850 research outputs found
Low-disturbance wind tunnels
During the past years, there was an extensive program under way at the Langley Research Center to upgrade the flow quality in several of the large wind tunnels. This effort has resulted in significant improvements in flow quality in these tunnels and has also increased the understanding of how and where changes in existing and new wind tunnels are most likely to yield the desired improvements. As part of this ongoing program, flow disturbance levels and spectra were measured in several Langley tunnels before and after modifications were made to reduce acoustic and vorticity fluctuations. A brief description of these disturbance control features is given for the Low-Turbulence Pressure Tunnel, the 4 x 7 Meter Tunnel, and the 8 Foot Transonic Pressure Tunnel. To illustrate typical reductions in disturbance levels obtained in these tunnels, data from hot-wire or acoustic sensors are presented. A concept for a subsonic quiet tunnel designed to study boundary layer stability and transition is also presented. Techniques developed at Langley in recent years to eliminate the high intensity and high-frequency acoustic disturbances present in all previous supersonic wind tunnels are described. In conclusion, the low-disturbance levels present in atmospheric flight can now be simulated in wind tunnels over the speed range from low subsonic through high supersonic
Design and preliminary test results at Mach 5 of an axisymmetric slotted sound shield
The basic theory and sound attenuation mechanisms, the design procedures, and preliminary experimental results are presented for a small axisymmetric sound shield for supersonic wind tunnels. The shield consists of an array of small diameter rods aligned nearly parallel to the entrance flow with small gaps between the rods for boundary layer suction. Results show that at the lowest test Reynolds number (based on rod diameter) of 52,000 the noise shield reduced the test section noise by about 60 percent ( or 8 db attenuation) but no attenuation was measured for the higher range of test reynolds numbers from 73,000 to 190,000. These results are below expectations based on data reported elsewhere on a flat sound shield model. The smaller attenuation from the present tests is attributed to insufficient suction at the gaps to prevent feedback of vacuum manifold noise into the shielded test flow and to insufficient suction to prevent transition of the rod boundary layers to turbulent flow at the higher Reynolds numbers. Schlieren photographs of the flow are shown
Primary and secondary eclipse spectroscopy with JWST: exploring the exoplanet parameter space
Eclipse exoplanet spectroscopy has yielded detection of H_2O, CH_4, CO_2 and
CO in the atmosphere of hot jupiters and neptunes. About 40 large terrestrial
planets are announced or confirmed, two of which are transiting, and another
deemed habitable. Hence the potential for eclipse spectroscopy of terrestrial
planets with James Webb Space Telescope (JWST) has become an active field of
study. We explore the parameter space (type of stars, planet orbital periods
and types, and instruments/wavelengths) in terms of the signal-to-noise ratio
(S/N) achievable on the detection of spectroscopic features. We use analytic
formula and model data for both the astrophysical scene and the instrument, to
plot S/N contour maps, while indicating how the S/N scales with the fixed
parameters. We systematically compare stellar photon noise-only figures with
ones including detailed instrumental and zodiacal noises. Likelihood of
occurring targets is based both on model and catalog star population of the
solar neighborhood. The 9.6 micron ozone band is detectable (S/N = 3) with
JWST, for a warm super-earth 6.7 pc away, using ~2% of the 5-year nominal
mission time (summing observations, M4V and lighter host star for primary
eclipses, M5V for secondary). If every star up to this mass limit and distance
were to host a habitable planet, there should be statistically ~1 eclipsing
case. Investigation of systematic noises in the co-addition of 5 years worth-,
tens of days separated-, hours-long observations is critical, complemented by
dedicated characterisation of the instruments, currently in integration phase.
