spin
coupling on the time-resolved X-ray absorption spectra of the photochemical
ring opening of furanone. We predict geometry dependent carbon K-edge signals
involving transitions from core orbitals to both singly and unoccupied
molecular orbitals. The most obvious features of the ring opening come from the
carbon atom directly involved in the bond breaking, through its transition to
both the newly formed SOMO and the available LUMO state. In addition to this
primary feature, the singlet spin coupling of four unpaired electrons that
arises in the core-to-LUMO states creates additional geometry dependence in
some spectral features, with both oscillator strengths and relative excitation
energies varying observably as a function of the ring opening. We attribute
this behavior to a spin-occupancy-induced selection rule, which occurs when
singlet spin coupling is enforced in the diradical state. Notably, one of these
geometry-sensitive core-to-LUMO transitions excites core electrons from a
backbone carbon not involved in the bond breaking, providing a novel non-local
X-ray probe of chemical dynamics arising from electron spin coupling.Comment: 52 Pages, 13 Figure
The scaling properties of dissipation in incompressible isotropic three-dimensional magnetohydrodynamic turbulence
The statistical properties of the dissipation process constrain the analysis
of large scale numerical simulations of three dimensional incompressible
magnetohydrodynamic (MHD) turbulence, such as those of Biskamp and Muller
[Phys. Plasmas 7, 4889 (2000)]. The structure functions of the turbulent flow
are expected to display statistical self-similarity, but the relatively low
Reynolds numbers attainable by direct numerical simulation, combined with the
finite size of the system, make this difficult to measure directly. However, it
is known that extended self-similarity, which constrains the ratio of scaling
exponents of structure functions of different orders, is well satisfied. This
implies the extension of physical scaling arguments beyond the inertial range
into the dissipation range. The present work focuses on the scaling properties
of the dissipation process itself. This provides an important consistency check
in that we find that the ratio of dissipation structure function exponents is
that predicted by the She and Leveque [Phys. Rev. Lett 72, 336 (1994)] theory
proposed by Biskamp and Muller. This supplies further evidence that the cascade
mechanism in three dimensional MHD turbulence is non-linear random eddy
scrambling, with the level of intermittency determined by dissipation through
the formation of current sheets.Comment: 9 pages, 6 figures. Figures embedded in text. Typos corrected in text
and references. Published in Physics of Plasmas. Abstract can be found
at:http://link.aip.org/link/?php/12/02230
Fetal liver blood flow distribution: role in human developmental strategy to prioritize fat deposition versus brain development
Among primates, human neonates have the largest brains but also the highest proportion of body fat. If placental nutrient supply is limited, the fetus faces a dilemma: should resources be allocated to brain growth, or to fat deposition for use as a potential postnatal energy reserve? We hypothesised that resolving this dilemma operates at the level of umbilical blood distribution entering the fetal liver. In 381 uncomplicated pregnancies in third trimester, we measured blood flow perfusing the fetal liver, or bypassing it via the ductus venosus to supply the brain and heart using ultrasound techniques. Across the range of fetal growth and independent of the mother's adiposity and parity, greater liver blood flow was associated with greater offspring fat mass measured by dual-energy X-ray absorptiometry, both in the infant at birth (r = 0.43, P<0.001) and at age 4 years (r = 0.16, P = 0.02). In contrast, smaller placentas less able to meet fetal demand for essential nutrients were associated with a brain-sparing flow pattern (r = 0.17, p = 0.02). This flow pattern was also associated with a higher degree of shunting through ductus venosus (P = 0.04). We propose that humans evolved a developmental strategy to prioritize nutrient allocation for prenatal fat deposition when the supply of conditionally essential nutrients requiring hepatic inter-conversion is limited, switching resource allocation to favour the brain if the supply of essential nutrients is limited. Facilitated placental transfer mechanisms for glucose and other nutrients evolved in environments less affluent than those now prevalent in developed populations, and we propose that in circumstances of maternal adiposity and nutrient excess these mechanisms now also lead to prenatal fat deposition. Prenatal developmental influences play important roles in the human propensity to deposit fa
Ultrafast X-ray Spectroscopy of Intersystem Crossing in Hexafluoroacetylacetone: Chromophore Photophysics and Spectral Changes in the Face of Electron Withdrawing Groups
Intersystem crossings between singlet and triplet states represent a crucial
relaxation pathway in photochemical processes. Herein, we probe the intersystem
crossing in hexafluoro-acetylacetone with ultrafast X-ray transient absorption
spectroscopy at the carbon K-edge. We observe the excited state dynamics
following excitation with 266 nm UV light to the (S) state
with element and site-specificity using a broadband soft X-ray pulse produced
by high harmonic generation. These results are compared to X-ray spectra
computed from orbital optimized density functional theory methods. It is found
that the electron withdrawing fluorine atoms decongest the X-ray absorption
spectrum by enhancing separation between features originating from different
carbon atoms. This facilitates the elucidation of structural and electronic
dynamics at the chromophore. The evolution of the core-to-valence resonances at
the carbon K-edge reveals an ultrafast population transfer between the
(S) and (T) states on a ps
timescale, which is similar to the 1.5 ps timescale earlier observed for
acetylacetone [J. Am. Chem. Soc. 139, 16576 (2017)]. It therefore appears that
terminal fluorination has little influence on the intersystem crossing rate of
the acetylacetone chromophore. In addition, the significant role of
hydrogen-bond opened and twisted rotational isomers is elucidated in the
excited state dynamics by comparison of the experimental transient X-ray
spectra with theory
Accurate prediction of core-level spectra of radicals at density functional theory cost via square gradient minimization and recoupling of mixed configurations
State-specific orbital optimized approaches are more accurate at predicting
core-level spectra than traditional linear-response protocols, but their
utility had been restricted on account of the risk of `variational collapse'
down to the ground state. We employ the recently developed square gradient
minimization (SGM, J. Chem. Theory Comput. 16, 1699-1710, 2020) algorithm to
reliably avoid variational collapse and study the effectiveness of orbital
optimized density functional theory (DFT) at predicting second period element
1s core-level spectra of open-shell systems. Several density functionals
(including SCAN, B3LYP and B97X-D3) are found to predict excitation
energies from the core to singly occupied levels to high accuracy ( eV
RMS error), against available experimental data. Higher excited states are
however more challenging by virtue of being intrinsically multiconfigurational.
We thus present a CI inspired route to self-consistently recouple single
determinant mixed configurations obtained from DFT, in order to obtain
approximate doublet states. This recoupling scheme is used to predict the C
K-edge spectra of the allyl radical, the O K-edge spectra of CO and the N
K-edge of NO to high accuracy relative to experiment, indicating
substantial promise in using this approach for computation of core-level
spectra for doublet species (vs more traditional time dependent DFT, EOM-CCSD
or using unrecoupled mixed configurations). We also present general guidelines
for computing core-excited states from orbital optimized DFT.Comment: Added more dat
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