266 research outputs found
Reduction of Altitude Diffuser Jet Noise Using Water Injection
A feasibility study on the effects of injecting water into the exhaust plume of an altitude rocket diffuser for the purpose of reducing the far-field acoustic noise has been performed. Water injection design parameters such as axial placement, angle of injection, diameter of injectors, and mass flow rate of water have been systematically varied during the operation of a subscale altitude test facility. The changes in acoustic far-field noise were measured with an array of free-field microphones in order to quantify the effects of the water injection on overall sound pressure level spectra and directivity. The results showed significant reductions in noise levels were possible with optimum conditions corresponding to water injection at or just upstream of the exit plane of the diffuser. Increasing the angle and mass flow rate of water injection also showed improvements in noise reduction. However, a limit on the maximum water flow rate existed as too large of flow rate could result in un-starting the supersonic diffuser
Reduction of Altitude Diffuser Jet Noise Using Water Injection
A feasibility study on the effects of injecting water into the exhaust plume of an altitude rocket diffuser for the purpose of reducing the far-field acoustic noise has been performed. Water injection design parameters such as axial placement, angle of injection, diameter of injectors, and mass flow rate of water have been systematically varied during the operation of a subscale altitude test facility. The changes in acoustic far-field noise were measured with an array of free-field microphones in order to quantify the effects of the water injection on overall sound pressure level spectra and directivity. The results showed significant reductions in noise levels were possible with optimum conditions corresponding to water injection at or just upstream of the exit plane of the diffuser. Increasing the angle and mass flow rate of water injection also showed improvements in noise reduction. However, a limit on the maximum water flow rate existed as too large of flow rate could result in un-starting the supersonic diffuser
Computational Analyses in Support of Sub-scale Diffuser Testing for the A-3 Facility
A unique assessment of acoustic similarity scaling laws and acoustic analogy methodologies in predicting the far-field acoustic signature from a sub-scale altitude rocket test facility at the NASA Stennis Space Center was performed. A directional, point-source similarity analysis was implemented for predicting the acoustic far-field. In this approach, experimental acoustic data obtained from "similar" rocket engine tests were appropriately scaled using key geometric and dynamic parameters. The accuracy of this engineering-level method is discussed by comparing the predictions with acoustic far-field measurements obtained. In addition, a CFD solver was coupled with a Lilley's acoustic analogy formulation to determine the improvement of using a physics-based methodology over an experimental correlation approach. In the current work, steady-state Reynolds-averaged Navier-Stokes calculations were used to model the internal flow of the rocket engine and altitude diffuser. These internal flow simulations provided the necessary realistic input conditions for external plume simulations. The CFD plume simulations were then used to provide the spatial turbulent noise source distributions in the acoustic analogy calculations. Preliminary findings of these studies will be discussed
A 14-Year Empirical Analysis of Undergraduates’ Pre- and Post-Test Scores in Three Introductory Communication Courses: Lessons Learned for Pedagogy and Assessment
Conducting long-term assessment of the impact of students’ participation in introductory communication courses is an important endeavor for enhancing pedagogy and understanding the contribution of communication instruction to the student experience. This 14-year study reports data from a campus-wide assessment program extending from 2004 to 2018. The study analyzed a large sample of undergraduate students’ self-reported pre- and post-test scores on critical variables related to student outcomes in three introductory communication courses. The variables examined were demographic characteristics, self-esteem and communication apprehension in both the public speaking course and the business communication course, and self-esteem and willingness to communicate in the interpersonal communication course. Across the 14-year period, 93% of the results of pre/post comparison scores showed a significant increase in students’ self-perceived levels of self-esteem and willingness to communicate and a significant decrease in communication apprehension (p \u3c .001). The usefulness of the results are discussed. Four general conclusions for engaging in introductory course assessment are outlined, along with specific lessons learned and best practices for consideration by basic course directors and faculty
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A Coherent FMCW LIDAR Mapping System for Automated Tissue Debridment
The Oak Ridge National Laboratory (ORNL) is developing a prototype 850-nm FMCW lidar system for mapping tissue damage in burn cases for the US Army Medical Research and Material Command. The laser system will provide a 3D-image map of the burn and surrounding area and provide tissue damage assessment
Estimate of dark halo ellipticity by lensing flexion
Aims. The predictions of the ellipticity of the dark matter halos from models
of structure formation are notoriously difficult to test with observations. A
direct measurement would give important constraints on the formation of
galaxies, and its effect on the dark matter distribution in their halos. Here
we show that galaxy-galaxy flexion provides a direct and potentially powerful
method for determining the ellipticity of (an ensemble of) elliptical lenses.
