12,081 research outputs found
Precessing Jets and Molecular Outflows: A 3-D Numerical Study
We present 3-D numerical hydrodynamical simulations of precessing supersonic
heavy jets to explore how well they serve as a model for generating molecular
outflows from Young Stellar Objects. The dynamics are studied with a number of
high resolution simulations on a Cartesian grid (128x128x128 zones) using a
high order finite difference method. A range of cone angles and precession
rates were included in the study. Two higher resolution runs (256x256x256
zones) were made for comparison in order to confirm numerical convergence of
global flow characteristics. Morphological, kinematical and dynamical
characteristics of precessing jets are described and compared to important
properties of straight jets and also to observations of YSOs. In order to
examine the robustness of precessing jets as a mean to produce molecular
outflows around Young Stellar Objects, ``synthetic observations'' of the
momentum distributions of the simulated precessing jets are compared to
observations of molecular outflows. It is found that precessing jets match
better the morphology, highly forward driven momentum and momentum
distributions along the long axis of molecular outflows than do wind-driven or
straight jet-driven flow models.Comment: Accepted by ApJ, 31 pages, using aasms.sty, Also available in
postscript with figures via a gzipped tar file at
ftp://s1.msi.umn.edu/pub/afrank/3DJet/3DJet.tar.gz . For information contact
[email protected]
Oblique MHD cosmic-ray modified shocks: Two-fluid numerical simulations
We present the first results of time dependent, two-fluid, cosmic-ray (CR) modified, MHD shock simulations. The calculations were carried out with a new numerical code for 1-D ideal MHD. By coupling this code with the CR energy transport equation we can simulate the time-dependent evolution of MHD shocks including the acceleration of the CR and their feedback on the shock structures. We report tests of the combined numerical method including comparisons with analytical steady state results published earlier by Webb, as well as internal consistency checks for more general MHD CR shock structures after they appear to have converged to dynamical steady states. We also present results from an initial time dependent simulation which extend the parameter space domain of previous analytical models. These new results support Webb's suggestion that equilibrium oblique shocks are less effective than parallel shocks in the acceleration of CR. However, for realistic models of anisotropic CR diffusion, oblique shocks may achieve dynamical equilibrium on shorter timescale than parallel shocks
A Divergence-Free Upwind Code for Multidimensional Magnetohydrodynamic Flows
A description is given for preserving {\bmsy\nabla}\cdot{\vec B}=0 in a
magnetohydrodynamic (MHD) code that employs the upwind, Total Variation
Diminishing (TVD) scheme and the Strang-type operator splitting for
multi-dimensionality. The method is based on the staggered mesh technique to
constrain the transport of magnetic field: the magnetic field components are
defined at grid interfaces with their advective fluxes on grid edges, while
other quantities are defined at grid centers. The magnetic field at grid
centers for the upwind step is calculated by interpolating the values from grid
interfaces. The advective fluxes on grid edges for the magnetic field evolution
are calculated from the upwind fluxes at grid interfaces. Then, the magnetic
field can be maintained with {\bmsy\nabla}\cdot{\vec B}=0 exactly, if this is
so initially, while the upwind scheme is used for the update of fluid
quantities. The correctness of the code is demonstrated through tests comparing
numerical solutions either with analytic solutions or with numerical solutions
from the code using an explicit divergence-cleaning method. Also the robustness
is shown through tests involving realistic astrophysical problems.Comment: 15 pages of text, 8 figures (in degraded gif format), to appear in
The Astrophysical Journal (Dec. 10, 1998), original quality figures available
via anonymous ftp at ftp://ftp.msi.umn.edu/pub/users/twj/mhddivb5.uu or
ftp://canopus.chungnam.ac.kr/ryu/mhddivb5.u
Undergraduate Teaching Assistants and their Use of Nonverbal Immediacy Behaviors in the Basic Communication Course
Over the past two decades, perhaps no instructional communication topic has been researched as thoroughly as teacher immediacy. However, one important area of the existing teacher immediacy literature that remains underdeveloped is how undergraduate teaching assistants enact immediacy behaviors, and how, if at all, students respond to these teaching assistants differently based on the enactment of these behaviors. Thus, the purpose of this investigation was to gain a clearer understanding as to what, if any, immediacy behaviors are used by undergraduate teaching assistants in the basic communication course at a large Midwestern university. The researchers conducted 50 hours of observation in an attempt to determine the different types of immediacy behaviors being displayed by undergraduate teaching assistants in the instructional context as well as examine the effects those behaviors have on students. The results of this analysis are discussed
The Evolution and Efficiency of Oblique MHD Cosmic-Ray Shocks: Two-Fluid Simulations
Using a new, second-order accurate numerical method we present dynamical
simulations of oblique MHD cosmic ray (CR) modified plane shock evolution using
the two-fluid model for diffusive particle acceleration. The numerical shocks
evolve to published analytical steady state properties. In order to probe the
dynamical role of magnetic fields we have explored for these time asymptotic
states the parameter space of upstream fast mode Mach number, , and plasma
, compiling the results into maps of dynamical steady state CR
acceleration efficiency, . These maps, along with additional
numerical experiments, show that is reduced through the action of
compressive work on tangential magnetic fields in CR-MHD shocks. Thus
in low , moderate shocks tends to be smaller in quasi
perpendicular shocks than it would be high shocks of the same .
