2,142 research outputs found
A photochemical flow reactor for large scale syntheses of aglain and rocaglate natural product analogues
Published in final edited form as: Bioorg Med Chem. 2017 Dec 1; 25(23): 6197–6202. Published online 2017 Jun 11. doi: [10.1016/j.bmc.2017.06.010]Herein, we report the development of continuous flow photoreactors for large scale ESIPT-mediated [3+2]-photocycloaddition of 2-(p-methoxyphenyl)-3-hydroxyflavone and cinnamate-derived dipolarophiles. These reactors can be efficiently numbered up to increase throughput two orders of magnitude greater than the corresponding batch reactions.Financial support from Boston University and National Institutes of Health (ABB R33AI105944) is gratefully acknowledged. We thank Dr. Norman Lee (Boston University) for high-resolution mass spectrometry data. NMR (CHE-0619339) and MS (CHE-0443618 facilities at Boston University are supported by the NSF. (Boston University; ABB R33AI105944 - National Institutes of Health; CHE-0619339 - NSF; CHE-0443618 - NSF)Accepted manuscrip
The optimal polarizations for achieving maximum contrast in radar images
There is considerable interest in determining the optimal polarizations that maximize contrast between two scattering classes in polarimetric radar images. A systematic approach is presented for obtaining the optimal polarimetric matched filter, i.e., that filter which produces maximum contrast between two scattering classes. The maximization procedure involves solving an eigenvalue problem where the eigenvector corresponding to the maximum contrast ratio is an optimal polarimetric matched filter. To exhibit the physical significance of this filter, it is transformed into its associated transmitting and receiving polarization states, written in terms of horizontal and vertical vector components. For the special case where the transmitting polarization is fixed, the receiving polarization which maximizes the contrast ratio is also obtained. Polarimetric filtering is then applies to synthetic aperture radar images obtained from the Jet Propulsion Laboratory. It is shown, both numerically and through the use of radar imagery, that maximum image contrast can be realized when data is processed with the optimal polarimeter matched filter
Detecting non-Markovian plasmonic band gaps in quantum dots using electron transport
Placing a quantum dot close to a metal nanowire leads to drastic changes in
its radiative decay behavior because of evanescent couplings to surface
plasmons. We show how two non-Markovian effects, band-edge and retardation,
could be observed in such a system. Combined with a quantum dot p-i-n junction,
these effects could be readout via current-noise measurements. We also discuss
how these effects can occur in similar systems with restricted geometries, like
phononic cavities and photonic crystal waveguides. This work links two
previously separate topics: surface-plasmons and current-noise measurements.Comment: 8 page
Experience report of physics-informed neural networks in fluid simulations: pitfalls and frustration
Though PINNs (physics-informed neural networks) are now deemed as a
complement to traditional CFD (computational fluid dynamics) solvers rather
than a replacement, their ability to solve the Navier-Stokes equations without
given data is still of great interest. This report presents our
not-so-successful experiments of solving the Navier-Stokes equations with PINN
as a replacement for traditional solvers. We aim to, with our experiments,
prepare readers for the challenges they may face if they are interested in
data-free PINN. In this work, we used two standard flow problems: 2D
Taylor-Green vortex at Re=100 and 2D cylinder flow at Re=200. The PINN method
solved the 2D Taylor-Green vortex problem with acceptable results, and we used
this flow as an accuracy and performance benchmark. About 32 hours of training
were required for the PINN method's accuracy to match the accuracy of a 16x16
finite-difference simulation, which took less than 20 seconds. The 2D cylinder
flow, on the other hand, did not produce a physical solution. The PINN method
behaved like a steady-flow solver and did not capture the vortex shedding
phenomenon. By sharing our experience, we would like to emphasize that the PINN
method is still a work-in-progress, especially in terms of solving flow
problems without any given data. More work is needed to make PINN feasible for
real-world problems in such applications.Comment: 8 pages, 9 figure
Predictive Limitations of Physics-Informed Neural Networks in Vortex Shedding
The recent surge of interest in physics-informed neural network (PINN)
methods has led to a wave of studies that attest to their potential for solving
partial differential equations (PDEs) and predicting the dynamics of physical
systems. However, the predictive limitations of PINNs have not been thoroughly
investigated. We look at the flow around a 2D cylinder and find that data-free
PINNs are unable to predict vortex shedding. Data-driven PINN exhibits vortex
shedding only while the training data (from a traditional CFD solver) is
available, but reverts to the steady state solution when the data flow stops.
We conducted dynamic mode decomposition and analyze the Koopman modes in the
solutions obtained with PINNs versus a traditional fluid solver (PetIBM). The
distribution of the Koopman eigenvalues on the complex plane suggests that PINN
is numerically dispersive and diffusive. The PINN method reverts to the steady
solution possibly as a consequence of spectral bias. This case study reaises
concerns about the ability of PINNs to predict flows with instabilities,
specifically vortex shedding. Our computational study supports the need for
more theoretical work to analyze the numerical properties of PINN methods. The
results in this paper are transparent and reproducible, with all data and code
available in public repositories and persistent archives; links are provided in
the paper repository at \url{https://github.com/barbagroup/jcs_paper_pinn}, and
a Reproducibility Statement within the paper
Reducing Perineal Tears: The Effect of Pushing Methods and Length of 2nd Stage of Labor
Objective: To evaluate the current literature evidence for the effects of either instructed or spontaneous pushing on perineal laceration incidence during delivery and the duration of second stage of labor.
