1,748 research outputs found
Acoustic GRIN Lens
Museum displays with audio explanations typically use headphones. A speaker’s sound at one display interferes with patrons at other displays, thus museums use headphones. These devices are either integrated into the display itself, or attached to an mp3 player. When integrated into the display, headphones can be unsanitary; when attached to an mp3 player, these devices are inconvenient. If speakers can direct sound toward an intended area, headphones can be eliminated. Acoustic horns direct sound toward a specific point, but require a large spatial footprint. Acoustic spherical concave lenses also direct sound toward a point, but exhibit excessive acoustic attenuation. Current speakers on the market that focus sound toward a location are costly compared to headphones. An acoustic gradient index of refraction lens, GRIN lens, accomplishes sound wave focusing while minimizing attenuation and decreasing material costs compared to current directive speakers. The goal of this project is to design and optimize a GRIN lens using numerical simulations in COMSOL Multiphysics and scripts developed in MATLAB. This GRIN lens must direct sound for museum exhibits; therefore, must operate in the human speech frequency band (300Hz - 3400Hz). An acoustic test chamber was designed and built in order to test and validate the physical prototype. The prototype was fabricated using PETG on the MakerGearM2 3D printer. This chamber is 1m length x 1m height x 1m depth and characterizes acoustic radiation pattern vs. azimuth angle (300Hz - 3400Hz) and the frequency response - microphone directly in front of speaker (300Hz - 10kHz) - for 3” maximum diameter speakers
The turbomachine blading design using S2-S1 approach
The boundary conditions corresponding to the design problem when the blades being simulated by the bound vorticity distribution are presented. The 3D flow is analyzed by the two steps S2 - S1 approach. In the first step, the number of blades is supposed to be infinite, the vortex distribution is transformed into an axisymmetric one, so that the flow field can be analyzed in a meridional plane. The thickness distribution of the blade producing the flow channel striction is taken into account by the modification of metric tensor in the continuity equation. Using the meridional stream function to define the flow field, the mass conservation is satisfied automatically. The governing equation is deduced from the relation between the azimuthal component of the vorticity and the meridional velocity. The value of the azimuthal component of the vorticity is provided by the hub to shroud equilibrium condition. This step leads to the determination of the axisymmetric stream sheets as well as the approximate camber surface of the blade. In the second step, the finite number of blades is taken into account, the inverse problem corresponding to the blade to blade flow confined in each stream sheet is analyzed. The momentum equation implies that the free vortex of the absolute velocity must be tangential to the stream sheet. The governing equation for the blade to blade flow stream function is deduced from this condition. At the beginning, the upper and the lower surfaces of the blades are created from the camber surface obtained from the first step with the assigned thickness distribution. The bound vorticity distribution and the penetrating flux conservation applied on the presumed blade surface constitute the boundary conditions of the inverse problem. The detection of this flux leads to the rectification of the geometry of the blades
Physiological responses to prolonged bed rest in humans: A compendium of research, 1981-1988
Clinical observations and results form more basic studies that help to elucidate the physiological mechanisms of the adaptation of humans to prolonged bed rest. If the authors' abstract or summary was appropriate, it was included. In some cases a more detailed synopsis was provided under the subheadings of purpose, methods, results, and conclusions
Perturbative Effective Theory in an Oscillator Basis?
The effective interaction/operator problem in nuclear physics is believed to
be highly nonperturbative, requiring extended high-momentum spaces for accurate
solution. We trace this to difficulties that arise at both short and long
distances when the included space is defined in terms of a basis of harmonic
oscillator Slater determinants. We show, in the simplest case of the deuteron,
that both difficulties can be circumvented, yielding highly perturbative
results in the potential even for modest (~6hw) included spaces.Comment: 10 pages, 4 figure
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Researching Environments for Early Learning (REEL) Study
No description supplie
Accretion in the Early Kuiper Belt II. Fragmentation
We describe new planetesimal accretion calculations in the Kuiper Belt that
include fragmentation and velocity evolution. All models produce two power law
cumulative size distributions, N_C propto r^{-q}, with q = 2.5 for radii less
than 0.3-3 km and q = 3 for radii exceeding 1-3 km. The power law indices are
nearly independent of the initial mass in the annulus, the initial eccentricity
of the planetesimal swarm, and the initial size distribution of the
planetesimal swarm. The transition between the two power laws moves to larger
radii as the initial eccentricity increases. The maximum size of objects
depends on their intrinsic tensile strength; Pluto formation requires a
strength exceeding 300 erg per gram. Our models yield formation timescales for
Pluto-sized objects of 30-40 Myr for a minimum mass solar nebula. The
production of several `Plutos' and more than 10^5 50 km radius Kuiper Belt
objects leaves most of the initial mass in 0.1-10 km radius objects that can be
collisionally depleted over the age of the solar system. These results resolve
the puzzle of large Kuiper Belt objects in a small mass Kuiper Belt.Comment: to appear in the Astronomical Journal (July 1999); 54 pages including
7 tables and 13 figure
Analysis of the rotational properties of Kuiper belt objects
We use optical data on 10 Kuiper Belt objects (KBOs) to investigate their
rotational properties. Of the 10, three (30%) exhibit light variations with
amplitude delta_m >= 0.15 mag, and 1 out of 10 (10%) has delta_m >= 0.40 mag,
which is in good agreement with previous surveys. These data, in combination
with the existing database, are used to discuss the rotational periods, shapes,
and densities of Kuiper Belt objects. We find that, in the sampled size range,
Kuiper Belt objects have a higher fraction of low amplitude lightcurves and
rotate slower than main belt asteroids. The data also show that the rotational
properties and the shapes of KBOs depend on size. If we split the database of
KBO rotational properties into two size ranges with diameter larger and smaller
than 400 km, we find that: (1) the mean lightcurve amplitudes of the two groups
are different with 98.5% confidence, (2) the corresponding power-law shape
distributions seem to be different, although the existing data are too sparse
to render this difference significant, and (3) the two groups occupy different
regions on a spin period vs. lightcurve amplitude diagram. These differences
are interpreted in the context of KBO collisional evolution.Comment: 15 pages, 14 figures, LaTeX. Astronomical Journal in pres
Present Constraints on the H-dibaryon at the Physical Point from Lattice QCD
The current constraints from lattice QCD on the existence of the H-dibaryon
are discussed. With only two significant lattice QCD calculations of the
H-dibaryon binding energy at approximately the same lattice spacing, the forms
of the chiral and continuum extrapolations to the physical point are not
determined. In this brief report, we consider the constraints on the H-dibaryon
imposed by two simple chiral extrapolations. In both instances, the
extrapolation to the physical pion mass allows for a bound H-dibaryon or a
near-threshold scattering state. Further lattice QCD calculations are required
to clarify this situation.Comment: 8 pages, 2 figures, 1 table; revised for the journa
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