8,643 research outputs found
A Comparison of Sampling Methods for Measuring Residual Stand Damage from Commercial Thinning
Four sampling methods were compared for accuracy and ease of implementation in measuring residual stand damage. Data were collected from young Douglas-fir (Pseudotsuga menziesii) stands, which were commercially thinned using three different logging systems in western Oregon. Systematic plot sampling consistently provided damage estimates similar to the results of a 100% survey; there was no significant difference between their accuracies in measuring stand damage. This method also took the least amount of time and effort for map layout and field plot location. Because measuring stand damage requires considerable effort in sample planning and implementation, an easier, quick-survey method should be developed to monitor residual stand damage for in-progress and post-thinning operations
New numerical approaches for modeling thermochemical convection in a compositionally stratified fluid
Seismic imaging of the mantle has revealed large and small scale
heterogeneities in the lower mantle; specifically structures known as large low
shear velocity provinces (LLSVP) below Africa and the South Pacific. Most
interpretations propose that the heterogeneities are compositional in nature,
differing in composition from the overlying mantle, an interpretation that
would be consistent with chemical geodynamic models. Numerical modeling of
persistent compositional interfaces presents challenges, even to
state-of-the-art numerical methodology. For example, some numerical algorithms
for advecting the compositional interface cannot maintain a sharp compositional
boundary as the fluid migrates and distorts with time dependent fingering due
to the numerical diffusion that has been added in order to maintain the upper
and lower bounds on the composition variable and the stability of the advection
method. In this work we present two new algorithms for maintaining a sharper
computational boundary than the advection methods that are currently openly
available to the computational mantle convection community; namely, a
Discontinuous Galerkin method with a Bound Preserving limiter and a
Volume-of-Fluid interface tracking algorithm. We compare these two new methods
with two approaches commonly used for modeling the advection of two distinct,
thermally driven, compositional fields in mantle convection problems; namely,
an approach based on a high-order accurate finite element method advection
algorithm that employs an artificial viscosity technique to maintain the upper
and lower bounds on the composition variable as well as the stability of the
advection algorithm and the advection of particles that carry a scalar quantity
representing the location of each compositional field. All four of these
algorithms are implemented in the open source FEM code ASPECT
Statistics of Advective Stretching in Three-dimensional Incompressible Flows
We present a method to quantify kinematic stretching in incompressible, unsteady, isoviscous, three-dimensional flows. We extend the method of Kellogg and Turcotte (J. Geophys. Res. 95:421–432, 1990) to compute the axial stretching/thinning experienced by infinitesimal ellipsoidal strain markers in arbitrary three-dimensional incompressible flows and discuss the differences between our method and the computation of Finite Time Lyapunov Exponent (FTLE). We use the cellular flow model developed in Solomon and Mezic (Nature 425:376–380, 2003) to study the statistics of stretching in a three-dimensional unsteady cellular flow. We find that the probability density function of the logarithm of normalised cumulative stretching (log S) for a globally chaotic flow, with spatially heterogeneous stretching behavior, is not Gaussian and that the coefficient of variation of the Gaussian distribution does not decrease with time as
. However, it is observed that stretching becomes exponential log S∼t and the probability density function of log S becomes Gaussian when the time dependence of the flow and its three-dimensionality are increased to make the stretching behaviour of the flow more spatially uniform. We term these behaviors weak and strong chaotic mixing respectively. We find that for strongly chaotic mixing, the coefficient of variation of the Gaussian distribution decreases with time as
. This behavior is consistent with a random multiplicative stretching process
Vanishing Hall Resistance at High Magnetic Field in a Double Layer Two-Dimensional Electron System
At total Landau level filling factor a double layer
two-dimensional electron system with small interlayer separation supports a
collective state possessing spontaneous interlayer phase coherence. This state
exhibits the quantized Hall effect when equal electrical currents flow in
parallel through the two layers. In contrast, if the currents in the two layers
are equal, but oppositely directed, both the longitudinal and Hall resistances
of each layer vanish in the low temperature limit. This finding supports the
prediction that the ground state at is an excitonic superfluid.Comment: 4 pages, 4 figure
On the Sensitivity of 3-D Thermal Convection Codes to Numerical Discretization: A Model Intercomparison