Background: Lacerations (tears) of the perineum are common among women during delivery, increasing pain, infection risk, and other problems for women. Furthermore, prolonged second stage of labor has been shown a risk factor for lacerations. Pushing methods could have an effect on the incidence of lacerations and duration of second stage of labor.
Methods: Thorough search of online databases for the highest levels of evidence relating to the topic within the last 5 years.
Results: Spontaneous pushing versus instructed pushing method may decrease laceration incidence, according to limited evidence; however, other studies do not find this effect to be statistically significant. In addition, spontaneous pushing results in longer second stage of labor, having a possible indirect effect on laceration incidence.
Conclusion: Since no evidence strongly supports either pushing method as beneficial for decreasing lacerations, further research on this topic seems warranted. For now, either method may be accepted for use in current medical practice
Instabilities and Clumping in Type Ia Supernova Remnants
We present two-dimensional high-resolution hydrodynamical simulations in
spherical polar coordinates of a Type Ia supernova interacting with a constant
density interstellar medium. The ejecta are assumed to be freely expanding with
an exponential density profile. The interaction gives rise to a double-shocked
structure susceptible to hydrodynamic instabilities. The Rayleigh-Taylor
instability initially grows, but the Kelvin-Helmholtz instability takes over,
producing vortex rings. The nonlinear instability initially evolves toward
longer wavelengths and eventually fades away when the reverse shock front is in
the flatter part of the supernova density distribution. Based on observations
of X-ray knots and the protrusion in the southeast outlin of Tycho's supernova
remnant, we include clumping in the ejecta. The clump interaction with the
reverse shock induces Rayleigh-Taylor and Kelvin-Helmholtz instabilities on the
clump surface that facilitate fragmentation. In order to survive crushing and
to have a bulging effect on the forward shock, the clump's initial density
ratio to the surrounding ejecta must be at least 100 for the conditions in
Tycho's remnant. The 56Ni bubble effect may be important for the development of
clumpiness in the ejecta. The observed presence of an Fe clump would then
require a non-radioactive origin for this Fe, possibly 54Fe. The large radial
distance of the X-ray emitting Si and S ejecta from the remnant center
indicates that they were initially in clumps.Comment: 27 pages, 4 postscript figures, 5 GIF figures submitted to
Astrophysical Journa
Spin alignment of stars in old open clusters
Stellar clusters form by gravitational collapse of turbulent molecular
clouds, with up to several thousand stars per cluster. They are thought to be
the birthplace of most stars and therefore play an important role in our
understanding of star formation, a fundamental problem in astrophysics. The
initial conditions of the molecular cloud establish its dynamical history until
the stellar cluster is born. However, the evolution of the cloud's angular
momentum during cluster formation is not well understood. Current observations
have suggested that turbulence scrambles the angular momentum of the
cluster-forming cloud, preventing spin alignment amongst stars within a
cluster. Here we use asteroseismology to measure the inclination angles of spin
axes in 48 stars from the two old open clusters NGC~6791 and NGC~6819. The
stars within each cluster show strong alignment. Three-dimensional
hydrodynamical simulations of proto-cluster formation show that at least 50 %
of the initial proto-cluster kinetic energy has to be rotational in order to
obtain strong stellar-spin alignment within a cluster. Our result indicates
that the global angular momentum of the cluster-forming clouds was efficiently
transferred to each star and that its imprint has survived after several
gigayears since the clusters formed.Comment: 14 pages, 3 figures, 1 table. Published in Nature Astronom
Clump Development by the Nickel Bubble Effect in Supernovae
We used one-dimensional radiative-transport radiation hydrodynamical (RHD)
simulations to investigate the formation of clumping in freely-expanding
supernova ejecta due to the radioactive heating from the Ni56 -> Co56 -> Fe56
decay sequence. The heating gives rise to an inflated Nickel bubble, which
induces a forward shock that compresses the outer ambient gas into a shell. The
radiative energy deposited by the radioactivity leaks out across the shock by
radiative diffusion, and we investigate its effect on the evolution of the
ejecta structure. Compared to the hydrodynamical adiabatic approximation with
gamma =4/3, the preshock gas becomes accelerated by the radiation outflow. The
shock is thus weakened and the shell becomes broader and less dense. The
thickness of the shell takes up <~ 4 % of the radius of the bubble, and the
structure of the shell can be approximately described by a self-similar
solution. We compared the properties of the shell components with those of the
ejecta clumps indicated by our previous hydrodynamical simulations for the
later interaction of clumps with the outer supernova remnant. The high density
contrast across the shell, chi ~ 100, is compatible with that of ejecta clumps
as indicated for Tycho's knots, but there is insufficient dense gas to cause a
pronounced protrusion on the outline of a core collapse supernova remnant, like
the bullets in the Vela remnant.Comment: accepted by Ap
Polarimetric clutter modeling: Theory and application
The two-layer anisotropic random medium model is used to investigate fully polarimetric scattering properties of earth terrain media. The polarization covariance matrices for the untilted and tilted uniaxial random medium are evaluated using the strong fluctuation theory and distorted Born approximation. In order to account for the azimuthal randomness in the growth direction of leaves in tree and grass fields, an averaging scheme over the azimuthal direction is also applied. It is found that characteristics of terrain clutter can be identified through the analysis of each element of the covariance matrix. Theoretical results are illustrated by the comparison with experimental data provided by MIT Lincoln Laboratory for tree and grass fields
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