137 research outputs found
Non-bilaterians as Model Systems for Tissue Mechanics
In animals, epithelial tissues are barriers against the external environment,
providing protection against biological, chemical, and physical damage.
Depending on the animal's physiology and behavior, these tissues encounter
different types of mechanical forces and need to provide a suitable adaptive
response to ensure success. Therefore, understanding tissue mechanics in
different contexts is an important research area. Here, we review recent tissue
mechanics discoveries in a few early-divergent non-bilaterian animals --
Trichoplax adhaerens, Hydra vulgaris, and Aurelia aurita. We highlight each
animal's simple body plan and biology, and unique, rapid tissue remodeling
phenomena that play a crucial role in its physiology. We also discuss the
emergent large-scale mechanics that arise from small-scale phenomena. Finally,
we emphasize the enormous potential of these non-bilaterian animals to be model
systems for further investigation in tissue mechanics.Comment: Review paper, Comments/suggestions are welcom
Energy spectra in turbulent bubbly flows
We conduct experiments in a turbulent bubbly flow to study the nature of the
transition between the classical 5/3 energy spectrum scaling for a
single-phase turbulent flow and the 3 scaling for a swarm of bubbles rising
in a quiescent liquid and of bubble-dominated turbulence. The bubblance
parameter, which measures the ratio of the bubble-induced kinetic energy to the
kinetic energy induced by the turbulent liquid fluctuations before bubble
injection, is often used to characterise the bubbly flow. We vary the bubblance
parameter from (pseudo-turbulence) to (single-phase flow)
over 2-3 orders of magnitude () to study its effect on the turbulent
energy spectrum and liquid velocity fluctuations. The probability density
functions (PDFs) of the liquid velocity fluctuations show deviations from the
Gaussian profile for , i.e. when bubbles are present in the system. The
PDFs are asymmetric with higher probability in the positive tails. The energy
spectra are found to follow the 3 scaling at length scales smaller than the
size of the bubbles for bubbly flows. This 3 spectrum scaling holds not only
in the well-established case of pseudo-turbulence, but surprisingly in all
cases where bubbles are present in the system (). Therefore, it is a
generic feature of turbulent bubbly flows, and the bubblance parameter is
probably not a suitable parameter to characterise the energy spectrum in bubbly
turbulent flows. The physical reason is that the energy input by the bubbles
passes over only to higher wave numbers, and the energy production due to the
bubbles can be directly balanced by the viscous dissipation in the bubble wakes
as suggested by Lance Bataille (1991). In addition, we provide an
alternative explanation by balancing the energy production of the bubbles with
viscous dissipation in the Fourier space.Comment: J. Fluid Mech. (in press
Three-dimensional Lagrangian Voronoi analysis for clustering of particles and bubbles in turbulence
Three-dimensional Voronoi analysis is used to quantify the clustering of
inertial particles in homogeneous isotropic turbulence using data from numerics
and experiments. We study the clustering behavior at different density ratios
and particle response times (i.e. Stokes numbers St). The Probability Density
Functions (PDFs) of the Voronoi cell volumes of light and heavy particles show
a different behavior from that of randomly distributed particles -i.e. fluid
tracers-implying that clustering is present. The standard deviation of the PDF
normalized by that of randomly distributed particles is used to quantify the
clustering. Light particles show maximum clustering for St around 1-2. The
results are consistent with previous investigations employing other approaches
to quantify the clustering. We also present the joint PDFs of enstrophy and
Voronoi volumes and their Lagrangian autocorrelations. The small Voronoi
volumes of light particles correspond to regions of higher enstrophy than those
of heavy particles, indicating that light particles cluster in higher vorticity
regions. The Lagrangian temporal autocorrelation function of Voronoi volumes
shows that the clustering of light particles lasts much longer than that of
heavy or neutrally buoyant particles. Due to inertial effects, the Lagrangian
autocorrelation time-scale of clustered light particles is even longer than
that of the enstrophy of the flow itself.Comment: J. Fluid Mech. 201
Lagrangian statistics of light particles in turbulence
We study the Lagrangian velocity and acceleration statistics of light
particles (micro-bubbles in water) in homogeneous isotropic turbulence.
