1,758 research outputs found
Issues Concerning Loop Corrections to the Primordial Power Spectra
We expound ten principles in an attempt to clarify the debate over infrared
loop corrections to the primordial scalar and tensor power spectra from
inflation. Among other things we note that existing proposals for nonlinear
extensions of the scalar fluctuation field introduce new ultraviolet
divergences which no one understands how to renormalize. Loop corrections and
higher correlators of these putative observables would also be enhanced by
inverse powers of the slow roll parameter . We propose an extension
which should be better behaved.Comment: 36 pages, uses LaTeX2e, version 3 revised for publication with a much
expanded section 4, proving that our proposed extension of the zeta-zeta
correlator absorbs the one loop infrared divergences from graviton
Three regularization models of the Navier-Stokes equations
We determine how the differences in the treatment of the subfilter-scale
physics affect the properties of the flow for three closely related
regularizations of Navier-Stokes. The consequences on the applicability of the
regularizations as SGS models are also shown by examining their effects on
superfilter-scale properties. Numerical solutions of the Clark-alpha model are
compared to two previously employed regularizations, LANS-alpha and Leray-alpha
(at Re ~ 3300, Taylor Re ~ 790) and to a DNS. We derive the Karman-Howarth
equation for both the Clark-alpha and Leray-alpha models. We confirm one of two
possible scalings resulting from this equation for Clark as well as its
associated k^(-1) energy spectrum. At sub-filter scales, Clark-alpha possesses
similar total dissipation and characteristic time to reach a statistical
turbulent steady-state as Navier-Stokes, but exhibits greater intermittency. As
a SGS model, Clark reproduces the energy spectrum and intermittency properties
of the DNS. For the Leray model, increasing the filter width decreases the
nonlinearity and the effective Re is substantially decreased. Even for the
smallest value of alpha studied, Leray-alpha was inadequate as a SGS model. The
LANS energy spectrum k^1, consistent with its so-called "rigid bodies,"
precludes a reproduction of the large-scale energy spectrum of the DNS at high
Re while achieving a large reduction in resolution. However, that this same
feature reduces its intermittency compared to Clark-alpha (which shares a
similar Karman-Howarth equation). Clark is found to be the best approximation
for reproducing the total dissipation rate and the energy spectrum at scales
larger than alpha, whereas high-order intermittency properties for larger
values of alpha are best reproduced by LANS-alpha.Comment: 21 pages, 8 figure
Diffusion and dispersion of passive tracers: Navier-Stokes versus MHD turbulence
A comparison of turbulent diffusion and pair-dispersion in homogeneous,
macroscopically isotropic Navier-Stokes (NS) and nonhelical magnetohydrodynamic
(MHD) turbulence based on high-resolution direct numerical simulations is
presented. Significant differences between MHD and NS systems are observed in
the pair-dispersion properties, in particular a strong reduction of the
separation velocity in MHD turbulence as compared to the NS case. It is shown
that in MHD turbulence the average pair-dispersion is slowed down for
, being
the Kolmogorov time, due to the alignment of the relative Lagrangian tracer
velocity with the local magnetic field. Significant differences in turbulent
single-particle diffusion in NS and MHD turbulence are not detected. The fluid
particle trajectories in the vicinity of the smallest dissipative structures
are found to be characterisically different although these comparably rare
events have a negligible influence on the statistics investigated in this work.Comment: Europhysics Letters, in prin
On the strength of the nonlinearity in isotropic turbulence
Turbulence governed by the Navier-Stokes equations shows a tendency to evolve
towards a state in which the nonlinearity is diminished. In fully developed
turbulence this tendency can be measured by comparing the variance of the
nonlinear term to the variance of the same quantity measured in a Gaussian
field with the same energy distribution. In order to study this phenomenon at
high Reynolds numbers, a version of the Direct Interaction Approximation is
used to obtain a closed expression for the statistical average of the
mean-square nonlinearity. The wavenumber spectrum of the mean-square nonlinear
term is evaluated and its scaling in the inertial range is investigated as a
function of the Reynolds number. Its scaling is dominated by the sweeping by
the energetic scales, but this sweeping is weaker than predicted by a random
sweeping estimate. At inertial range scales, the depletion of nonlinearity as a
function of the wavenumber is observed to be constant. At large it is observed
that the mean-square nonlinearity is larger than its Gaussian estimate, which
is shown to be related to the non-Gaussianity of the Reynolds-stress
fluctuations at these scales.Comment: Accepted for publication in J. Fluid Mec
Nonlocal and local wind forcing dependence of the Atlantic meridional overturning circulation and its depth scale
We use wind sensitivity experiments to understand the wind forcing dependencies of the level of no motion and the e-folding pycnocline scale as well as their relationship to northward transport of the mid-depth Atlantic meridional overturning circulation (AMOC) south and north of the equator. In contrast to previous studies, we investigate the interplay of nonlocal and local wind effects on a decadal timescale. We use 30-year simulations with a high-resolution ocean general circulation model (OGCM) which is an eddy-resolving version of the Max Planck Institute Ocean Model (MPIOM). Our findings deviate from the common perspective that the AMOC is a nonlocal phenomenon only, because northward transport in the inter-hemispheric cell can only be understood by analyzing nonlocal Southern Ocean wind effects and local wind effects in the northern hemisphere downwelling region where Ekman pumping takes place. Southern Ocean wind forcing predominantly determines the magnitude of the pycnocline scale throughout the basin, whereas northern hemisphere winds additionally influence the level of no motion locally. In that respect, the level of no motion is a better proxy for northward transport and mid-depth velocity profiles despite the Ekman return flow which is found to be baroclinic. We compare our results inferred from the wind experiments and a 100-year global warming experiment in which the atmospheric CO2 concentration is quadrupled, using MPIOM coupled to an atmospheric model. We find that the evolution of the level of no motion in response to global warming represents changes in vertical velocity profiles or northward transport, whereas the changes of the pycnocline scale are opposite to the changes of the level of no motion over time. Using the level of no motion as depth scale, the analysis of the wind experiments and the warming experiment suggests a hemisphere-dependent scaling of the strength of AMOC. Furthermore, we put forward the idea that the ability of numerical models to capture the spatial and temporal variations of the level of no motion is crucial to reproduce the mid-depth cell in an appropriate wa
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