The census of nearby transiting habitable planets must be complete before the
beginning of science operations.Comment: Accepted for publication in A&A, 16 pages, 19 figure
Critical Protoplanetary Core Masses in Protoplanetary Disks and the Formation of Short-Period Giant Planets
We study a solid protoplanetary core of 1-10 earth masses migrating through a
disk. We suppose the core luminosity is generated as a result of planetesimal
accretion and calculate the structure of the gaseous envelope assuming
equilibrium. This is a good approximation when the core mass is less than the
critical value, M_{crit}, above which rapid gas accretion begins. We model the
structure of the protoplanetary nebula as an accretion disk with constant
\alpha. We present analytic fits for the steady state relation between disk
surface density and mass accretion rate as a function of radius r. We calculate
M_{crit} as a function of r, gas accretion rate through the disk, and
planetesimal accretion rate onto the core \dot{M}. For a fixed \dot{M},
M_{crit} increases inwards, and it decreases with \dot{M}. We find that \dot{M}
onto cores migrating inwards in a time 10^3-10^5 yr at 1 AU is sufficient to
prevent the attainment of M_{crit} during the migration process. Only at small
radii where planetesimals no longer exist can M_{crit} be attained. At small
radii, the runaway gas accretion phase may become longer than the disk lifetime
if the core mass is too small. However, massive cores can be built-up through
the merger of additional incoming cores on a timescale shorter than for in situ
formation. Therefore, feeding zone depletion in the neighborhood of a fixed
orbit may be avoided. Accordingly, we suggest that giant planets may begin to
form early in the life of the protostellar disk at small radii, on a timescale
that may be significantly shorter than for in situ formation. (abridged)Comment: 24 pages (including 9 figures), LaTeX, uses emulateapj.sty, to be
published in ApJ, also available at http://www.ucolick.org/~ct/home.htm
High-resolution study of a star-forming cluster in the Cep-A HW2 region
Due to its relatively small distance (725 pc), the Cepheus A East
star-forming region is an ideal laboratory to study massive star formation
processes. Based on its morphology, it has been suggested that the flattened
molecular gas distribution around the YSO HW2 may be a 350-AU-radius massive
protostellar disk. Goal of our work is to ascertain the nature of this
structure. We have employed the Plateau de Bure Interferometer to acquire
(sub-)arcsecond-resolution imaging of high-density and shock tracers, such as
methyl cyanide (CH3CN) and silicon monoxide (SiO), towards the HW2 position. On
the 1-arcsecond (about 725 AU) scale, the flattened distribution of molecular
gas around HW2 appears to be due to the projected superposition, on the plane
of the sky, of at least three protostellar objects, of which at least one is
powering a molecular outflow at a small angle with respect to the line of
sight. The presence of a protostellar disk around HW2 is not ruled out, but
such structure is likely to be detected on a smaller spatial scale, or using
different molecular tracers.Comment: 6 pages, 5 figures, accepted for publication in Astronomy &
Astrophysic
Formation and structure of the three Neptune-mass planets system around HD69830
Since the discovery of the first giant planet outside the solar system in
1995 (Mayor & Queloz 1995), more than 180 extrasolar planets have been
discovered. With improving detection capabilities, a new class of planets with
masses 5-20 times larger than the Earth, at close distance from their parent
star is rapidly emerging. Recently, the first system of three Neptune-mass
planets has been discovered around the solar type star HD69830 (Lovis et al.
2006). Here, we present and discuss a possible formation scenario for this
planetary system based on a consistent coupling between the extended core
accretion model and evolutionary models (Alibert et al. 2005a, Baraffe et al.
2004,2006). We show that the innermost planet formed from an embryo having
started inside the iceline is composed essentially of a rocky core surrounded
by a tiny gaseous envelope. The two outermost planets started their formation
beyond the iceline and, as a consequence, accrete a substantial amount of water
ice during their formation. We calculate the present day thermodynamical
conditions inside these two latter planets and show that they are made of a
rocky core surrounded by a shell of fluid water and a gaseous envelope.Comment: Accepted in AA Letter
Evolution and Impact of Bars over the Last Eight Billion Years: Early Results from GEMS
Bars drive the dynamical evolution of disk galaxies by redistributing mass
and angular momentum, and they are ubiquitous in present-day spirals. Early
studies of the Hubble Deep Field reported a dramatic decline in the rest-frame
optical bar fraction f_opt to below 5% at redshifts z>0.7, implying that disks
at these epochs are fundamentally different from present-day spirals. The GEMS
bar project, based on ~8300 galaxies with HST-based morphologies and accurate
redshifts over the range 0.2-1.1, aims at constraining the evolution and impact
of bars over the last 8 Gyr. We present early results indicating that f_opt
remains nearly constant at ~30% over the range z=0.2-1.1,corresponding to
lookback times of ~2.5-8 Gyr. The bars detected at z>0.6 are primarily strong
