9,820 research outputs found
Modeling Particle Transport Distances as a Function of Slope and Surface Roughness
Significant effort has been put into modeling the evolution of hillslope profiles through time. The models use a continuum approach and are commonly deterministic. Early models assumed a linear relationship between hillslope angle and sediment flux. This relationship produces hillslope profiles that increase in steepness from crest to base. However, hillslopes observed in the field are commonly planar downslope of their convex crests. Recently, non-linear sediment transport equations have been developed that produce hillslope profiles closer to those which are observed in nature, yet the mid-slope sections are not entirely planar. Currently, there is interest in using a non-deterministic approach where transport distances follow probability distributions that depend on hillslope angle. In order to qualitatively and quantitatively characterize this probabilistic relationship, the transport distances of individual particles released into a dry ravel flume with a rough surface were measured as a function of flume angle. Using the inputs of flume angle and surface roughness, the results of the experiments were replicated with a discrete element model in which the motion of the particles was modeled with the momentum equation. The implication of this study is that this method can be used with inputs measured from the field to model the evolution of entire hillslopes
Coupled DEM-LBM method for the free-surface simulation of heterogeneous suspensions
The complexity of the interactions between the constituent granular and
liquid phases of a suspension requires an adequate treatment of the
constituents themselves. A promising way for numerical simulations of such
systems is given by hybrid computational frameworks. This is naturally done,
when the Lagrangian description of particle dynamics of the granular phase
finds a correspondence in the fluid description. In this work we employ
extensions of the Lattice-Boltzmann Method for non-Newtonian rheology, free
surfaces, and moving boundaries. The models allows for a full coupling of the
phases, but in a simplified way. An experimental validation is given by an
example of gravity driven flow of a particle suspension
Studying the Variation of the Fine Structure Constant Using Emission Line Multiplets
As an extension of the method by Bahcall et al. (2004) to investigate the
time dependence of the fine structure constant, we describe an approach based
on new observations of forbidden line multiplets from different ionic species.
We obtain optical spectra of fine structure transitions in [Ne III], [Ne V], [O
III], [OI], and [SII] multiplets from a sample of 14 Seyfert 1.5 galaxies in
the low-z range 0.035 < z < 0.281. Each source and each multiplet is
independently analyzed to ascertain possible errors. Averaging over our sample,
we obtain a conservative value alpha^2(t)/\alpha^2(0) = 1.0030+-0.0014.
However, our sample is limited in size and our fitting technique simplistic as
we primarily intend to illustrate the scope and strengths of emission line
studies of the time variation of the fine structure constant. The approach can
be further extended and generalized to a "many-multiplet emission line method"
analogous in principle to the corresponding method using absorption lines. With
that aim, we note that the theoretical limits on emission line ratios of
selected ions are precisely known, and provide well constrained selection
criteria. We also discuss several other forbidden and allowed lines that may
constitute the basis for a more rigorous study using high-resolution
instruments on the next generation of 8 m class telescopes.Comment: 20 pages, 4 figures, sumbitted to A
Exclusive W + photon production in proton-antiproton collisions I: general formalism
We present a detailed computation of the fully exclusive cross section of p +
antip --> W + photon + X with X = 0 and 1 jet in the framework of the
factorization theorem and dimensional regularization. Order alpha-strong and
photon bremsstrahlung contributions are discussed in the MS-bar mass
factorization scheme. The resulting expressions are ready to be implemented
numerically using Monte Carlo techniques to compute single and double
differential cross sections and correlations between outgoing pairs of
particles.Comment: ITP-SB-93-72, 40 pages, LateX. 3*4 figures in separate file.
([email protected]) ([email protected]
Financial panic and emerging market funds
This article studies equity investment of emerging-market funds based on the 2003â2009 weekly data and compares the dynamics of flow and return between tranquil period and financial panic based on the experience of the latest 2008â2009 global financial crisis. First, we find that the well-documented positive feedback trading is a tranquil-period phenomenon such that it is more difficult in general for emerging-market funds to attract new investment in financial panic. Second, the predictive power of flow on return is driven by a combination of price pressure and information effects in tranquil period, while the information effect dominates in financial panic. Third, the underlying co-movements or contagion of flow across the emerging-market funds influence the association between flow and return. Overall, the findings highlight the importance of accounting for state-dependent dynamics as well as cross-regional co-movements in the analysis of flow and return
Behavior of confined granular beds under cyclic thermal loading
We investigate the mechanical behavior of a confined granular packing of
irregular polyhedral particles under repeated heating and cooling cycles by
means of numerical simulations with the Non-Smooth Contact Dynamics method.
Assuming a homogeneous temperature distribution as well as constant temperature
rate, we study the effect of the container shape, and coefficients of thermal
expansions on the pressure buildup at the confining walls and the density
evolution. We observe that small changes in the opening angle of the
confinement can lead to a drastic peak pressure reduction. Furthermore, the
displacement fields over several thermal cycles are obtained and we discover
the formation of convection cells inside the granular material having the shape
of a torus. The root mean square of the vorticity is then calculated from the
displacement fields and a quadratic dependency on the ratio of thermal
expansion coefficients is established
Updated opacities from the opacity project
Using the code autostructure, extensive calculations of inner-shell atomic data have been made for the chemical elements He, C, N, O, Ne, Na, Mg, Al, Si, S, Ar, Ca, Cr, Mn, Fe and Ni. The results are used to obtain updated opacities from the Opacity Project (OP). A number of other improvements on earlier work have also been included. Rosseland-mean opacities from the OP are compared with those from OPAL. Differences of 5-10 per cent occur. The OP gives the 'Z-bump', at log(T) 5.2, to be shifted to slightly higher temperatures. The opacities from the OP, as functions of temperature and density, are smoother than those from OPAL. The accuracy of the integrations used to obtain mean opacities can depend on the frequency mesh used. Tests involving variation of the numbers of frequency points show that for typical chemical mixtures the OP integrations are numerically correct to within 0.1 per cent. The accuracy of the interpolations used to obtain mean opacities for any required values of temperature and density depends on the temperature-density meshes used. Extensive tests show that, for all cases of practical interest, the OP interpolations give results correct to better than 1 per cent. Prior to a number of recent investigations which have indicated a need for downward revisions in the solar abundances of oxygen and other elements, there was good agreement between properties of the Sun deduced from helioseismology and from stellar evolution models calculated using OPAL opacities. The revisions destroy that agreement. In a recent paper, Bahcall et al. argue that the agreement would be restored if opacities for the regions of the Sun with 2 Ă 106T 5 Ă 106 K (0.7-0.4 R) were larger than those given by OPAL by about 10 per cent. In the region concerned, the present results from the OP do not differ from those of OPAL by more than 2.5 per cent
- âŠ