152 research outputs found
Dynamics of small heavy particles in homogeneous turbulence: a Lagrangian experimental study
We investigate the behavior of microscopic heavy particles settling in
homogeneous air turbulence. The regimes are relevant to the airborne transport
of dust and droplets: the Taylor-microscale Reynolds number is Re = 289 - 462,
the Kolmogorov-scale Stokes number is St = 1.2 - 13, and the Kolmogorov
acceleration is comparable to the gravitational acceleration (i.e., the Froude
number Fr = O(1)). We use high-speed laser imaging to track the particles and
simultaneously characterize the air velocity field, resolving all relevant
spatio-temporal scales. The role of the flow sampled by the particles is
spotlighted. In the present range of parameters, the particle settling velocity
is enhanced proportionally to the velocity scale of the turbulence. Both
gravity and inertia reduce the velocity fluctuations of the particles compared
to the fluid; while they have competing effect on the particle acceleration,
through the crossing trajectories and inertial filtering mechanisms,
respectively. The preferential sampling of high-strain/low-vorticity regions is
measurable, but its impact on the global statistics is moderate. The inertial
particles have large relative velocity at small separations, which increases
their pair dispersion; however, gravity offsets this effect by causing them to
experience fluid velocities that decorrelate faster in time compared to
tracers. Based on the observations, we derive an analytical model to predict
the particle velocity and acceleration variances for arbitrary St, Fr, and Re.
This agrees well with the present observations and previous simulations and
captures the respective effects of inertia and gravity, both of which play
crucial roles in the transport
Dynamics and Scaling of Particle Streaks in High-Reynolds-Number Turbulent Boundary Layers
Inertial particles in wall-bounded turbulence are known to form streaks, but experimental evidence and predictive understanding of this phenomenon is lacking, especially in regimes relevant to atmospheric flows. We carry out wind tunnel measurements to investigate this process, characterizing the transport of microscopic particles suspended in turbulent boundary layers. The friction Reynolds number Re = O(104) allows for significant scale separation and the emergence of large-scale motions, while the range of viscous Stokes number St+ = 18â870 is relevant to the transport of dust and fine sand in the atmospheric surface layer. We perform simultaneous imaging of both carrier and dispersed phases along wall-parallel planes in the logarithmic layer, demonstrating that streamwise particle streaks largely overlap with large-scale low-speed flow regions. The fluidâparticle slip velocity indicates that with increasing inertia, the particle streaks outlive the low-speed fluid streaks. Moreover, two-point statistics show that the width of the particle streaks increases linearly with Stokes number, bounded by the size of the coherent flow structures. Finally, the particle-sampled flow topology suggests that particle streaks reside between the legs of hairpin packets. From these observations, we infer a conceptual view of the formation of particle streaks in the frame of the attached eddy model. A scaling for the particle streaksâ width is derived as a function of Re and St+, which reproduces the measured trends and predicts widths O(0.1) m in the atmospheric surface layer, comparable to aeolian streamers observed in the field
Square Root {LASSO}: well-posedness, Lipschitz stability and the tuning trade off
This paper studies well-posedness and parameter sensitivity of the Square
Root LASSO (SR-LASSO), an optimization model for recovering sparse solutions to
linear inverse problems in finite dimension. An advantage of the SR-LASSO
(e.g., over the standard LASSO) is that the optimal tuning of the
regularization parameter is robust with respect to measurement noise. This
paper provides three point-based regularity conditions at a solution of the
SR-LASSO: the weak, intermediate, and strong assumptions. It is shown that the
weak assumption implies uniqueness of the solution in question. The
intermediate assumption yields a directionally differentiable and locally
Lipschitz solution map (with explicit Lipschitz bounds), whereas the strong
assumption gives continuous differentiability of said map around the point in
question. Our analysis leads to new theoretical insights on the comparison
between SR-LASSO and LASSO from the viewpoint of tuning parameter sensitivity:
noise-robust optimal parameter choice for SR-LASSO comes at the "price" of
elevated tuning parameter sensitivity. Numerical results support and showcase
the theoretical findings
LASSO reloaded: a variational analysis perspective with applications to compressed sensing
This paper provides a variational analysis of the unconstrained formulation
of the LASSO problem, ubiquitous in statistical learning, signal processing,
and inverse problems. In particular, we establish smoothness results for the
optimal value as well as Lipschitz properties of the optimal solution as
functions of the right-hand side (or measurement vector) and the regularization
parameter. Moreover, we show how to apply the proposed variational analysis to
study the sensitivity of the optimal solution to the tuning parameter in the
context of compressed sensing with subgaussian measurements. Our theoretical
findings are validated by numerical experiments
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Successful Supercooled Liver Storage for 4 Days
