112,768 research outputs found
A coupled approximate deconvolution and dynamic mixed scale model for large-eddy simulation
Large-eddy simulations of incompressible Newtonian fluid flows with
approximate deconvolution models based on the van Cittert method are reported.
The Legendre spectral element method is used for the spatial discretization to
solve the filtered Navier--Stokes equations. A novel variant of approximate
deconvolution models blended with a mixed scale model using a dynamic
evaluation of the subgrid-viscosity constant is proposed. This model is
validated by comparing the large-eddy simulation with the direct numerical
simulation of the flow in a lid-driven cubical cavity, performed at a Reynolds
number of 12'000. Subgrid modeling in the case of a flow with coexisting
laminar, transitional and turbulent zones such as the lid-driven cubical cavity
flow represents a challenging problem. Moreover, the coupling with the spectral
element method having very low numerical dissipation and dispersion builds a
well suited framework to analyze the efficiency of a subgrid model. First- and
second-order statistics obtained using this new model are showing very good
agreement with the direct numerical simulation. Filtering operations rely on an
invertible filter applied in a modal basis and preserving the C0-continuity
across elements. No clipping on dynamic parameters was needed to preserve
numerical stability
Departures From Axisymmetric Morphology and Dynamics in Spiral Galaxies
New HI synthesis data have been obtained for six face-on galaxies with the
Very Large Array. These data and reanalyses of three additional data sets make
up a sample of nine face-on galaxies analyzed for deviations from axisymmetry
in morphology and dynamics. This sample represents a subsample of galaxies
already analyzed for morphological symmetry properties in the R-band. Four
quantitative measures of dynamical nonaxisymmetry are compared to one another
and to the quantitative measures of morphological asymmetry in HI and R-band to
investigate the relationships between nonaxisymmetric morphology and dynamics.
We find no significant relationship between asymmetric morphology and most of
the dynamical measures in our sample. A possible relationship is found,
however, between morphology and dynamical position angle differences between
approaching and receding sides of the galaxy.Comment: 24 pages, 19 figures, AASTeX, accepted for publication in AJ,
postscript figures available at
ftp://culebra.tn.cornell.edu/pub/david/figures.tar.g
Effect of lateral displacement of a high-altitude platform on cellular interference and handover
A method for predicting movements in cellular coverage caused by lateral drift of a high-altitude platform (a quasi-stationary platform in the stratosphere) is developed. Cells are produced by spot beams generated by horn-type antennas on the platform. It is shown how the carrier-to-interference ratio (CIR) across these cells varies when the antenna payload is steered to accommodate the lateral movement of the platform. The geometry of the antenna beam footprint on the ground is first developed and then applied to a system of many cochannel beams. Pointing strategies are examined, where the pointing angle is calculated to keep, for example, a center cell or an edge cell in the same nominal position before and after the platform drift, and the CIR distribution is calculated. It is shown that the optimum pointing angle depends on the desired level of CIR across the service area, typically lying between 3 +/- 0.75 degrees for a platform drift of 2'km and corresponding to a cell in the middle ring. It is shown that it is necessary for a significant proportion of users to perform a handover to maintain a given CIR after platform drift. The analysis reveals that there is an optimum pointing angle that minimizes the probability of handover for a particular value of drift and CIR
Morphology Development in Model Polyethylene via Two-Dimensional Correlation Analysis
Two-dimensional (2D) correlation analysis is applied to synchrotron X-ray scattering data to characterize
morphological regimes during nonisothermal crystallization of a model ethylene copolymer (hydrogenated polybutadiene,
HPBD). The 2D correlation patterns highlight relationships
among multiple characteristics of structure evolution, particularly the extent to which separate features change simultaneously versus sequentially. By visualizing these relationships during cooling, evidence is obtained for two separate physical processes occurring in what is known as “irreversible crystallization” in random ethylene copolymers. Initial growth of primarily lamellae into unconstrained melt (“primary-irreversible crystallization”) is distinguished from subsequent secondary lamellae formation in the constrained, noncrystalline regions
between the primary lamellae (“secondary-irreversible crystallization”). At successively lower temperatures (“reversible crystallization”), growth of the crystalline reflections is found to occur simultaneously with the change in shape of the amorphous halo, which is inconsistent with the formation of an additional phase. Rather, the synchronous character supports the view that growth of frustrated crystals distorts the adjacent noncrystalline material. Furthermore, heterocorrelation analysis of small-angle and wideangle X-ray scattering data from the reversible crystallization regime reveals that the size of new crystals is consistent with fringedmicellar structures (~9 nm). Thus, 2D correlation analysis provides new insights into morphology development in polymeric systems
Orphan penumbrae: Submerging horizontal fields
We investigate the properties of orphan penumbrae, which are photospheric
filamentary structures observed in active regions near polarity inversion lines
that resemble the penumbra of regular sunspots but are not connected to any
umbra. We use Hinode data from the Solar Optical Telescope to determine the
properties of orphan penumbrae. Spectropolarimetric data are employed to obtain
the vector magnetic field and line-of-sight velocities in the photosphere.
