26,440 research outputs found
The structure of hypersonic shock waves using Navier-Stokes equations modified to include mass diffusion
Howard Brenner has recently proposed modifications to the Navier-Stokes
equations that relate to a diffusion of fluid volume that would be significant
for flows with high density gradients. In a previous paper (Greenshields &
Reese, 2007), we found these modifications gave good predictions of the viscous
structure of shock waves in argon in the range Mach 1.0-12.0 (while
conventional Navier-Stokes equations are known to fail above about Mach 2).
However, some areas of concern with this model were a somewhat arbitrary choice
of modelling coefficient, and potentially unphysical and unstable solutions. In
this paper, we therefore present slightly different modifications to include
molecule mass diffusion fully in the Navier-Stokes equations. These
modifications are shown to be stable and produce physical solutions to the
shock problem of a quality broadly similar to those from the family of extended
hydrodynamic models that includes the Burnett equations. The modifications
primarily add a diffusion term to the mass conservation equation, so are at
least as simple to solve as the Navier-Stokes equations; there are none of the
numerical implementation problems of conventional extended hydrodynamics
models, particularly in respect of boundary conditions. We recommend further
investigation and testing on a number of different benchmark non-equilibrium
flow cases.Comment: written for the 2nd European Conference on AeroSpace Sciences
(EUCASS), Belgium, 200
Quasi-static granular flow of ice mélange
We use Landsat 8 imagery to generate ice mélange velocity fields at Greenland’s three most
productive outlet glaciers: Jakobshavn Isbræ, Helheim Glacier, and Kangerdlugssuaq Glacier. Winter velocity
fields are generally steady and highly uniform. Summer velocity fields, on the other hand, tend to be much
more variable and can be uniform, compressional, or extensional. We rarely observe compressional flow
at Jakobshavn Isbræ or extensional flow at Helheim Glacier, while both are observed at Kangerdlugssuaq
Glacier. Transverse velocity profiles from all three locations are suggestive of viscoplastic flow, in which
deformation occurs primarily in shear zones along the fjord walls. We analyze the transverse profiles in
the context of quasi-static flow using continuum rheologies for granular materials and find that the force
per unit width that ice mélange exerts on glacier termini increases exponentially with the ice mélange
length-to-width ratio and the effective coefficient of friction. Our estimates of ice mélange resistance are
consistent with other independent estimates and suggest that ice mélange may be capable of inhibiting
iceberg calving events, especially during winter. Moreover, our results provide geophysical-scale support for
constitutive relationships for granular materials and suggest a potential avenue for modeling ice mélange
dynamics with continuum models.From acknowledgments:
Funding for this project was provided by the U.S. National Science Foundation (DMR-1506446 and DMR-1506307).
Digital elevation models were provided by the Polar Geospatial Center under the U.S. National Science Foundation (OPP-1043681, OPP-1559691, and OPP-1542736)Ye
The structure of shock waves as a test of Brenner's modifications to the Navier-Stokes equations
Brenner has recently proposed modifications to the Navier-Stokes equations
that are based on theoretical arguments but supported only by experiments
having a fairly limited range. These modifications relate to a diffusion of
fluid volume that would be significant for flows with high density gradients.
So the viscous structure of shock waves in gases should provide an excellent
test case for this new model. In this paper we detail the shock structure
problem and propose exponents for the gas viscosity-temperature relation based
on empirical viscosity data that is independent of shock experiments. We then
simulate shocks in the range Mach 1.0-12.0 using the Navier-Stokes equations,
both with and without Brenner's modifications. Initial simulations showed
Brenner's modifications display unphysical behaviour when the coefficient of
volume diffusion exceeds the kinematic viscosity. Our subsequent analyses
attribute this behaviour to both an instability to temporal disturbances and a
spurious phase velocity-frequency relationship. On equating the volume
diffusivity to the kinematic viscosity, however, we find the results with
Brenner's modifications are significantly better than those of the standard
Navier-Stokes equations, and broadly similar to those from the family of
extended hydrodynamic models that includes the Burnett equations. Brenner's
modifications add only two terms to the Navier-Stokes equations, and the
numerical implementation is much simpler than conventional extended
hydrodynamic models, particularly in respect of boundary conditions. We
recommend further investigation and testing on a number of different benchmark
non-equilibrium flow cases
Quasi-static granular flow of ice mélange
We use Landsat 8 imagery to generate ice mélange velocity fields at Greenland’s three most
productive outlet glaciers: Jakobshavn Isbræ, Helheim Glacier, and Kangerdlugssuaq Glacier. Winter velocity
fields are generally steady and highly uniform. Summer velocity fields, on the other hand, tend to be much
more variable and can be uniform, compressional, or extensional. We rarely observe compressional flow
at Jakobshavn Isbræ or extensional flow at Helheim Glacier, while both are observed at Kangerdlugssuaq
Glacier. Transverse velocity profiles from all three locations are suggestive of viscoplastic flow, in which
deformation occurs primarily in shear zones along the fjord walls. We analyze the transverse profiles in
the context of quasi-static flow using continuum rheologies for granular materials and find that the force
per unit width that ice mélange exerts on glacier termini increases exponentially with the ice mélange
length-to-width ratio and the effective coefficient of friction. Our estimates of ice mélange resistance are
consistent with other independent estimates and suggest that ice mélange may be capable of inhibiting
iceberg calving events, especially during winter. Moreover, our results provide geophysical-scale support for
constitutive relationships for granular materials and suggest a potential avenue for modeling ice mélange
dynamics with continuum models.From acknowledgments:
Funding for this project was provided by the U.S. National Science Foundation (DMR-1506446 and DMR-1506307).
