4,736 research outputs found
Wind tunnel supplementary Mach number minimum section insert
A device is described which changes the Mach number capability of a wind tunnel without permanently altering the existing nozzle of the tunnel. An insert is removably attached to the wall of the existing nozzle expansion area thereby creating a second minimum section upstream of the model test section. The added insert may be removed without complicated and expensive changes to the basic wind tunnel. In one embodiment, a removable insert is disposed within wind tunnel nozzle walls with a portion of the flow boundary layer being bled off from the tunnel via passageway and tunnel exit to reduce the extent of separated flow normally occuring upstream of the insert contraction section
The Role of Turbulence in Neutrino-Driven Core-Collapse Supernova Explosions
The neutrino-heated "gain layer" immediately behind the stalled shock in a
core-collapse supernova is unstable to high-Reynolds-number turbulent
convection. We carry out and analyze a new set of 19 high-resolution
three-dimensional (3D) simulations with a three-species neutrino
leakage/heating scheme and compare with spherically-symmetric (1D) and
axisymmetric (2D) simulations carried out with the same methods. We study the
postbounce supernova evolution in a - progenitor star and vary the
local neutrino heating rate, the magnitude and spatial dependence of
asphericity from convective burning in the Si/O shell, and spatial resolution.
Our simulations suggest that there is a direct correlation between the strength
of turbulence in the gain layer and the susceptability to explosion. 2D and 3D
simulations explode at much lower neutrino heating rates than 1D simulations.
This is commonly explained by the fact that nonradial dynamics allows accreting
material to stay longer in the gain layer. We show that this explanation is
incomplete. Our results indicate that the effective turbulent ram pressure
exerted on the shock plays a crucial role by allowing multi-D models to explode
at a lower postshock thermal pressure and thus with less neutrino heating than
1D models. We connect the turbulent ram pressure with turbulent energy at large
scales and in this way explain why 2D simulations are erroneously exploding
more easily than 3D simulations.Comment: 13 pages, 8 figures, accepted by Ap
The Otto-engine-equivalent vehicle concept
A vehicle comparison methodology based on the Otto-Engine Equivalent (OEE) vehicle concept is described. As an illustration of this methodology, the concept is used to make projections of the fuel economy potential of passenger cars using various alternative power systems. Sensitivities of OEE vehicle results to assumptions made in the calculational procedure are discussed. Factors considered include engine torque boundary, rear axle ratio, performance criteria, engine transient response, and transmission shift logic
Aspherical Core-Collapse Supernovae in Red Supergiants Powered by Nonrelativistic Jets
We explore the observational characteristics of jet-driven supernovae by
simulating bipolar-jet-driven explosions in a red supergiant progenitor. We
present results of four models in which we hold the injected kinetic energy at
a constant ergs across all jet models but vary the specific
characteristics of the jets to explore the influence of the nature of jets on
the structure of the supernova ejecta. We evolve the explosions past
shock-breakout and into quasi-homologous expansion of the supernova envelope
into a red supergiant wind. The oppositely-directed, nickel-rich jets give a
large-scale asymmetry that may account for the non-spherical excitation and
substructure of spectral lines such as H and He I 10830\AA. Jets with a
large fraction of kinetic to thermal energy punch through the progenitor
envelope and give rise to explosions that would be observed to be asymmetric
from the earliest epochs, inconsistent with spectropolarimetric measurements of
Type II supernovae. Jets with higher thermal energy fractions result in
explosions that are roughly spherical at large radii but are significantly
elongated at smaller radii, deep inside the ejecta, in agreement with the
polarimetric observations. We present shock breakout light curves that indicate
that strongly aspherical shock breakouts are incompatible with recent {\it
GALEX} observations of shock breakout from red supergiant stars. Comparison
with observations indicates that jets must deposit their kinetic energy
efficiently throughout the ejecta while in the hydrogen envelope. Thermal
energy-dominated jets satisfy this criterion and yield many of the
observational characteristics of Type II supernovae.Comment: 21 pages, 19 figures, submitted to ApJ on 4 Nov 200
Implicit large eddy simulations of anisotropic weakly compressible turbulence with application to core-collapse supernovae
(Abridged) In the implicit large eddy simulation (ILES) paradigm, the
dissipative nature of high-resolution shock-capturing schemes is exploited to
provide an implicit model of turbulence. Recent 3D simulations suggest that
turbulence might play a crucial role in core-collapse supernova explosions,
however the fidelity with which turbulence is simulated in these studies is
unclear. Especially considering that the accuracy of ILES for the regime of
interest in CCSN, weakly compressible and strongly anisotropic, has not been