Fully 3-D numerical simulations of thermal convection in a spherical shell have become a standard for studying the dynamics of pattern formation and its stability under perturbations to various parameter values. The question arises as to how does the discretization of the governing equations affect the outcome and thus any physical interpretation. This work demonstrates the impact of numerical discretization on the observed patterns, the value at which symmetry is broken, and how stability and stationary behavior is dependent upon it. Motivated by numerical simulations of convection in the Earth\u27s mantle, we consider isoviscous Rayleigh-Bénard convection at infinite Prandtl number, where the aspect ratio between the inner and outer shell is 0.55. We show that the subtleties involved in development mantle convection models are considerably more delicate than has been previously appreciated, due to the rich dynamical behavior of the system. Two codes with different numerical discretization schemes: an established, community-developed, and benchmarked finite element code (CitcomS) and a novel spectral method that combines Chebyshev polynomials with radial basis functions (RBF) are compared. A full numerical study is investigated for the following three cases. The first case is based on the cubic (or octahedral) initial condition (spherical harmonics of degree ℓ =4). How variations in the behavior of the cubic pattern to perturbations in the initial condition and Rayleigh number between the two numerical discrezations is studied. The second case investigates the stability of the dodecahedral (or icosahedral) initial condition (spherical harmonics of degree ℓ = 6). Although both methods converge first to the same pattern, this structure is ultimately unstable and systematically degenerates to cubic or tetrahedral symmetries, depending on the code used. Lastly, a new steady state pattern is presented as a combination of order 3 and 4 spherical harmonics leading to a five cell or a hexahedral pattern and stable up to 70 times the critical Rayleigh number. This pattern can provide the basis for a new accuracy benchmark for 3-D spherical mantle convection codes
Putting theory oriented evaluation into practice
Evaluations of gaming simulations and business games as teaching devices are typically end-state driven. This emphasis fails to detect how the simulation being evaluated does or does not bring about its desired consequences. This paper advances the use of a logic model approach which possesses a holistic perspective that aims at including all elements associated with the situation created by a game. The use of the logic model approach is illustrated as applied to Simgame, a board game created for secondary school level business education in six European Union countries
The TWA 3 Young Triple System: Orbits, Disks, Evolution
We have characterized the spectroscopic orbit of the TWA 3A binary and
provide preliminary families of probable solutions for the TWA 3A visual orbit
as well as for the wide TWA 3A--B orbit. TWA 3 is a hierarchical triple located
at 34 pc in the 10 Myr old TW Hya association. The wide component
separation is 1."55; the close pair was first identified as a possible binary
almost 20 years ago. We initially identified the 35-day period orbital solution
using high-resolution infrared spectroscopy which angularly resolved the A and
B components. We then refined the preliminary orbit by combining the infrared
data with a re-analysis of our high-resolution optical spectroscopy. The
orbital period from the combined spectroscopic solution is 35 days, the
eccentricity is 0.63, and the mass ratio is 0.84; although this
high mass ratio would suggest that optical spectroscopy alone should be
sufficient to identify the orbital solution, the presence of the tertiary B
component likely introduced confusion in the blended optical spectra. Using
millimeter imaging from the literature, we also estimate the inclinations of
the stellar orbital planes with respect to the TWA 3A circumbinary disk
inclination and find that all three planes are likely misaligned by at least
30 degrees. The TWA 3A spectroscopic binary components have spectral
types of M4.0 and M4.5; TWA 3B is an M3. We speculate that the system formed as
a triple, is bound, and that its properties were shaped by dynamical
interactions between the inclined orbits and disk.Comment: Accepted to Ap
Chandra detection of extended X-ray emission from the recurrent nova RS Ophiuchi
Radio, infrared, and optical observations of the 2006 eruption of the
symbiotic recurrent nova RS Ophiuchi (RS Oph) showed that the explosion
produced non-spherical ejecta. Some of this ejected material was in the form of
bipolar jets to the east and west of the central source. Here we describe Xray
observations taken with the Chandra X-ray Observatory one and a half years
after the beginning of the outburst that reveal narrow, extended structure with
a position angle of approximately 300 degrees (east of north). Although the
orientation of the extended feature in the X-ray image is consistent with the
readout direction of the CCD detector, extensive testing suggests that the
feature is not an artifact. Assuming it is not an instrumental effect, the
extended X-ray structure shows hot plasma stretching more than 1,900 AU from
the central binary (taking a distance of 1.6 kpc). The X-ray emission is
elongated in the northwest direction - in line with the extended infrared
emission and some minor features in the published radio image. It is less
consistent with the orientation of the radio jets and the main bipolar optical
structure. Most of the photons in the extended X-ray structure have energies of
less than 0.8 keV. If the extended X-ray feature was produced when the nova
explosion occurred, then its 1".2 length as of 2007 August implies that it
expanded at an average rate of more than 2 mas/d, which corresponds to a flow
speed of greater than 6,000 km/s (d/1.6 kpc) in the plane of the sky. This
expansion rate is similar to the earliest measured expansion rates for the
radio jets.Comment: accepted in Ap
Anisotropy of Growth of the Close-Packed Surfaces of Silver
The growth morphology of clean silver exhibits a profound anisotropy: The
growing surface of Ag(111) is typically very rough while that of Ag(100) is
smooth and flat. This serious and important difference is unexpected, not
understood, and hitherto not observed for any other metal. Using density
functional theory calculations of self-diffusion on flat and stepped Ag(100) we
find, for example, that at flat regions a hopping mechanism is favored, while
across step edges diffusion proceeds by an exchange process. The calculated
microscopic parameters explain the experimentally reported growth properties.Comment: RevTeX, 4 pages, 3 figures in uufiles form, to appear in Phys. Rev.
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