504 research outputs found
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
Recent Enhancements to the Development of CFD-Based Aeroelastic Reduced-Order Models
Recent enhancements to the development of CFD-based unsteady aerodynamic and aeroelastic reduced-order models (ROMs) are presented. These enhancements include the simultaneous application of structural modes as CFD input, static aeroelastic analysis using a ROM, and matched-point solutions using a ROM. The simultaneous application of structural modes as CFD input enables the computation of the unsteady aerodynamic state-space matrices with a single CFD execution, independent of the number of structural modes. The responses obtained from a simultaneous excitation of the CFD-based unsteady aerodynamic system are processed using system identification techniques in order to generate an unsteady aerodynamic state-space ROM. Once the unsteady aerodynamic state-space ROM is generated, a method for computing the static aeroelastic response using this unsteady aerodynamic ROM and a state-space model of the structure, is presented. Finally, a method is presented that enables the computation of matchedpoint solutions using a single ROM that is applicable over a range of dynamic pressures and velocities for a given Mach number. These enhancements represent a significant advancement of unsteady aerodynamic and aeroelastic ROM technology
TW Hya: Spectral Variability, X-Rays, and Accretion Diagnostics
The nearest accreting T Tauri star, TW Hya was observed with spectroscopic
and photometric measurements simultaneous with a long se gmented exposure using
the CHANDRA satellite. Contemporaneous optical photometry from WASP-S indicates
a 4.74 day period was present during this time. Absence of a similar
periodicity in the H-alpha flux and the total X-ray flux points to a different
source of photometric variations. The H-alpha emission line appears
intrinsically broad and symmetric, and both the profile and its variability
suggest an origin in the post-shock cooling region. An accretion event,
signaled by soft X-rays, is traced spectroscopically for the first time through
the optical emission line profiles. After the accretion event, downflowing
turbulent material observed in the H-alpha and H-beta lines is followed by He I
(5876A) broadening. Optical veiling increases with a delay of about 2 hours
after the X-ray accretion event. The response of the stellar coronal emission
to an increase in the veiling follows about 2.4 hours later, giving direct
evidence that the stellar corona is heated in part by accretion. Subsequently,
the stellar wind becomes re-established. We suggest a model that incorporates
this sequential series of events: an accretion shock, a cooling downflow in a
supersonically turbulent region, followed by photospheric and later, coronal
heating. This model naturally explains the presence of broad optical and
ultraviolet lines, and affects the mass accretion rates determined from
emission line profiles.Comment: 61 pages; 22 figures; to appear in The Astrophysical Journa
Circumbinary Gas Accretion onto a Central Binary: Infrared Molecular Hydrogen Emission from GG Tau A
We present high spatial resolution maps of ro-vibrational molecular hydrogen
emission from the environment of the GG Tau A binary component in the GG Tau
quadruple system. The H2 v= 1-0 S(1) emission is spatially resolved and
encompasses the inner binary, with emission detected at locations that should
be dynamically cleared on several hundred-year timescales. Extensions of H2 gas
emission are seen to ~100 AU distances from the central stars. The v = 2-1 S(1)
emission at 2.24 microns is also detected at ~30 AU from the central stars,
with a line ratio of 0.05 +/- 0.01 with respect to the v = 1-0 S(1) emission.
Assuming gas in LTE, this ratio corresponds to an emission environment at ~1700
K. We estimate that this temperature is too high for quiescent gas heated by
X-ray or UV emission from the central stars. Surprisingly, we find that the
brightest region of H2 emission arises from a spatial location that is exactly
coincident with a recently revealed dust "streamer" which seems to be
transferring material from the outer circumbinary ring around GG Tau A into the
inner region. As a result, we identify a new excitation mechanism for
ro-vibrational H2 stimulation in the environment of young stars. The H2 in the
GG Tau A system appears to be stimulated by mass accretion infall as material
in the circumbinary ring accretes onto the system to replenish the inner
circumstellar disks. We postulate that H2 stimulated by accretion infall could
be present in other systems, particularly binaries and "transition disk"
systems which have dust cleared gaps in their circumstellar environments.Comment: 18 pages, including 4 figures. Accepted for publication in Ap
HST/NICMOS Imaging of Disks and Envelopes Around Very Young Stars
We present HST/NICMOS observations with 0.1" (15 AU) resolution of six young
stellar objects in the Taurus star-formation region. The targets of our survey
are three Class I IRAS sources (IRAS 04016+2610, IRAS 04248+2612, and IRAS
04302+2247) and three low-luminosity stars (DG Tau B, Haro 6-5B, and CoKu
Tau/1) associated with Herbig Haro jets. The broad-band images show that the
near-infrared radiation from these sources is dominated by light scattered from
dusty circumstellar material distributed in a region 10 - 15 times the size of
our solar system. Although the detailed morphologies of the individual objects
are unique, the observed young stellar objects share common features. All of
the circumstellar reflection nebulae are crossed by dark lanes from 500 - 900
AU in extent and from less than 50 to 350 AU in apparent thickness. The
absorption lanes extend perpendicular to known optical and millimeter outflows
in these sources. We interpret the dark lanes as optically thick circumstellar
disks seen in silhouette against bright reflection nebulosity. The bipolar
reflection nebulae extending perpendicular to the dust lanes appear to be
produced by scattering from the upper and lower surfaces of the disks and from
dusty material within or on the walls of the outflow cavities. Out of five
objects in which the central source is directly detected, two are found to be
subarcsecond binaries. This mini-survey is the highest resolution near-infrared
study to date of circumstellar environments around solar-type stars with age <=
1 Myr.Comment: 34 pages, 4 figures; also available at
http://spider.ipac.caltech.edu/staff/brandner/topics/disks/disks.html ;
accepted for publication in AJ (March 1999 issue
Star formation history in the SMC: the case of NGC602
Deep HST/ACS photometry of the young cluster NGC 602, located in the remote
low density "wing" of the Small Magellanic Cloud, reveals numerous pre-main
sequence stars as well as young stars on the main sequence. The resolved
stellar content thus provides a basis for studying the star formation history
into recent times and constraining several stellar population properties, such
as the present day mass function, the initial mass function and the binary
fraction. To better characterize the pre-main sequence population, we present a
new set of model stellar evolutionary tracks for this evolutionary phase with
metallicity appropriate for the Small Magellanic Cloud (Z = 0.004). We use a
stellar population synthesis code, which takes into account a full range of
stellar evolution phases to derive our best estimate for the star formation
history in the region by comparing observed and synthetic color-magnitude
diagrams. The derived present day mass function for NGC 602 is consistent with
that resulting from the synthetic diagrams. The star formation rate in the
region has increased with time on a scale of tens of Myr, reaching in the last 2.5 Myr, comparable to what is
found in Galactic OB associations. Star formation is most complete in the main
cluster but continues at moderate levels in the gas-rich periphery of the
nebula.Comment: 24 pages. Accepted for publication in A
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