Micro-bubbles with a diameter of 340 microns and Stokes number from 0.02 to
0.09 are dispersed in a turbulent water tunnel operated at Taylor-Reynolds
numbers (Re) ranging from 160 to 265. We reconstruct the bubble trajectories by
employing three-dimensional particle tracking velocimetry (PTV). It is found
that the probability density functions (PDFs) of the micro-bubble acceleration
show a highly non-Gaussian behavior with flatness values in the range 23-30.
The acceleration flatness values show an increasing trend with Re, consistent
with previous experiments (Voth et al., JFM, 2002) and numerics (Ishihara et
al., JFM, 2007). These acceleration PDFs show a higher intermittency compared
to tracers (Ayyalasomayajula et al., Phys. Fluids, 2008) and heavy particles
(Ayyalasomayajula et al., Phys. Rev. Lett., 2006) in wind tunnel experiments.
In addition, the micro-bubble acceleration autocorrelation function
decorrelates slower with increasing Re. We also compare our results with
experiments in von Karman flows and point-particle direct numerical simulations
with periodic boundary conditions.Comment: 13 pages, 9 figures, revised manuscrip
Sloan Digital Sky Survey IV: Mapping the Milky Way, Nearby Galaxies, and the Distant Universe
We describe the Sloan Digital Sky Survey IV (SDSS-IV), a project encompassing three major spectroscopic programs. The Apache Point Observatory Galactic Evolution Experiment 2 (APOGEE-2) is observing hundreds of thousands of Milky Way stars at high resolution and high signal-to-noise ratios in the near-infrared. The Mapping Nearby Galaxies at Apache Point Observatory (MaNGA) survey is obtaining spatially resolved spectroscopy for thousands of nearby galaxies (median ). The extended Baryon Oscillation Spectroscopic Survey (eBOSS) is mapping the galaxy, quasar, and neutral gas distributions between and 3.5 to constrain cosmology using baryon acoustic oscillations, redshift space distortions, and the shape of the power spectrum. Within eBOSS, we are conducting two major subprograms: the SPectroscopic IDentification of eROSITA Sources (SPIDERS), investigating X-ray AGNs and galaxies in X-ray clusters, and the Time Domain Spectroscopic Survey (TDSS), obtaining spectra of variable sources. All programs use the 2.5 m Sloan Foundation Telescope at the Apache Point Observatory; observations there began in Summer 2014. APOGEE-2 also operates a second near-infrared spectrograph at the 2.5 m du Pont Telescope at Las Campanas Observatory, with observations beginning in early 2017. Observations at both facilities are scheduled to continue through 2020. In keeping with previous SDSS policy, SDSS-IV provides regularly scheduled public data releases; the first one, Data Release 13, was made available in 2016 July
The 16th Data Release of the Sloan Digital Sky Surveys: First Release from the APOGEE-2 Southern Survey and Full Release of eBOSS Spectra
This paper documents the 16th data release (DR16) from the Sloan Digital Sky Surveys (SDSS), the fourth and penultimate from the fourth phase (SDSS-IV). This is the first release of data from the Southern Hemisphere survey of the Apache Point Observatory Galactic Evolution Experiment 2 (APOGEE-2); new data from APOGEE-2 North are also included. DR16 is also notable as the final data release for the main cosmological program of the Extended Baryon Oscillation Spectroscopic Survey (eBOSS), and all raw and reduced spectra from that project are released here. DR16 also includes all the data from the Time Domain Spectroscopic Survey and new data from the SPectroscopic IDentification of ERosita Survey programs, both of which were co-observed on eBOSS plates. DR16 has no new data from the Mapping Nearby Galaxies at Apache Point Observatory (MaNGA) survey (or the MaNGA Stellar Library "MaStar"). We also preview future SDSS-V operations (due to start in 2020), and summarize plans for the final SDSS-IV data release (DR17)
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