Methods. We decompose the spin-1 flexion into a radial and a tangential
component. Using the ratio of tangential-to- radial flexion, which is
independent of the radial mass profile, the mass ellipticity can be estimated.
Results. An estimator for the ellipticity of the mass distribution is derived
and tested with simulations. We show that the estimator is slightly biased. We
quantify this bias, and provide a method to reduce it. Furthermore, a
parametric fitting of the flexion ratio and orientation provides another
estimate for the dark halo ellipticity, which is more accurate for individual
lenses Overall, galaxy-galaxy flexion appears as a powerful tool for
constraining the ellipticity of mass distributions.Comment: 6 pages,5 figures, submitted to AA, comments welcom
Quantifying the heart of darkness with GHALO - a multi-billion particle simulation of our galactic halo
We perform a series of simulations of a Galactic mass dark matter halo at
different resolutions, our largest uses over three billion particles and has a
mass resolution of 1000 M_sun. We quantify the structural properties of the
inner dark matter distribution and study how they depend on numerical
resolution. We can measure the density profile to a distance of 120 pc (0.05%
of R_vir) where the logarithmic slope is -0.8 and -1.4 at (0.5% of R_vir). We
propose a new two parameter fitting function that has a linearly varying
logarithmic density gradient which fits the GHALO and VL2 density profiles
extremely well. Convergence in the density profile and the halo shape scales as
N^(-1/3), but the shape converges at a radius three times larger at which point
the halo becomes more spherical due to numerical resolution. The six
dimensional phase-space profile is dominated by the presence of the
substructures and does not follow a power law, except in the smooth
under-resolved inner few kpc.Comment: 6 pages, 4 figures, submitted to MNRAS Letters, for full sized
images, see http://www.itp.uzh.ch/news.htm
Intrinsic galaxy shapes and alignments I: Measuring and modelling COSMOS intrinsic galaxy ellipticities
The statistical properties of the ellipticities of galaxy images depend on
how galaxies form and evolve, and therefore constrain models of galaxy
morphology, which are key to the removal of the intrinsic alignment
contamination of cosmological weak lensing surveys, as well as to the
calibration of weak lensing shape measurements. We construct such models based
on the halo properties of the Millennium Simulation and confront them with a
sample of 90,000 galaxies from the COSMOS Survey, covering three decades in
luminosity and redshifts out to z=2. The ellipticity measurements are corrected
for effects of point spread function smearing, spurious image distortions, and
measurement noise. Dividing galaxies into early, late, and irregular types, we
find that early-type galaxies have up to a factor of two lower intrinsic
ellipticity dispersion than late-type galaxies. None of the samples shows
evidence for redshift evolution, while the ellipticity dispersion for late-type
galaxies scales strongly with absolute magnitude at the bright end. The
simulation-based models reproduce the main characteristics of the intrinsic
ellipticity distributions although which model fares best depends on the
selection criteria of the galaxy sample. We observe fewer close-to-circular
late-type galaxy images in COSMOS than expected for a sample of randomly
oriented circular thick disks and discuss possible explanations for this
deficit.Comment: 18 pages, 8 figures; updated simulations and galaxy sample
definition, more galaxy samples analysed; matches version published in MNRA
The Halo Shape and Evolution of Polar Disc Galaxies
We examine the properties and evolution of a simulated polar disc galaxy.
This galaxy is comprised of two orthogonal discs, one of which contains old
stars (old stellar disc), and the other, containing both younger stars and the
cold gas (polar disc) of the galaxy. By exploring the shape of the inner region
of the dark matter halo, we are able to confirm that the halo shape is a oblate
ellipsoid flattened in the direction of the polar disc. We also note that there
is a twist in the shape profile, where the innermost 3 kpc of the halo flattens
in the direction perpendicular to the old disc, and then aligns with the polar
disc out until the virial radius. This result is then compared to the halo
shape inferred from the circular velocities of the two discs. We also use the
temporal information of the simulation to track the system's evolution, and
identify the processes which give rise to this unusual galaxy type. We confirm
the proposal that the polar disc galaxy is the result of the last major merger,
where the angular moment of the interaction is orthogonal to the angle of the
infalling gas. This merger is followed by the resumption of coherent gas
infall. We emphasise that the disc is rapidly restored after the major merger
and that after this event the galaxy begins to tilt. A significant proportion
of the infalling gas comes from filaments. This infalling gas from the filament
gives the gas its angular momentum, and, in the case of the polar disc galaxy,
the direction of the gas filament does not change before or after the last
major merger.Comment: Accepted for publication in MNRAS; 14 pages; 14 figure
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