459 research outputs found

    An implicit method for the calculation of inlet flow fields

    Get PDF
    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

    Full text link
    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

    Get PDF
    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

    Full text link
    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 Λ\LambdaCDM model without adding extra heating sources can fit well the observed distributions of X-ray luminosity versus temperature (LxL_{\rm x} vs. TT) of galaxy groups and is also consistent with the distributions of X-ray luminosity versus velocity dispersion (LxL_{\rm x} vs. σ\sigma). Because the baryonic gas is multiphase, the LxTL_{\rm x}-T and LxσL_{\rm x}-\sigma distributions are significantly scattered. If we describe the relationships by power laws LxTαLTL_{\rm x}\propto T^{\alpha_{LT}} and LxσαLVL_{\rm x}\propto \sigma^{\alpha_{LV}}, we find αLT>2.5\alpha_{LT}>2.5 and αLV>2.1\alpha_{LV}>2.1. The X-ray background in the soft 0.520.5-2 keV band emitted by the baryonic gas in the temperature range 105<T<10710^5<T<10^7 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 106.510^{6.5} K, and (2) no more than 7% is from the region of dark matter with mass density ρdm<50ρˉdm\rho_{\rm dm}<50 \bar{\rho}_{\rm dm}. The region of ρdm>50ρˉdm\rho_{\rm dm}>50\bar{\rho}_{\rm dm} 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

    Get PDF
    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

    Full text link
    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 (Reλ200Re_\lambda \gtrsim 200) with up to 10910^9 inertial particles are performed for Stokes numbers ranging from 0.050.05 to 5.05.0. 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 ReλRe_\lambda and Stokes number StSt is assessed. For St1.0St \sim 1.0, both the scale-dependent skewness and flatness values become larger as the scale decreases, suggesting intermittent clustering at small scales. For St0.2St \le 0.2, 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 StSt 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 ReλRe_\lambda increases, the flatness increases slightly. For Reλ328Re_\lambda \ge 328, 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

    Get PDF
    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

    Analysis of a finite difference grid

    Get PDF
    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

    On parameterized deformations and unsupervised learning

    Get PDF
    corecore