Digital elevation models were provided by the Polar Geospatial Center under the U.S. National Science Foundation (OPP-1043681, OPP-1559691, and OPP-1542736)Ye
A concurrent precursor inflow method for Large Eddy Simulations and applications to finite length wind farms
In order to enable simulations of developing wind turbine array boundary
layers with highly realistic inflow conditions a concurrent precursor method
for Large Eddy Simulations is proposed. In this method we consider two domains
simultaneously, i.e. in one domain a turbulent Atmospheric Boundary Layer (ABL)
without wind turbines is simulated in order to generate the turbulent inflow
conditions for a second domain in which the wind turbines are placed. The
benefit of this approach is that a) it avoids the need for large databases in
which the turbulent inflow conditions are stored and the correspondingly slow
I/O operations and b) we are sure that the simulations are not negatively
affected by statically swept fixed inflow fields or synthetic fields lacking
the proper ABL coherent structures. Sample applications are presented, in
which, in agreement with field data a strong decrease of the power output of
downstream wind-turbines with respect to the first row of wind-turbines is
observed for perfectly aligned inflow.Comment: 13 pages, 5 figure
Evidence for Correlated Titanium and Deuterium Depletion in the Galactic ISM
Current measurements indicate that the deuterium abundance in diffuse
interstellar gas varies spatially by a factor of ~4 among sightlines extending
beyond the Local Bubble. One plausible explanation for the scatter is the
variable depletion of D onto dust grains. To test this scenario, we have
obtained high signal-to-noise, high resolution profiles of the refractory ion
TiII along seven Galactic sightlines with D/H ranging from 0.65 to 2.1x10^-5.
These measurements, acquired with the recently upgraded Keck/HIRES
spectrometer, indicate a correlation between Ti/H and D/H at the >95% c.l.
Therefore, our observations support the interpretation that D/H scatter is
associated with differential depletion. We note, however, that Ti/H values
taken from the literature do not uniformly show the correlation. Finally, we
identify significant component-to-component variations in the depletion levels
among individual sightlines and discuss complications arising from this
behavior.Comment: 4 pages; Accepted to Astrophysical Journal Letter
Entanglement of two qubits in a relativistic orbit
The creation and destruction of entanglement between a pair of interacting
two-level detectors accelerating about diametrically opposite points of a
circular path is investigated. It is found that any non-zero acceleration has
the effect of suppressing the vacuum entanglement and enhancing the
acceleration radiation thereby reducing the entangling capacity of the
detectors. Given that for large accelerations the acceleration radiation is the
dominant effect, we investigate the evolution of a two detector system
initially prepared in a Bell state using a perturbative mater equation and
treating the vacuum fluctuations as an unobserved environment. A general
function for the concurrence is obtained for stationary and symmetric
worldlines in flatspace. The entanglement sudden death time is computed.Comment: v2: Some typo's fixed, figures compressed to smaller filesize and
added some references
The Magellanic Bridge: The Nearest Purely Tidal Stellar Population
We report on observations of the stellar populations in twelve fields
spanning the region between the Magellanic Clouds, made with the Mosaic-II
camera on the 4-meter telescope at the Cerro-Tololo Inter-American Observatory.
The two main goals of the observations are to characterize the young stellar
population (which presumably formed in situ in the Bridge and therefore
represents the nearest stellar population formed from tidal debris), and to
search for an older stellar component (which would have been stripped from
either Cloud as stars, by the same tidal forces which formed the gaseous
Bridge). We determine the star-formation history of the young inter-Cloud
population, which provides a constraint on the timing of the gravitational
interaction which formed the Bridge. We do not detect an older stellar
population belonging to the Bridge in any of our fields, implying that the
material that was stripped from the Clouds to form the Magellanic Bridge was
very nearly a pure gas.Comment: 19 pages, 9 figures. Accepted to Ap
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