94 research outputs found
Magnetized Ekman Layer and Stewartson Layer in a Magnetized Taylor-Couette Flow
In this paper we present axisymmetric nonlinear simulations of magnetized
Ekman and Stewartson layers in a magnetized Taylor-Couette flow with a
centrifugally stable angular-momemtum profile and with a magnetic Reynolds
number below the threshold of magnetorotational instability. The magnetic field
is found to inhibit the Ekman suction. The width of the Ekman layer is reduced
with increased magnetic field normal to the end plate. A uniformly-rotating
region forms near the outer cylinder. A strong magnetic field leads to a steady
Stewartson layer emanating from the junction between differentially rotating
rings at the endcaps. The Stewartson layer becomes thinner with larger Reynolds
number and penetrates deeper into the bulk flow with stronger magnetic field
and larger Reynolds number. However, at Reynolds number larger than a critical
value , axisymmetric, and perhaps also nonaxisymmetric, instabilities
occur and result in a less prominent Stewartson layer that extends less far
from the boundary.Comment: 24 pages, 12 figures, accepted by PRE, revision according to referee
Towards Precision LSST Weak-Lensing Measurement - I: Impacts of Atmospheric Turbulence and Optical Aberration
The weak-lensing science of the LSST project drives the need to carefully
model and separate the instrumental artifacts from the intrinsic lensing
signal. The dominant source of the systematics for all ground based telescopes
is the spatial correlation of the PSF modulated by both atmospheric turbulence
and optical aberrations. In this paper, we present a full FOV simulation of the
LSST images by modeling both the atmosphere and the telescope optics with the
most current data for the telescope specifications and the environment. To
simulate the effects of atmospheric turbulence, we generated six-layer phase
screens with the parameters estimated from the on-site measurements. For the
optics, we combined the ray-tracing tool ZEMAX and our simulated focal plane
data to introduce realistic aberrations and focal plane height fluctuations.
Although this expected flatness deviation for LSST is small compared with that
of other existing cameras, the fast f-ratio of the LSST optics makes this focal
plane flatness variation and the resulting PSF discontinuities across the CCD
boundaries significant challenges in our removal of the systematics. We resolve
this complication by performing PCA CCD-by-CCD, and interpolating the basis
functions using conventional polynomials. We demonstrate that this PSF
correction scheme reduces the residual PSF ellipticity correlation below 10^-7
over the cosmologically interesting scale. From a null test using HST/UDF
galaxy images without input shear, we verify that the amplitude of the galaxy
ellipticity correlation function, after the PSF correction, is consistent with
the shot noise set by the finite number of objects. Therefore, we conclude that
the current optical design and specification for the accuracy in the focal
plane assembly are sufficient to enable the control of the PSF systematics
required for weak-lensing science with the LSST.Comment: Accepted to PASP. High-resolution version is available at
http://dls.physics.ucdavis.edu/~mkjee/LSST_weak_lensing_simulation.pd
The Buffer Gas Beam: An Intense, Cold, and Slow Source for Atoms and Molecules
Beams of atoms and molecules are stalwart tools for spectroscopy and studies
of collisional processes. The supersonic expansion technique can create cold
beams of many species of atoms and molecules. However, the resulting beam is
typically moving at a speed of 300-600 m/s in the lab frame, and for a large
class of species has insufficient flux (i.e. brightness) for important
applications. In contrast, buffer gas beams can be a superior method in many
cases, producing cold and relatively slow molecules in the lab frame with high
brightness and great versatility. There are basic differences between
supersonic and buffer gas cooled beams regarding particular technological
advantages and constraints. At present, it is clear that not all of the
possible variations on the buffer gas method have been studied. In this review,
we will present a survey of the current state of the art in buffer gas beams,
and explore some of the possible future directions that these new methods might
take
Generalized Whittle-Matrn random field as a model of correlated fluctuations
This paper considers a generalization of Gaussian random field with
covariance function of Whittle-Matrn family. Such a random
field can be obtained as the solution to the fractional stochastic differential
equation with two fractional orders. Asymptotic properties of the covariance
functions belonging to this generalized Whittle-Matrn family
are studied, which are used to deduce the sample path properties of the random
field. The Whittle-Matrn field has been widely used in
modeling geostatistical data such as sea beam data, wind speed, field
temperature and soil data. In this article we show that generalized
Whittle-Matrn field provides a more flexible model for wind
speed data.Comment: 22 pages, 10 figures, accepted by Journal of Physics
Adomian decomposition method simulation of Von Kármán swirling bioconvection nanofluid flow
The study reveals analytically on the 3-dimensional viscous time-dependent gyrotactic bioconvection in
swirling nanofluid flow past from a rotating disk. It is known that the deformation of the disk is along the radial
direction. In addition to that Stefan blowing is considered. The Buongiorno nanofluid model is taken care of assuming
the fluid to be dilute and we find Brownian motion and thermophoresis have dominant role on nanoscale unit. The
primitive mass conservation equation, radial, tangential and axial momentum, heat, nano-particle concentration and
micro-organism density function are developed in a cylindrical polar coordinate system with appropriate wall (disk
surface) and free stream boundary conditions. This highly nonlinear, strongly coupled system of unsteady partial
differential equations is normalized with the classical Von Kármán and other transformations to render the boundary
value problem into an ordinary differential system. The emerging 11th order system features an extensive range of
dimensionless flow parameters i.e. disk stretching rate, Brownian motion, thermophoresis, bioconvection Lewis number,
unsteadiness parameter, ordinary Lewis number, Prandtl number, mass convective Biot number, Péclet number and
Stefan blowing parameter. Solutions of the system are obtained with developed semi-analytical technique i.e. Adomian
decomposition method. Validation of the said problem is also conducted with earlier literature computed by
Runge-Kutta shooting technique
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