3,831 research outputs found
Patterns and bifurcations in low-Prandtl number Rayleigh-Benard convection
We present a detailed bifurcation structure and associated flow patterns for
low-Prandtl number () Rayleigh-B\'{e}nard
convection near its onset. We use both direct numerical simulations and a
30-mode low-dimensional model for this study. We observe that low-Prandtl
number (low-P) convection exhibits similar patterns and chaos as zero-P
convection \cite{pal:2009}, namely squares, asymmetric squares, oscillating
asymmetric squares, relaxation oscillations, and chaos. At the onset of
convection, low-P convective flows have stationary 2D rolls and associated
stationary and oscillatory asymmetric squares in contrast to zero-P convection
where chaos appears at the onset itself. The range of Rayleigh number for which
stationary 2D rolls exist decreases rapidly with decreasing Prandtl number. Our
results are in qualitative agreement with results reported earlier
Computational Geometry Column 34
Problems presented at the open-problem session of the 14th Annual ACM
Symposium on Computational Geometry are listed
Identifying Keystone Species in the Human Gut Microbiome from Metagenomic Timeseries using Sparse Linear Regression
Human associated microbial communities exert tremendous influence over human
health and disease. With modern metagenomic sequencing methods it is possible
to follow the relative abundance of microbes in a community over time. These
microbial communities exhibit rich ecological dynamics and an important goal of
microbial ecology is to infer the interactions between species from sequence
data. Any algorithm for inferring species interactions must overcome three
obstacles: 1) a correlation between the abundances of two species does not
imply that those species are interacting, 2) the sum constraint on the relative
abundances obtained from metagenomic studies makes it difficult to infer the
parameters in timeseries models, and 3) errors due to experimental uncertainty,
or mis-assignment of sequencing reads into operational taxonomic units, bias
inferences of species interactions. Here we introduce an approach, Learning
Interactions from MIcrobial Time Series (LIMITS), that overcomes these
obstacles. LIMITS uses sparse linear regression with boostrap aggregation to
infer a discrete-time Lotka-Volterra model for microbial dynamics. We tested
LIMITS on synthetic data and showed that it could reliably infer the topology
of the inter-species ecological interactions. We then used LIMITS to
characterize the species interactions in the gut microbiomes of two individuals
and found that the interaction networks varied significantly between
individuals. Furthermore, we found that the interaction networks of the two
individuals are dominated by distinct "keystone species", Bacteroides fragilis
and Bacteroided stercosis, that have a disproportionate influence on the
structure of the gut microbiome even though they are only found in moderate
abundance. Based on our results, we hypothesize that the abundances of certain
keystone species may be responsible for individuality in the human gut
microbiome
Simultaneous EUV and Radio Observations of Bidirectional Plasmoids Ejection During Magnetic Reconnection
We present a multiwavelength study of the X-class flare, which occurred in
active region (AR) NOAA 11339 on 3 November 2011. The EUV images recorded by
SDO/AIA show the activation of a remote filament (located north of the AR) with
footpoint brightenings about 50 min prior to the flare occurrence. The kinked
filament rises-up slowly and after reaching a projected height of ~49 Mm, it
bends and falls freely near the AR, where the X-class flare was triggered.
Dynamic radio spectrum from the Green Bank Solar Radio Burst Spectrometer
(GBSRBS) shows simultaneous detection of both positive and negative drifting
pulsating structures (DPSs) in the decimetric radio frequencies (500-1200 MHz)
during the impulsive phase of the flare. The global negative DPSs in solar
flares are generally interpreted as a signature of electron acceleration
related to the upward moving plasmoids in the solar corona. The EUV images from
AIA 94 \AA reveal the ejection of multiple plasmoids, which move simultaneously
upward and downward in the corona during the magnetic reconnection. The
estimated speeds of the upward and downward moving plasmoids are ~152-362 and
~83-254 km/s, respectively. These observations strongly support the recent
numerical simulations of the formation and interaction of multiple plasmoids
due to tearing of the current-sheet structure. On the basis of our analysis, we
suggest that the simultaneous detection of both the negative and positive DPSs
is most likely generated by the interaction/coalescence of the multiple
plasmoids moving upward and downward along the current-sheet structure during
the magnetic reconnection process. Moreover, the differential emission measure
(DEM) analysis of the active region reveals presence of a hot flux-rope
structure (visible in AIA 131 and 94 \AA) prior to the flare initiation and
ejection of the multi-temperature plasmoids during the flare impulsive phase.Comment: A&A (accepted), 13 pages, 9 figure
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