with ellipticities of 0.4-0.8. Remarkably, the bar fraction and range of bar
sizes observed at z>0.6 appear to be comparable to the values measured in the
local Universe for bars of corresponding strengths. Implications for bar
evolution models are discussed.Comment: Submitted June 25, 2004. 10 pages 5 figures. To appear in Penetrating
Bars through Masks of Cosmic Dust: The Hubble Tuning Fork Strikes a New Note,
eds. D. Block, K. Freeman, R. Groess, I. Puerari, & E.K. Block (Dordrecht:
Kluwer), in pres
Recommended from our members
Structure refinement upon ultrasonic melt treatment in a DC-casting launder
2020 The Author(s). This work focuses on ultrasonic melt treatment (UST) in a launder upon pilotscale direct chill (DC) casting of 152-mm-diameter billets from an AA6XXX alloy with Zr addition. Two casting temperatures (650 C and 665 C) were used to assess their effect on the resulting microstructure (grain size, particle size, and number density). Structure refinement results show the feasibility of UST in the DC casting launder. This is quantified through the corresponding reduction of grain size by around 50% in the billet center, or more towards the billet surface, reduction of the average Al3Zr particle size, and increase in the particle number density. A higher Al3Zr particle density was obtained when the alloy was cast at 665 C. Numerical simulation results and suggestions on how to improve the treatment quality of UST in DC casting launder are also provided.EPSRC (UK) projects UltraMelt2 (EP/R011001/1, EP/R011044/1,and EP/R011095/1)
Multiphysics Modelling of Ultrasonic Melt Treatment in the Hot-Top and Launder during Direct-Chill Casting: Path to Indirect Microstructure Simulation
Copyright: © 2021 by the authors. This study concerns the numerical simulation of two competing ultrasonic treatment (UST) strategies for microstructure refinement in the direct-chill (DC) casting of aluminium alloys. In the first, more conventional, case, the sonotrode vibrating at 17.3 kHz is immersed in the hop-top to treat the sump melt pool, in the second case, the sonotrode is inserted between baffles in the launder. It is known that microstructure refinement depends on the intensity of acoustic cavitation and the residence time of the treated fluid in the cavitation zone. The geometry, acoustic field intensity, induced flow velocities, and local temperature are factors which affect this treatment. The mathematical model developed in this work couples flow velocity, acoustics modified by cavitation, heat transfer, and solidification at the macroscale, with Lagrangian refiner particles, used to determine: (a) their residence time in the active zones, and (b) their eventual distribution in the sump as a function of the velocity field. This is the first attempt at using particle models as an efficient, though indirect, alternative to microstructure simulation, and the results indicate that UST in the launder, assisted with baffle separators, yields a more uniform distribution of refining particles, avoiding the strong acoustic streaming jet that, otherwise, accompanies hot-top treatment, and may lead to the strong segregation of refining particles. Experiments conducted in parallel to the numerical studies in this work appeared to support the results obtained in the simulation.EPSRC (UK), under the UltraMelt 2 project, grant numbers EP/R011001/1, EP/R011044/1, and EP/R011095/1
The c2d Spitzer Spectroscopic Survey of Ices Around Low-Mass Young Stellar Objects. IV. NH3 and CH3OH
NH3 and CH3OH are key molecules in astrochemical networks leading to the
formation of more complex N- and O-bearing molecules, such as CH3CN and
HCOOCH3. Despite a number of recent studies, little is known about their
abundances in the solid state. (...) In this work, we investigate the ~ 8-10
micron region in the Spitzer IRS (InfraRed Spectrograph) spectra of 41 low-mass
young stellar objects (YSOs). These data are part of a survey of interstellar
ices in a sample of low-mass YSOs studied in earlier papers in this series. We
used both an empirical and a local continuum method to correct for the
contribution from the 10 micron silicate absorption in the recorded spectra. In
addition, we conducted a systematic laboratory study of NH3- and
CH3OH-containing ices to help interpret the astronomical spectra. We clearly
detect a feature at ~9 micron in 24 low-mass YSOs. Within the uncertainty in
continuum determination, we identify this feature with the NH3 nu_2 umbrella
mode, and derive abundances with respect to water between ~2 and 15%.
Simultaneously, we also revisited the case of CH3OH ice by studying the nu_4
C-O stretch mode of this molecule at ~9.7 micron in 16 objects, yielding
abundances consistent with those derived by Boogert et al. 2008 (hereafter
paper I) based on a simultaneous 9.75 and 3.53 micron data analysis. Our study
indicates that NH3 is present primarily in H2O-rich ices, but that in some
cases, such ices are insufficient to explain the observed narrow FWHM. The
laboratory data point to CH3OH being in an almost pure methanol ice, or mixed
mainly with CO or CO2, consistent with its formation through hydrogenation on
grains. Finally, we use our derived NH3 abundances in combination with
previously published abundances of other solid N-bearing species to find that
up to 10-20 % of nitrogen is locked up in known ices.Comment: 31 pages, 15 figures, accepted for publication in Ap
- …