The realization of longâterm human organ preservation will have groundbreaking effects on the current practice of transplantation. Herein we present a novel technique based on subâzero nonâfreezing tissue preservation and extracorporeal machine perfusion that allows transplantation of rat livers preserved for up to 4 days, thereby tripling the viable preservation duration
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Supercooling as a Viable Non-Freezing Cell Preservation Method of Rat Hepatocytes
Supercooling preservation holds the potential to drastically extend the preservation time of organs, tissues and engineered tissue products, and fragile cell types that do not lend themselves well to cryopreservation or vitrification. Here, we investigate the effects of supercooling preservation (SCP at -4oC) on primary rat hepatocytes stored in cryovials and compare its success (high viability and good functional characteristics) to that of static cold storage (CS at +4oC) and cryopreservation. We consider two prominent preservation solutions a) Hypothermosol (HTS-FRS) and b) University of Wisconsin solution (UW) and a range of preservation temperatures (-4 to -10 oC). We find that there exists an optimum temperature (-4oC) for SCP of rat hepatocytes which yields the highest viability; at this temperature HTS-FRS significantly outperforms UW solution in terms of viability and functional characteristics (secretions and enzymatic activity in suspension and plate culture). With the HTS-FRS solution we show that the cells can be stored for up to a week with high viability (~56%); moreover we also show that the preservation can be performed in large batches (50 million cells) with equal or better viability and no loss of functionality as compared to smaller batches (1.5 million cells) performed in cryovials
Average Properties of a Large Sample of z_abs ~ z_em associated Mg II Absorption Line Systems
We have studied a sample of 415 associated (z_ab z_em; relative velocity with
respect to QSO <3000km/s) Mg II absorption systems with 1.0<=z_ab<=1.86, in the
spectra of SDSS DR3 QSOs, to determine the dust content and ionization state in
the absorbers. We studied the dependence of these properties on the properties
of the QSOs and also, compared the properties with those of a similarly
selected sample of 809 intervening systems (apparent relative velocity with
respect to the QSO of >3000km/s), so as to understand their origin. From the
analysis of the composite spectra, as well as from the comparison of measured
equivalent widths in individual spectra, we conclude that the associated Mg II
absorbers have higher apparent ionization, measured by the strength of the C IV
absorption lines compared to the Mg II absorption lines, than the intervening
absorbers. The ionization so measured appears to be related to apparent
ejection velocity, being lower as the apparent ejection velocity is more and
more positive. There is clear evidence, from the composite spectra, for SMC
like dust attenuation in these systems; the 2175AA absorption feature is not
present. The extinction is almost twice that observed in the similarly selected
sample of intervening systems. We reconfirm that QSOs with non-zero FIRST radio
flux are intrinsically redder than the QSOs with no detection in the FIRST
survey. The incidence of associated Mg II systems in QSOs with non-zero FIRST
radio flux is 1.7 times that in the QSOs with no detection in the FIRST survey.
The associated absorbers in radio-detected QSOs which comprise about 12% of our
sample, cause 3 times more reddening than the associated absorbers in
radio-undetected QSOs. This excess reddening possibly suggests an intrinsic
nature for the associated absorbers in radio-detected QSOs.Comment: Accepted for publication in Ap
LSST: from Science Drivers to Reference Design and Anticipated Data Products
(Abridged) We describe here the most ambitious survey currently planned in
the optical, the Large Synoptic Survey Telescope (LSST). A vast array of
science will be enabled by a single wide-deep-fast sky survey, and LSST will
have unique survey capability in the faint time domain. The LSST design is
driven by four main science themes: probing dark energy and dark matter, taking
an inventory of the Solar System, exploring the transient optical sky, and
mapping the Milky Way. LSST will be a wide-field ground-based system sited at
Cerro Pach\'{o}n in northern Chile. The telescope will have an 8.4 m (6.5 m
effective) primary mirror, a 9.6 deg field of view, and a 3.2 Gigapixel
camera. The standard observing sequence will consist of pairs of 15-second
exposures in a given field, with two such visits in each pointing in a given
night. With these repeats, the LSST system is capable of imaging about 10,000
square degrees of sky in a single filter in three nights. The typical 5
point-source depth in a single visit in will be (AB). The
project is in the construction phase and will begin regular survey operations
by 2022. The survey area will be contained within 30,000 deg with
, and will be imaged multiple times in six bands, ,
covering the wavelength range 320--1050 nm. About 90\% of the observing time
will be devoted to a deep-wide-fast survey mode which will uniformly observe a
18,000 deg region about 800 times (summed over all six bands) during the
anticipated 10 years of operations, and yield a coadded map to . The
remaining 10\% of the observing time will be allocated to projects such as a
Very Deep and Fast time domain survey. The goal is to make LSST data products,
including a relational database of about 32 trillion observations of 40 billion
objects, available to the public and scientists around the world.Comment: 57 pages, 32 color figures, version with high-resolution figures
available from https://www.lsst.org/overvie
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