Magnetograms are used to study the overall evolution of these structures, and
G-band and Ca II H filtergrams are to investigate their brightness and apparent
horizontal motions. Orphan penumbrae form between regions of opposite polarity
in places with horizontal magnetic fields. Their magnetic configuration is that
of -shaped flux ropes. In the two cases studied here, the
opposite-polarity regions approach each other with time and the whole structure
submerges as the penumbral filaments disappear. Orphan penumbrae are very
similar to regular penumbrae, including the existence of strong gas flows.
Therefore, they could have a similar origin. The main difference between them
is the absence of a "background" magnetic field in orphan penumbrae. This could
explain most of the observed differences. The fast flows we detect in orphan
penumbrae may be caused by the siphon flow mechanism. Based on the similarities
between orphan and regular penumbrae, we propose that the Evershed flow is also
a manifestation of siphon flows.Comment: 15 pages, 15 figure
Study of flare energy release using events with numerous type III-like bursts in microwaves
The analysis of narrowband drifting of type III-like structures in radio
bursts dynamic spectra allows to obtain unique information about primary energy
release mechanisms in solar flares. The SSRT spatially resolved images and a
high spectral and temporal resolution allow direct determination not only the
positions of its sources but also the exciter velocities along the flare loop.
Practically, such measurements are possible during some special time intervals
when the SSRT (about 5.7 GHz) is observing the flare region in two high-order
fringes; thus, two 1D scans are recorded simultaneously at two frequency bands.
The analysis of type III-like bursts recorded during the flare 14 Apr 2002 is
presented. Using-muliwavelength radio observations recorded by SSRT, SBRS,
NoRP, RSTN we study an event with series of several tens of drifting microwave
pulses with drift rates in the range from -7 to 13 GHz/s. The sources of the
fast-drifting bursts were located near the top of the flare loop in a volume of
a few Mm in size. The slow drift of the exciters along the flare loop suggests
a high pitch-anisotropy of the emitting electrons.Comment: 16 pages, 6 figures, Solar Physics, in press, 201
A thermodynamic analysis of forced convection through porous media using pore scale modeling
The flow thorough porous media is analyzed from a thermodynamic perspective, with a particular focus on the
entropy generation inside the porous media, using a pore scale modeling approach. A single representative
elementary volume was utilized to reduce the CPU time. Periodic boundary conditions were employed for the
vertical boundaries, by re-injecting the velocity and temperature profiles from the outlet to the inlet and iterating.
The entropy generation was determined for both circular and square cross-sectional configurations, and the effects
of different Reynolds numbers, assuming Darcy and Forchheimer regimes, were also taken into account. Three
porosities were evaluated and discussed for each cross-sectional configuration, and streamlines, isothermal lines and
the local entropy generation rate contours were determined and compared. The local entropy generation rate
contours indicated that the highest entropy generation regions were close to the inlet for low Reynolds flows and
near the central cylinder for high Reynolds flows. Increasing Reynolds number from 100 to 200 reveals disturbances
in the dimensionless volume averaged entropy generation rate trend that may be due to a change in the fluid flow
regime. According to Bejan number evaluation for both cross-section configurations, it is demonstrated that is
mainly provoked by the heat transfer irreversibility. A performance evaluation criterion parameter was calculated for
different case-studies. By this parameter, conditions for obtaining the least entropy generation and the highest
Nusselt number could be achieved simultaneously. Indeed, this parameter utilizes both the first and the second laws
of thermodynamics to present the best case-study. According to the performance evaluation criterion, it is indicated
that the square cross-section configuration with o=0.64
exhibits better thermal performance for low Reynolds
number flows. A comparison between the equal porosity cases for two different cross-sectional configurations
indicated that the square cross-section demonstrated a higher performance evaluation criterion than the circular
cross-section, for a variety of different Reynolds numbers
First Results of the 74 MHz VLA-Pie Town Link. Hercules A at Low Frequencies
We present the results of the first successful observations of the Pie Town
link with the Very Large Array (VLA) at 74 MHz on Hercules A. The improvement
in resolution from 25 arcsec to 10 arcsec resolves the helical- and ring-like
features seen at higher frequencies. We also present new high dynamic range
images of this powerful radio galaxy at 325 MHz. Our low frequency observations
confirm the multiple outburst interpretation of the spectral index differences
at high frequencies. Comparison between our radio and ROSAT X-ray data does not
reveal any association between the X-ray emission from the cluster and the
radio lobes. There are no extra regions of radio emission at 74 MHz.Comment: 9 pages, 7 figures, accepted for publication in MNRA
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