1,862 research outputs found
Turbulence modulation in particle-laden stationary homogeneous shear turbulence using one-dimensional turbulence
Turbulence modulation in particle-laden stationary homogeneous shear turbulence (HST) is investigated using one-dimensional turbulence (ODT), a low-dimensional stochastic flow simulation model. For this purpose, an ODT formulation previously used to study turbulence modulation in forced homogeneous isotropic turbulence (HIT) is extended, so that the model emulates the anisotropic character of HST and, potentially, anisotropic turbulence in general. This is done by limiting the kinetic-energy redistribution during an eddy event to an exchange involving two velocity components, where the three possible choices of the omitted component define three eddy types whose relative likelihoods control the anisotropy. Comparisons of ODT and direct-numerical-simulation results with reference to signatures of turbulence modulation are the basis of a broader ODT parameter study that is reported. Owing to the reduced dimensionality of ODT, it is found that the fidelity of the model for single-phase HST does not extend to particle effects on flow anisotropy, but for quantities averaged over components, parametric trends are captured. The consistent approach to case comparisons that was introduced in the HIT study to evaluate sensitivities to particle-phase parameters in a given flow configuration is extended here to a cross-comparison of HST and HIT model results, and its efficacy is again confirmed. The results provide an overall characterization of the potential for ODT to support the incorporation of particle-induced turbulence modulation into subgrid-scale closures of large-eddy simulations
Bottom dissipation of subinertial currents at the Atlantic zonal boundaries
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/90515/1/jgr_bbldiss_wrightetal_2012.pd
A surface-aware projection basis for quasigeostrophic flow
Recent studies indicate that altimetric observations of the ocean's mesoscale
eddy field reflect the combined influence of surface buoyancy and interior
potential vorticity anomalies. The former have a surface-trapped structure,
while the latter have a more grave form. To assess the relative importance of
each contribution to the signal, it is useful to project the observed field
onto a set of modes that separates their influence in a natural way. However,
the surface-trapped dynamics are not well-represented by standard baroclinic
modes; moreover, they are dependent on horizontal scale.
Here we derive a modal decomposition that results from the simultaneous
diagonalization of the energy and a generalisation of potential enstrophy that
includes contributions from the surface buoyancy fields. This approach yields a
family of orthonomal bases that depend on two parameters: the standard
baroclinic modes are recovered in a limiting case, while other choices provide
modes that represent surface and interior dynamics in an efficient way.
For constant stratification, these modes consist of symmetric and
antisymmetric exponential modes that capture the surface dynamics, and a series
of oscillating modes that represent the interior dynamics. Motivated by the
ocean, where shears are concentrated near the upper surface, we also consider
the special case of a quiescent lower surface. In this case, the interior modes
are independent of wavenumber, and there is a single exponential surface mode
that replaces the barotropic mode. We demonstrate the use and effectiveness of
these modes by projecting the energy in a set of simulations of baroclinic
turbulence
A Simple Passive Scalar Advection-Diffusion Model
This paper presents a simple, one-dimensional model of a randomly advected
passive scalar. The model exhibits anomalous inertial range scaling for the
structure functions constructed from scalar differences. The model provides a
simple computational test for recent ideas regarding closure and scaling for
randomly advected passive scalars. Results suggest that high order structure
function scaling depends on the largest velocity eddy size, and hence scaling
exponents may be geometry-dependent and non-universal.Comment: 30 pages, 11 figure
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Effect of ICU Strain on Timing of Limitations in Life-Sustaining Therapy and Death
Purpose: The effect of capacity strain in an ICU on the timing of end-of-life decision making is unknown. We sought to determine how changes in strain impact timing of new DNR orders and of death.
Methods: Retrospective cohort study of 9,891 patients dying in the hospital following an ICU stay ≥ 72 hours in Project IMPACT, 2001-2008. We examined the effect of ICU capacity strain (measured by standardized census, proportion of new admissions, and average patient acuity) on time to initiation of DNR orders and time to death for all ICU decedents using fixed-effects linear regression.
Results: Increases in strain were associated with shorter time to DNR for patients with limitations in therapy (predicted time to DNR 6.11 days for highest versus 7.70 days for lowest quintile of acuity, p=0.02; 6.50 days for highest versus 7.77 days for lowest quintile of admissions, p<0.001), and shorter time to death (predicted time to death 7.64 days for highest versus 9.05 days for lowest quintile of admissions, p<0.001; 8.28 days for highest versus 9.06 days for lowest quintile of census, only in closed ICUs, p=0.006). Time to DNR order significantly mediated relationships between acuity and admissions and time to death, explaining the entire effect of acuity, and 65% of the effect of admissions. There was no association between strain and time to death for decedents without a limitation in therapy.
