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Testing the assumptions underlying ocean mixing methodologies using direct numerical simulations
AbstractDirect numerical simulations of stratified turbulence are used to test several fundamental assumptions involved in the Osborn, Osborn–Cox, and Thorpe methods commonly used to estimate the turbulent diffusivity from field measurements. The forced simulations in an idealized triply periodic computational domain exhibit characteristic features of stratified turbulence including intermittency and layer formation. When calculated using the volume-averaged dissipation rates from the simulations, the vertical diffusivities inferred from the Osborn and Osborn–Cox methods are within 40% of the value diagnosed using the volume-averaged buoyancy flux for all cases, while the Thorpe-scale method performs similarly well in the simulation with a relatively large buoyancy Reynolds number (Reb ≃ 240) but significantly overestimates the vertical diffusivity in simulations with Reb < 60. The methods are also tested using a limited number of vertical profiles randomly selected from the computational volume. The Osborn, Osborn–Cox, and Thorpe-scale methods converge to their respective estimates based on volume-averaged statistics faster than the vertical diffusivity calculated directly from the buoyancy flux, which is contaminated with reversible contributions from internal waves. When applied to a small number of vertical profiles, several assumptions underlying the Osborn and Osborn–Cox methods are not well supported by the simulation data. However, the vertical diffusivity inferred from these methods compares reasonably well to the exact value from the simulations and outperforms the assumptions underlying these methods in terms of the relative error. Motivated by a recent theoretical development, it is speculated that the Osborn method might provide a reasonable approximation to the diffusivity associated with the irreversible buoyancy flux.</jats:p
Robust identification of dynamically distinct regions in stratified turbulence
We present a new robust method for identifying three dynamically distinct regions in a stratified turbulent flow, which we characterise as quiescent flow, intermittent layers and turbulent patches. The method uses the cumulative filtered distribution function of the local density gradient to identify each region. We apply it to data from direct numerical simulations of homogeneous stratified turbulence, with unity Prandtl number, resolved on up to grid points. In addition to classifying regions consistently with contour plots of potential enstrophy, our method identifies quiescent regions as regions where \unicode[STIX]{x1D716}/\unicode[STIX]{x1D708}N^{2}\sim O(1), layers as regions where \unicode[STIX]{x1D716}/\unicode[STIX]{x1D708}N^{2}\sim O(10) and patches as regions where \unicode[STIX]{x1D716}/\unicode[STIX]{x1D708}N^{2}\sim O(100). Here, \unicode[STIX]{x1D716} is the dissipation rate of turbulence kinetic energy, \unicode[STIX]{x1D708} is the kinematic viscosity and is the (overall) buoyancy frequency. By far the highest local dissipation and mixing rates, and the majority of dissipation and mixing, occur in patch regions, even when patch regions occupy only 5 % of the flow volume. We conjecture that treating stratified turbulence as an instantaneous assemblage of these different regions in varying proportions may explain some of the apparently highly scattered flow dynamics and statistics previously reported in the literature.The research activities of G.D.P. and S.dB.K. were funded by the US Office of Naval Research via grant N00014-15-1-2248. Additional support to G.D.P. and S.dB.K. was provided from the UK Engineering and Physical Sciences Research Council grant EP/K034529/1 entitled ‘Mathematical Underpinnings of Stratified Turbulence’, which also funds the research activity of J.R.T. and C.P.C. H.S. gratefully acknowledges the award of a Crighton Fellowship at the Department of Applied Mathematics & Theoretical Physics, University of Cambridge. High-performance computing resources were provided through the US Department of Defense High Performance Computing Modernization Program by the Army Engineer Research and Development Center and the Army Research Laboratory under Frontier Project FP-CFD-FY14-007.This is the author accepted manuscript. The final version is available from Cambridge University Press via https://doi.org/10.1017/jfm.2016.61
Unified force law for granular impact cratering
Experiments on the low-speed impact of solid objects into granular media have
been used both to mimic geophysical events and to probe the unusual nature of
the granular state of matter. Observations have been interpreted in terms of
conflicting stopping forces: product of powers of projectile depth and speed;
linear in speed; constant, proportional to the initial impact speed; and
proportional to depth. This is reminiscent of high-speed ballistics impact in
the 19th and 20th centuries, when a plethora of empirical rules were proposed.
