636 research outputs found
A Multi-Epoch HST Study of the Herbig-Haro Flow from XZ Tauri
We present nine epochs of Hubble Space Telescope optical imaging of the
bipolar outflow from the pre-main sequence binary XZ Tauri. Our data monitors
the system from 1995-2005 and includes emission line images of the flow. The
northern lobe appears to be a succession of bubbles, the outermost of which
expanded ballistically from 1995-1999 but in 2000 began to deform and
decelerate along its forward edge. It reached an extent of 6" from the binary
in 2005. A larger and fainter southern counterbubble was detected for the first
time in deep ACS images from 2004. Traces of shocked emission are seen as far
as 20" south of the binary. The bubble emission nebulosity has a low excitation
overall, as traced by the [S II]/H-alpha line ratio, requiring a nearly
comoving surrounding medium that has been accelerated by previous ejections or
stellar winds.
Within the broad bubbles there are compact emission knots whose alignments
and proper motions indicate that collimated jets are ejected from each binary
component. The jet from the southern component, XZ Tau A, is aligned with the
outflow axis of the bubbles and has tangential knot velocities of 70-200 km/s.
Knots in the northern flow are seen to slow and brighten as they approach the
forward edge of the outermost bubble. The knots in the jet from the other star,
XZ Tau B, have lower velocities of ~100 km/s
An embedded grid formulation applied to delta wings
An embedded grid algorithm for the Euler and/or Navier-Stokes equations is developed and applied to delta wings at high angles of attack in low speed flow. The Navier-Stokes code is an implicit, finite volume algorithm, using flux difference splitting for the convective and pressure terms and central differencing for the viscous and heat transfer terms. Calculations are compared with detailed experimental results over an angle of attack range up to and beyond the maximum lift coefficient, corresponding to vortex breakdown at the trailing edge, for a delta wing nominally of unit aspect ratio. The results indicate that the overall flowfield, including surface pressures, surface streamlines, and vortex trajectories, can be simulated accurately with the global grid version of the present algorithm. However, comparison of computed velocities and vorticity with experimentally measured off-body values at an angle of attack of 20.5 deg indicates the core region is substantially more diffuse in the computations than that measured with either a five-hole probe or a laser velocimeter. Embedded grids, used to improve the numerical discretization in the core region, are formulated within the framework of the implicit, upwind-biased multi-grid algorithm. Structured levels of local nested refinements are made. Three-dimensional results for both Euler and Navier-Stokes calculations are shown, with up to 3 levels of embedded refinement. The embedding procedure was effective in eliminating a crossflow secondary separation produced in the Euler solutions on coarse grids
Turbulence Model Implementation and Verification in the SENSEI CFD Code
This paper outlines the implementation and verification of the negative Spalart-Allmaras turbulence model into the SENSEI CFD code. The SA-neg turbulence model is implemented in a flexible, object-oriented framework where additional turbulence models can be easily added. In addition to outlining the new turbulence modeling framework in SENSEI, an overview of the other general improvements to SENSEI is provided. The results for four 2D test cases are compared to results from CFL3D and FUN3D to verify that the turbulence models are implemented properly. Several differences in the results from SENSEI, CFL3D, and FUN3D are identified and are attributed to differences in the implementation and discretization order of the boundary conditions as well as the order of discretization of the turbulence model. When a solid surface is located near or intersects an inflow or outflow boundary, higher order boundary conditions should be used to limit their effect on the forces on the surface. When the turbulence equations are discretized using second order spatial accuracy, the edge of the eddy viscosity profile seems to be sharper than when a first order discretization is used. However, the discretization order of the turbulence equation does not have a significant impact on output quantities of interest, such as pressure and viscous drag, for the cases studied
CFL3D User's Manual (Version 5.0)
This document is the User's Manual for the CFL3D computer code, a thin-layer Reynolds-averaged Navier-Stokes flow solver for structured multiple-zone grids. Descriptions of the code's input parameters, non-dimensionalizations, file formats, boundary conditions, and equations are included. Sample 2-D and 3-D test cases are also described, and many helpful hints for using the code are provided
HST and Spitzer Observations of the HD 207129 Debris Ring
A debris ring around the star HD 207129 (G0V; d = 16.0 pc) has been imaged in
scattered visible light with the ACS coronagraph on the Hubble Space Telescope
and in thermal emission using MIPS on the Spitzer Space Telescope at 70 microns
(resolved) and 160 microns (unresolved). Spitzer IRS (7-35 microns) and MIPS
(55-90 microns) spectrographs measured disk emission at >28 microns. In the HST
image the disk appears as a ~30 AU wide ring with a mean radius of ~163 AU and
is inclined by 60 degrees from pole-on. At 70 microns it appears partially
resolved and is elongated in the same direction and with nearly the same size
as seen with HST in scattered light. At 0.6 microns the ring shows no
significant brightness asymmetry, implying little or no forward scattering by
its constituent dust. With a mean surface brightness of V=23.7 mag per square
arcsec, it is the faintest disk imaged to date in scattered light.Comment: 28 pages, 8 figure
Measuring Gravitational Lensing Flexions in Abell 1689 Using an Analytic Image Model
Measuring dark matter substructure within galaxy cluster haloes is a
fundamental probe of the Lambda-CDM model of structure formation. Gravitational
lensing is a technique for measuring the total mass distribution which is
independent of the nature of the gravitating matter, making it a vital tool for
studying these dark-matter dominated objects. We present a new method for
measuring weak gravitational lensing flexions, the gradients of the lensing
shear field, to measure mass distributions on small angular scales. While
previously published methods for measuring flexions focus on measuring derived
properties of the lensed images, such as shapelet coefficients or surface
brightness moments, our method instead fits a
mass-sheet-transformation-invariant Analytic Image Model (AIM) to the each
galaxy image. This simple parametric model traces the distortion of lensed
image isophotes and constrains the flexion fields. We test the AIM method using
simulated data images with realistic noise and a variety of unlensed image
properties, and show that it successfully reproduces the input flexion fields.
We also apply the AIM method for flexion measurement to Hubble Space Telescope
observations of Abell 1689, and detect mass structure in the cluster using
flexions measured with the AIM method.Comment: 44 pages, 4 figures, 3 tables. Accepted to ApJ. V2 (published
version) has minor changes from V1; ApJ 736 (2011
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