459 research outputs found
An implicit method for the calculation of inlet flow fields
Inlet flow fields are calculated by an implicit, time marching procedure to solve the thin layer Navier-Stokes equations formulated in body fitted coordinates. Because the method can be used for a flow field with both subsonic and supersonic regions, it is applicable to subcritical as well as supercritical inlet operation. Results are presented and discussed for an inlet of current design practice. Results include inviscid calculations performed for supercritical inlet operation with uniform and nonuniform inflow boundary conditions as well as for subcritical inlet operation with uniform inflow boundary conditions. Results for viscous calculations performed for supercritical inlet operation with uniform inflow boundary conditions are also discussed
Dynamical Effect of the Turbulence of IGM on the Baryon Fraction Distribution
We investigate the dynamical effect of the turbulence in baryonic
intergalactic medium (IGM) on the baryon fraction distribution. In the fully
developed nonlinear regime, the IGM will evolve into the state of turbulence,
containing strong and curved shocks, vorticity and complex structures.
Turbulence would lead to the density and velocity fields of the IGM to be
different from those of underlying collisionless dark matter. Consequently, the
baryon fraction f_b will deviate from its cosmic mean . We study these
phenomena with simulation samples produced by the weighted essentially
non-oscillatory (WENO) hybrid cosmological hydrodynamic/N-body code, which is
effective of capturing shocks and complex structures. We find that the
distribution of baryon fraction is highly nonuniform on scales from hundreds
kpc to a few of Mpc, and f_b varies from as low as 1% to a few times of the
cosmic mean. We further show that the turbulence pressure in the IGM is weakly
scale-dependent and comparable to the gravitational energy density of halos
with mass around 10^11 h-1 M\odot . The baryon fraction in halos with mass
equal to or smaller than 10^11 h^-1 M\odot should be substantially lower than
f_b^cosmic. Numerical results show that f_b is decreasing from 0.8 f_b^cosmic
at halo mass scales around 10^12 h^-1 M\odot to 0.3f_b^cosmic at 10^11 h^-1
M\odot and shows further decrease when halo mass is less than 10^11 h^-1
M\odot. The strong mass dependence of f_b is similar to the observed results.
Although the simulated f_b in halos are higher than the observed value by a
factor of 2, the turbulence of the
IGM should be an important dynamical reason leading to the remarkable missing
of baryonic matter in halos with mass \leq 10^12 h^-1 M\odot.Comment: Accepted for publication in MNRAS, 12 pages, 10 figure
Preconditioning harmonic unsteady potential flow calculations
This paper considers finite element discretisations of the Helmholtz equation and its generalisation arising from harmonic acoustics perturbations to a non-uniform steady potential flow. A novel elliptic, positive definite preconditioner, with a multigrid implementation, is used to accelerate the iterative convergence of Krylov subspace solvers. Both theory and numerical results show that for a model 1D Helmholtz test problem the preconditioner clusters the discrete system's eigenvalues and lowers its condition number to a level independent of grid resolution. For the 2D Helmholtz equation, grid independent convergence is achieved using a QMR Krylov solver, significantly outperforming the popular SSOR preconditioner. Impressive results are also presented on more complex domains, including an axisymmetric aircraft engine inlet with non-stagnant mean flow and modal boundary conditions
X-ray Emission of Baryonic Gas in the Universe: Luminosity-Temperature Relationship and Soft-Band Background
We study the X-ray emission of baryon fluid in the universe using the WIGEON
cosmological hydrodynamic simulations. It has been revealed that cosmic baryon
fluid in the nonlinear regime behaves like Burgers turbulence, i.e. the fluid
field consists of shocks. Like turbulence in incompressible fluid, the Burgers
turbulence plays an important role in converting the kinetic energy of the
fluid to thermal energy and heats the gas. We show that the simulation sample
of the CDM model without adding extra heating sources can fit well the
observed distributions of X-ray luminosity versus temperature ( vs.
) of galaxy groups and is also consistent with the distributions of X-ray
luminosity versus velocity dispersion ( vs. ). Because the
baryonic gas is multiphase, the and
distributions are significantly scattered. If we describe the relationships by
power laws and , we find and . The
X-ray background in the soft keV band emitted by the baryonic gas in
the temperature range K has also been calculated. We show that of
the total background, (1) no more than 2% comes from the region with
temperature less than K, and (2) no more than 7% is from the region
of dark matter with mass density . The
region of is generally clustered and
discretely distributed. Therefore, almost all of the soft X-ray background
comes from clustered sources, and the contribution from truly diffuse gas is
probably negligible. This point agrees with current X-ray observations.Comment: 32 pages including 14 figures and 2 tables. Final version for
publication in Ap
A Study of Multigrid Preconditioners Using Eigensystem Analysis
The convergence properties of numerical schemes for partial differential equations are studied by examining the eigensystem of the discrete operator. This method of analysis is very general, and allows the effects of boundary conditions and grid nonuniformities to be examined directly. Algorithms for the Laplace equation and a two equation model hyperbolic system are examined
Scale-dependent statistics of inertial particle distribution in high Reynolds number turbulence
Multiscale statistical analyses of inertial particle distributions are
presented to investigate the statistical signature of clustering and void
regions in particle-laden incompressible isotropic turbulence.