To make progress, we developed a means to measure projectile dynamics with 100
nm and 20 us precision. For a 1-inch diameter steel sphere dropped from a wide
range of heights into non-cohesive glass beads, we reproduce prior observations
either as reasonable approximations or as limiting behaviours. Furthermore, we
demonstrate that the interaction between projectile and medium can be
decomposed into the sum of velocity-dependent inertial drag plus
depth-dependent friction. Thus we achieve a unified description of low-speed
impact phenomena and show that the complex response of granular materials to
impact, while fundamentally different from that of liquids and solids, can be
simply understood
Supersymmetric constraints from Bs -> mu+mu- and B -> K* mu+mu- observables
We study the implications of the recent LHCb limit and results on Bs ->
mu+mu- and B -> K* mu+mu- observables in the constrained SUSY scenarios. After
discussing the Standard Model predictions and carefully estimating the
theoretical errors, we show the constraining power of these observables in
CMSSM and NUHM. The latest limit on BR(Bs -> mu+mu-), being very close to the
SM prediction, constrains strongly the large tan(beta) regime and we show that
the various angular observables from B -> K* mu+mu- decay can provide
complementary information in particular for moderate tan(beta) values.Comment: 30 pages, 14 figure
Parsec-Scale Images of Flat-Spectrum Radio Sources in Seyfert Galaxies
We present high angular resolution (~2 mas) radio continuum observations of
five Seyfert galaxies with flat-spectrum radio nuclei, using the VLBA at 8.4
GHz. The goal of the project is to test whether these flat-spectrum cores
represent thermal emission from the accretion disk, as inferred previously by
Gallimore et al. for NGC 1068, or non-thermal, synchrotron self-absorbed
emission, which is believed to be responsible for more powerful, flat-spectrum
nuclear sources in radio galaxies and quasars. In four sources (T0109-383, NGC
2110, NGC 5252, Mrk 926), the nuclear source is detected but unresolved by the
VLBA, indicating brightness temperatures in excess of 10^8 K and sizes, on
average, less than 1 pc. We argue that the radio emission is non-thermal and
synchrotron self-absorbed in these galaxies, but Doppler boosting by
relativistic outflows is not required. Synchrotron self-absorption brightness
temperatures suggest intrinsic source sizes smaller than ~0.05-0.2 pc, for
these four galaxies, the smallest of which corresponds to a light-crossing time
of ~60 light days or 10^4 gravitational radii for a 10^8 M_sun black hole.
We also present MERLIN and VLA observations of NGC 4388, which was undetected
by the VLBA, and argue that the observed, flat-spectrum, nuclear radio emission
in this galaxy represents optically thin, free-free radiation from dense
thermal gas on scales ~0.4 to a few pc. It is notable that the two Seyfert
galaxies with detected thermal nuclear radio emission (NGC 1068 and NGC 4388)
both have large X-ray absorbing columns, suggesting that columns in excess of
\~10^{24} cm^{-2} are needed for such disks to be detectable. (Abridged)Comment: 36 pages including 5 tables and 4 figures; accepted for publication
in Ap
The Nuclear Regions of the Seyfert Galaxy NGC 4151 - Parsec-scale HI Absorption and a Remarkable Radio Jet
Sensitive high angular and linear resolution radio images of the 240-pc radio
jet in NGC4151, imaged at linear resolutions of 0.3 to 2.6 pc using the VLBA
and phased VLA at 21 cm, are presented and reveal for the first time a faint,
highly collimated jet (diameter <1.4 pc) underlying discrete components, seen
in lower resolution MERLIN and VLA images, that appear to be shock-like
features associated with changes in direction as the jet interacts with small
gas clouds within the central 100 pc of the galaxy. In addition, 21-cm spectral
line imaging of the neutral hydrogen in the nuclear region reveals the spatial
location, distribution and kinematics of the neutral gas detected previously in
a lower resolution MERLIN study. Neutral hydrogen absorption is detected
against component C4W (E+F) as predicted by Mundell et al, but the absorption,
extending over 3 pc, is spatially and kinematically complex on sub-parsec
scales, suggesting the presence of small, dense gas clouds with a wide range of
velocities and column densities. The main absorption component matches that
detected in the MERLIN study and is consistent with absorption through a clumpy
neutral gas layer in the putative obscuring torus, with higher velocity blue-
and red-shifted systems with narrow linewidths also detected across E+F. The
spatial location and distribution of the absorbing gas across component E+F
rules out component E as the location of the AGN (as suggested by Ulvestad et
al.) and, in combination with the well-collimated continuum structures seen in
component D, suggests that component D is the most likely location for the AGN.