systematically assessed before. In this paper we assess the accuracy of ILES
using numerical methods most commonly employed in computational astrophysics by
means of a number of local simulations of driven, weakly compressible,
anisotropic turbulence. We report a detailed analysis of the way in which the
turbulent cascade is influenced by the numerics. Our results suggest that
anisotropy and compressibility in CCSN turbulence have little effect on the
turbulent kinetic energy spectrum and a Kolmogorov scaling is
obtained in the inertial range. We find that, on the one hand, the kinetic
energy dissipation rate at large scales is correctly captured even at
relatively low resolutions, suggesting that very high effective Reynolds number
can be achieved at the largest scales of the simulation. On the other hand, the
dynamics at intermediate scales appears to be completely dominated by the
so-called bottleneck effect, \ie the pile up of kinetic energy close to the
dissipation range due to the partial suppression of the energy cascade by
numerical viscosity. An inertial range is not recovered until the point where
relatively high resolution , which would be difficult to realize in
global simulations, is reached. We discuss the consequences for CCSN
simulations.Comment: 17 pages, 9 figures, matches published versio
Multidimensional Simulations of Rotating Pair Instability Supernovae
We study the effects of rotation on the dynamics, energetics and Ni-56
production of Pair Instability Supernova explosions by performing rotating
two-dimensional ("2.5-D") hydrodynamics simulations. We calculate the evolution
of eight low metallicity (Z = 10^-3, 10^-4 Zsun) massive (135-245 Msun) PISN
progenitors with initial surface rotational velocities 50% that of the critical
Keplerian value using the stellar evolution code MESA. We allow for both the
inclusion and the omission of the effects of magnetic fields in the angular
momentum transport and in chemical mixing, resulting in slowly-rotating and
rapidly-rotating final carbon-oxygen cores, respectively. Increased rotation
for carbon-oxygen cores of the same mass and chemical stratification leads to
less energetic PISN explosions that produce smaller amounts of Ni-56 due to the
effect of the angular momentum barrier that develops and slows the dynamical
collapse. We find a non-monotonic dependence of Ni-56 production on rotational
velocity in situations when smoother composition gradients form at the outer
edge of the rotating cores. In these cases, the PISN energetics are determined
by the competition of two factors: the extent of chemical mixing in the outer
layers of the core due to the effects of rotation in the progenitor evolution
and the development of angular momentum support against collapse. Our 2.5-D
PISN simulations with rotation are the first presented in the literature. They
reveal hydrodynamic instabilities in several regions of the exploding star and
increased explosion asymmetries with higher core rotational velocity.Comment: 31 pages, 23 figures, accepted for publication in the Ap
Effect of blockage ratio on drag and pressure distributions for bodies of revolution at transonic speeds
Experimental data were obtained in two wind tunnels for 13 models over a Mach number range from 0.70 to 1.02. Effects of increasing test-section blockage ratio in the transonic region near a Mach number of 1.0 included change in the shape of the drag curves, premature drag creep, delayed drag divergence, and a positive increment of pressures on the model afterbodies. Effects of wall interference were apparent in the data even for a change in blockage ratio from a very low 0.000343 to an even lower 0.000170. Therefore, models having values of blockage ratio of 0.0003 - an order of magnitude below the previously considered safe value of 0.0050 - had significant errors in the drag-coefficient values obtained at speeds near a Mach number of 1.0. Furthermore, the flow relief afforded by slots or perforations in test-section walls - designed according to previously accepted criteria for interference-free subsonic flow - does not appear to be sufficient to avoid significant interference of the walls with the model flow field for Mach numbers very close to 1.0
Exploratory wind tunnel tests of a shock-swallowing air data sensor at a Mach number of approximately 1.83
The test probe was designed to measure free-stream Mach number and could be incorporated into a conventional airspeed nose boom installation. Tests were conducted in the Langley 4-by 4-foot supersonic pressure tunnel with an approximate angle of attack test range of -5 deg to 15 deg and an approximate angle of sideslip test range of + or - 4 deg. The probe incorporated a variable exit area which permitted internal flow. The internal flow caused the bow shock to be swallowed. Mach number was determined with a small axially movable internal total pressure tube and a series of fixed internal static pressure orifices. Mach number error was at a minimum when the total pressure tube was close to the probe tip. For four of the five tips tested, the Mach number error derived by averaging two static pressures measured at horizontally opposed positions near the probe entrance were least sensitive to angle of attack changes. The same orifices were also used to derive parameters that gave indications of flow direction
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