This result supports earlier conclusions that strong, oblique magnetic fields
inhibit diffusive shock acceleration. For quasi parallel shocks with , on the other hand, seems to be increased at a given when
compared to high shocks. The apparent contradiction to the first
conclusion results, however, from the fact that for small quasi
parallel shocks, the fast mode Mach number is not a good measure of compression
through the shock. That is better reflected in the sonic Mach number, which is
greater. Acceleration efficiencies for high and low having comparable
sonic Mach numbers are more similar. Time evolution of CR-MHD shocks is
qualitatively similar to CR-gasdynamical shocks. However, several potentially
interesting differences are apparent.Comment: 23 pages with 8 figures (uuencoded compressed tar file of text and
figures available via anonymous ftp from directory /pub at
sirius.chungnam.ac.kr), submitted to The Astrophysical Journal, CNU-AST-94-
Numerical Magnetohydrodynamics in Astrophysics: Algorithm and Tests for Multi-Dimensional Flow
We present for astrophysical use a multi-dimensional numerical code to solve
the equations for ideal magnetohydrodynamics (MHD). It is based on an explicit
finite difference method on an Eulerian grid, called the Total Variation
Diminishing (TVD) scheme, which is a second-order-accurate extension of the
Roe-type upwind scheme. Multiple spatial dimensions are treated through a
Strang-type operator splitting. The constraint of a divergence-free field is
enforced exactly by calculating a correction via a gauge transformation in each
time step.
Results from two-dimensional shock tube tests show that the code captures
correctly discontinuities in all three MHD waves families as well as contact
discontinuities. The numerical viscosities and resistivity in the code, which
are useful in order to understand simulations involving turbulent flows, are
estimated through the decay of two-dimensional linear waves. Finally, the
robustness of the code in two-dimensions is demonstrated through calculations
of the Kelvin-Helmholtz instability and the Orszag-Tang vortex.Comment: ApJ accepted, 20 pages, uuencoded, compressed postscript file, 8
figures available by anonymous ftp at ftp://sirius.chungnam.ac.kr/pub/mhd2d/
or available upon request to [email protected]
National Outreach Programming for Landowners—Natural Resource Enterprises
We conducted a survey of Mississippi landowners to determine revenues collected and expenditures incurred during 1996-1998 for fee hunting on their properties (inflated to 2011 estimates). Study findings revealed that respondents diversified incomes derived through fee hunting enterprises on their lands. This information has been used to design a series of multi-state landowner workshops about natural resource enterprises development and conservation practices on private lands. With state and local collaborator assistance, we have conducted over 50 landowner workshops in nine U.S. states and Sweden and have received requests to expand outreach programming to other U.S. states
Faculty Observables and Self-Reported Responsiveness to Academic Dishonesty
Prior to 2009, a mid-sized public institution in the southeast had a faculty-driven honor policy characterized by little education about the policy and no tracking of repeat offenders. An updated code, implemented in August of 2009, required that students sign an honor pledge, created a formal student honor board, and developed a process to track and hold accountable, repeat offenders. Self-reported data on faculty vigilance to detect and punish cheating is collected both prior to and after a change in the honor code at a mid-sized public institution in the southeast. We find that, at the time of the first survey, full professors and faculty with a longer duration of employment were more likely to claim vigilance in cheating detection and harshness in punishing cheaters than newer, untenured faculty. The relationship between these factors and detection and harshness diminished when the honor code was enhanced
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