We suggest that components C and E are shocks produced in the jet as the plasma
encounters, and is deviated by, dense clouds with diameters smaller than ~1.4
pc. (Abridged)Comment: 30 pages including 7 figures and 2 tables; accepted for publication
in Ap
Spitzer and JCMT Observations of the Active Galactic Nucleus in the Sombrero Galaxy (NGC 4594)
We present Spitzer 3.6-160 micron images, Spitzer mid-infrared spectra, and
JCMT SCUBA 850 micron images of the Sombrero Galaxy (NGC 4594), an Sa galaxy
with a 10^9 M_solar low luminosity active galactic nucleus (AGN). The brightest
infrared sources in the galaxy are the nucleus and the dust ring. The spectral
energy distribution of the AGN demonstrates that, while the environment around
the AGN is a prominent source of mid-infrared emission, it is a relatively weak
source of far-infrared emission, as had been inferred for AGN in previous
research. The weak nuclear 160 micron emission and the negligible polycyclic
aromatic hydrocarbon emission from the nucleus also implies that the nucleus is
a site of only weak star formation activity and the nucleus contains relatively
little cool interstellar gas needed to fuel such activity. We propose that this
galaxy may be representative of a subset of low ionization nuclear emission
region galaxies that are in a quiescent AGN phase because of the lack of gas
needed to fuel circumnuclear star formation and Seyfert-like AGN activity.
Surprisingly, the AGN is the predominant source of 850 micron emission. We
examine the possible emission mechanisms that could give rise to the 850 micron
emission and find that neither thermal dust emission, CO line emission,
bremsstrahlung emission, nor the synchrotron emission observed at radio
wavelengths can adequately explain the measured 850 micron flux density by
themselves. The remaining possibilities for the source of the 850 micron
emission include a combination of known emission mechanisms, synchrotron
emission that is self-absorbed at wavelengths longer than 850 microns, or
unidentified spectral lines in the 850 micron band.Comment: Accepted to ApJ, 200
Impact of Forest Seral Stage on use of Ant Communities for Rapid Assessment of Terrestrial Ecosystem Health
Bioassessment evaluates ecosystem health by using the responses of a community of organisms that integrate all aspects of the ecosystem. A variety of bioassessment methods have been applied to aquatic ecosystems; however, terrestrial methods are less advanced. The objective of this study was to examine baseline differences in ant communities at different seral stages from clear cut to mature pine plantation as a precursor to developing a broader terrestrial bioassessment protocol. Comparative sampling was conducted at nine sites having four seral stages: clearcut, 5 year recovery, 15 year recovery, and mature stands. Soil and vegetation data were also collected at each site. Ants were identified to genus. Analysis of the ant data indicated that ants respond strongly to habitat changes that accompany ecological succession in managed pine forests, and both individual genera and ant community structure can be used as indicators of successional change. Ants exhibited relatively high diversity in both early and mature seral stages. High ant diversity in mature seral stages was likely related to conditions on the forest floor favoring litter dwelling and cold climate specialists. While ants may be very useful in identifying environmental stress in managed pine forests, adjustments must be made for seral stage when comparing impacted and unimpacted forests
Genomewide Association Study for Determinants of HIV-1 Acquisition and Viral Set Point in HIV-1 Serodiscordant Couples with Quantified Virus Exposure
Host genetic factors may be important determinants of HIV-1 sexual acquisition. We performed a genome-wide association study (GWAS) for host genetic variants modifying HIV-1 acquisition and viral control in the context of a cohort of African HIV-1 serodiscordant heterosexual couples. To minimize misclassification of HIV-1 risk, we quantified HIV-1 exposure, using data including plasma HIV-1 concentrations, gender, and condom use.We matched couples without HIV-1 seroconversion to those with seroconversion by quantified HIV-1 exposure risk. Logistic regression of single nucleotide polymorphisms (SNPs) for 798 samples from 496 HIV-1 infected and 302 HIV-1 exposed, uninfected individuals was performed to identify factors associated with HIV-1 acquisition. In addition, a linear regression analysis was performed using SNP data from a subset (n = 403) of HIV-1 infected individuals to identify factors predicting plasma HIV-1 concentrations.After correcting for multiple comparisons, no SNPs were significantly associated with HIV-1 infection status or plasma HIV-1 concentrations.This GWAS controlling for HIV-1 exposure did not identify common host genotypes influencing HIV-1 acquisition. Alternative strategies, such as large-scale sequencing to identify low frequency variation, should be considered for identifying novel host genetic predictors of HIV-1 acquisition
Optimal Uses of Antiretrovirals for Prevention in HIV-1 Serodiscordant Heterosexual Couples in South Africa: A Modelling Study
Hallett et al use a mathematical model to examine the long-term impact and cost-effectiveness of different pre-exposure prophylaxis (PrEP) strategies for HIV prevention in serodiscordant couples
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