Conclusions: Strains in ICU capacity are associated with end-of-life decision making, with shorter times to placement of DNR orders and death for patients admitted during high-strain days
A Description of Local and Nonlocal Eddy–Mean Flow Interaction in a Global Eddy-Permitting State Estimate
The assumption that local baroclinic instability dominates eddy–mean flow interactions is tested on a global scale using a dynamically consistent eddy-permitting state estimate. Interactions are divided into local and nonlocal. If all the energy released from the mean flow through eddy–mean flow interaction is used to support eddy growth in the same region, or if all the energy released from eddies through eddy–mean flow interaction is used to feed back to the mean flow in the same region, eddy–mean flow interaction is local; otherwise, it is nonlocal. Different regions have different characters: in the subtropical region studied in detail, interactions are dominantly local. In the Southern Ocean and Kuroshio and Gulf Stream Extension regions, they are mainly nonlocal. Geographical variability of dominant eddy–eddy and eddy–mean flow processes is a dominant factor in understanding ocean energetics.Woods Hole Oceanographic InstitutionUnited States. National Aeronautics and Space Administration (Grant NNX09AI87G)United States. National Aeronautics and Space Administration (Grant NNX08AR33G
Impact of synoptic atmospheric forcing on the mean ocean circulation
The impact of synoptic atmospheric forcing on the mean ocean circulation is investigated by comparing simulations of a global eddy-permitting ocean-sea ice model forced with and without synoptic atmospheric phenomena. Consistent with previous studies, transient atmospheric motions such as weather systems are found to contribute significantly to the time-mean wind stress and surface heat loss at mid and high latitudes owing to the nonlinear nature of air-sea turbulent fluxes. Including synoptic atmospheric forcing in the model has led to a number of significant changes. For example, wind power input to the ocean increases by about 50%, which subsequently leads to a similar percentage increase in global eddy kinetic energy. The wind-driven subtropical gyre circulations are strengthened by about 10-15%, whereas even greater increases in gyre strength are found in the subpolar oceans. Deep convection in the northern North Atlantic becomes significantly more vigorous, which in turn leads to an increase in the Atlantic Meridional Overturning Circulation (AMOC) by as much as 55%. As a result of the strengthened horizontal gyre circulations and the AMOC, the maximum global northward heat transport increases by almost 50%. Results from this study show that synoptic atmospheric phenomena such as weather systems play a vital role in driving the global ocean circulation and heat transport, and therefore should be properly accounted for in paleo and future climate studies
Carbon and climate system coupling on timescales from the Precambrian to the Anthropocene
Author Posting. © Annual Reviews, 2007. This is the author's version of the work. It is posted here by permission of Annual Reviews for personal use, not for redistribution. The definitive version was published in Annual Review of Environment and Resources 32 (2007): 31-66, doi:10.1146/annurev.energy.32.041706.124700.The global carbon and climate systems are closely intertwined, with
biogeochemical processes responding to and driving climate variations. Over a range of
geological and historical time-scales, warmer climate conditions are associated with
higher atmospheric levels of CO2, an important climate-modulating greenhouse gas. The
atmospheric CO2-temperature relationship reflects two dynamics, the planet’s climate
sensitivity to a perturbation in atmospheric CO2 and the stability of non-atmospheric
carbon reservoirs to evolving climate. Both exhibit non-linear behavior, and coupled
carbon-climate interactions have the potential to introduce both stabilizing and
destabilizing feedback loops into the Earth System. Here we bring together evidence
from a wide range of geological, observational, experimental and modeling studies on the
dominant interactions between the carbon cycle and climate. The review is organized by
time-scale, spanning interannual to centennial climate variability, Holocene millennial
variations and Pleistocene glacial-interglacial cycles, and million year and longer
variations over the Precambrian and Phanerozoic. Our focus is on characterizing and,
where possible quantifying, the emergent behavior internal to the coupled carbon-climate
system as well as the responses of the system to external forcing from tectonics, orbital
dynamics, catastrophic events, and anthropogenic fossil fuel emissions. While there are
many unresolved uncertainties and complexity in the carbon cycle, one emergent
property is clear across time scales: while CO2 can increase in the atmosphere quickly,
returning to lower levels through natural processes is much slower, so the consequences
of the human perturbation will far outlive the emissions that caused them.S. Doney acknowledges support from the NSF Geosciences Carbon and Water program
(NSF ATM-0628582) and the WHOI W. Van Alan Clark Sr. Chair. D. Schimel
acknowledges support from the NSF Biocomplexity in the Environment program (NSF
EAR-0321918)
Skill tests of three-dimensional tidal currents in a global ocean model: A look at the North Atlantic
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/92458/1/jgr_timkoetal_northatlatnictidalcurrentskilltest_2012.pd
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