Three-dimensional direct numerical simulations of homogeneous isotropic
turbulence at high Reynolds number () with up to
inertial particles are performed for Stokes numbers ranging from to
. Orthogonal wavelet analysis is then applied to the computed particle
number density fields. Scale-dependent skewness and flatness values of the
particle number density distributions are calculated and the influence of
Reynolds number and Stokes number is assessed. For , both the scale-dependent skewness and flatness values become larger as
the scale decreases, suggesting intermittent clustering at small scales. For
, the flatness at intermediate scales, i.e. for scales larger than
the Kolmogorov scale and smaller than the integral scale of the flow, increases
as increases, and the skewness exhibits negative values at the
intermediate scales. The negative values of the skewness are attributed to void
regions. These results indicate that void regions at the intermediate sales are
pronounced and intermittently distributed for such small Stokes numbers. As
increases, the flatness increases slightly. For , the skewness shows negative values at large scales, suggesting that void
regions are pronounced at large scales, while clusters are pronounced at small
scales.Comment: 26 pages, 9 figure
Geostatistical integration of geophysical, well bore and outcrop data for flow modeling of a deltaic reservoir analogue
Significant world oil and gas reserves occur in deltaic reservoirs. Characterization of deltaic reservoirs requires understanding sedimentary and diagenetic heterogeneity at the submeter scale in three dimensions. However, deltaic facies architecture is complex and poorly understood. Moreover, precipitation of extensive calcite cement during diagenesis can modify the depositional permeability of sandstone reservoir and affect fluid flow. Heterogeneity contributes to trapping a significant portion of mobile oil in deltaic reservoirs analogous of Cretaceous Frontier Formation, Powder River Basin, Wyoming. This dissertation focuses on 3D characterization of an ancient deltaic lobe. The Turonian Wall Creek Member in central Wyoming has been selected for the present study, which integrates outcrop digitized image analysis, 2D and 3D interpreted ground penetrating radar surveys, outcrop gamma ray measurements, well logs, permeameter logs and transects, and other data for 3D reservoir characterization and flow modeling. Well log data are used to predict the geological facies using beta-Bayes method and classic multivariate statistic methods, and predictions are compared with the outcrop description. Geostatistical models are constructed for the size, orientation, and shape of the concretions using interpreted GPR, well, and outcrop data. The spatial continuity of concretions is quantified using photomosaic derived variogram analysis. Relationships among GRP attributes, well data, and outcrop data are investigated, including calcite concretion occurrence and permeability measurements from outcrop. A combination of truncated Gaussian simulation and Bayes rule predicts 3D concretion distributions. Comparisons between 2D flow simulations based on outcrop observations and an ensemble of geostatistical models indicates that the proposed approach can reproduce essential aspects of flow behavior in this system. Experimental design, analysis of variance, and flow simulations examine the effects of geological variability on breakthrough time, sweep efficiency and upscaled permeability. The proposed geostatistical and statistical methods can improve prediction of flow behavior even if conditioning data are sparse and radar data are noisy. The derived geostatistical models of stratigraphy, facies and diagenesis are appropriate for analogous deltaic reservoirs. Furthermore, the results can guide data acquisition, improve performance prediction, and help to upscale models
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Presynaptic Mechanisms of Lead Neurotoxicity: Effects on Vesicular Release, Vesicle Clustering and Mitochondria Number
Childhood lead (Pb2+) intoxication is a global public health problem and accounts for 0.6% of the global burden of disease associated with intellectual disabilities. Despite the recognition that childhood Pb2+ intoxication contributes significantly to intellectual disabilities, there is a fundamental lack of knowledge on presynaptic mechanisms by which Pb2+ disrupts synaptic function. In this study, using a well-characterized rodent model of developmental Pb2+ neurotoxicity, we show that Pb2+ exposure markedly inhibits presynaptic vesicular release in hippocampal Schaffer collateral-CA1 synapses in young adult rats. This effect was associated with ultrastructural changes which revealed a reduction in vesicle number in the readily releasable/docked vesicle pool, disperse vesicle clusters in the resting pool, and a reduced number of presynaptic terminals with multiple mitochondria with no change in presynaptic calcium influx. These studies provide fundamental knowledge on mechanisms by which Pb2+ produces profound inhibition of presynaptic vesicular release that contribute to deficits in synaptic plasticity and intellectual development
Analysis of a finite difference grid
Some means of assessing the suitability of a mesh network for a finite difference calculation are investigated in this study. This has been done by a study of the nonlinear truncation errors of the scheme. It turns out that the mesh can not be properly assessed a priori. The effect of the mesh on the numerical solution depends on several factors including the mesh itself, the numerical algorithm, and the solution. Several recommendations are made with regard to generating the mesh and to assessing its suitability for a particular numerical calculation
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