16,737 research outputs found
A study of methods to predict and measure the transmission of sound through the walls of light aircraft. A survey of techniques for visualization of noise fields
A survey of the most widely used methods for visualizing acoustic phenomena is presented. Emphasis is placed on acoustic processes in the audible frequencies. Many visual problems are analyzed on computer graphic systems. A brief description of the current technology in computer graphics is included. The visualization technique survey will serve as basis for recommending an optimum scheme for displaying acoustic fields on computer graphic systems
Interaction of weak shock waves with cylindrical and spherical gas inhomogeneities
The interaction of a plane weak shock wave with a single discrete gaseous inhomogeneity is studied as a model of the mechanisms by which finite-amplitude waves in random media generate turbulence and intensify mixing. The experiments are treated as an example of the shock-induced Rayleigh-Taylor instability. or Richtmyer-Meshkov instability, with large initial distortions of the gas interfaces. The inhomogeneities are made by filling large soap bubbles and cylindrical refraction cells (5 cm diameter) whose walls are thin plastic membranes with gases both lighter and heavier than the ambient air in a square (8.9 cm side shock-tube text section. The wavefront geometry and the deformation of the gas volume are visualized by shadowgraph photography. Wave configurations predicted by geometrical acoustics, including the effects of refraction, reflection and diffraction, are compared to the observations. Departures from the predictions of acoustic theory are discussed in terms of gasdynamic nonlinearity. The pressure field on the axis of symmetry downstream of the inhomogeneity is measured by piezoelectric pressure transducers. In the case of a cylindrical or spherical volume filled with heavy low-sound-speed gas the wave which passes through the interior focuses just behind the cylinder. On the other hand, the wave which passes through the light high-sound-speed volume strongly diverges. Visualization of the wavefronts reflected from and diffracted around the inhomogeneities exhibit many features known in optical and acoustic scattering. Rayleigh-Taylor instability induced by shock acceleration deforms the initially circular cross-section of the volume. In the case of the high-sound-speed sphere, a strong vortex ring forms and separates from the main volume of gas. Measurements of the wave and gas-interface velocities are compared to values calculated for one-dimensional interactions and for a simple model of shock-induced Rayleigh-Taylor instability. The circulation and Reynolds number of the vortical structures are calculated from the measured velocities by modeling a piston vortex generator. The results of the flow visualization are also compared with contemporary numerical simulations
Apparatus for real-time acoustic imaging of Rayleigh-Benard convection
We have designed and built an apparatus for real-time acoustic imaging of
convective flow patterns in optically opaque fluids. This apparatus takes
advantage of recent advances in two-dimensional ultrasound transducer array
technology; it employs a modified version of a commercially available
ultrasound camera, similar to those employed in non-destructive testing of
solids. Images of convection patterns are generated by observing the lateral
variation of the temperature dependent speed of sound via refraction of
acoustic plane waves passing vertically through the fluid layer. The apparatus
has been validated by observing convection rolls in both silicone oil and
ferrofluid.Comment: 20 pages, 11 figures, submitted to the Review of Scientific
Instrument
Time-delay Cosmography: Increased Leverage with Angular Diameter Distances
Strong lensing time-delay systems constrain cosmological parameters via the
so-called time-delay distance and the angular diameter distance to the lens. In
previous studies, only the former information was used. In this paper, we show
that the cosmological constraints improve significantly when the latter
information is also included. Specifically, the angular diameter distance plays
a crucial role in breaking the degeneracy between the curvature of the Universe
and the time-varying equation of state of dark energy. Using a mock sample of
55 bright quadruple lens systems based on expectations for ongoing/future
imaging surveys, we find that adding the angular diameter distance information
to the time-delay distance information and the cosmic microwave background data
of Planck improves the constraint on the constant equation of state by 30%, on
the time variation in the equation of state by a factor of two, and on the
Hubble constant in the flat CDM model by a factor of two. Therefore,
previous forecasts for the statistical power of time-delay systems were
significantly underestimated, i.e., time-delay systems are more powerful than
previously appreciated.Comment: [v2] 18 pages, 12 figures, submitted to JCAP. An error in the fisher
matrix for SNIa fixed; conclusions unchange
Towards cost-efficient prospection and 3D visualization of underwater structures using compact ROVs
The deployment of Remotely Operated Vehicles (ROV) for underwater prospection and 3D visualization has grown significantly in civil applications for a few decades. The demand for a wide range of optical and physical parameters of underwater environments is explained by an increasing complexity of the monitoring requirements of these environments. The prospection of engineering constructions (e.g. quay walls or enclosure doors) and underwater heritage (e.g. wrecks or sunken structures) heavily relies on ROV systems. Furthermore, ROVs offer a very flexible platform to measure the chemical content of the water. The biggest bottleneck of currently available ROVs is the cost of the systems. This constrains the availability of ROVs to a limited number of companies and institutes. Fortunately, as with the recent introduction of cost-efficient Unmanned Aerial Vehicles on the consumer market, a parallel development is expected for ROVs. The ability to participate in this new field of expertise by building Do It Yourself (DIY) kits and by adapting and adding on-demand features to the platform will increase the range of this new technology.
In this paper, the construction of a DIY OpenROV kit and its implementation in bathymetric research projects are elaborated. The original platform contains a modified webcam for visual underwater prospection and a Micro ElectroMechanical System (MEMS) based depth sensor, allowing relative positioning. However, the performance of the standard camera is limited and an absolute positioning system is absent. It is expected that 3D visualizations with conventional photogrammetric qualities are limited with the current system. Therefore, modifications to improve the standard platform are foreseen, allowing the development of a cost-efficient underwater platform. Preliminary results and expectations on these challenges are reported in this paper
Three-dimensional phenomena in microbubble acoustic streaming
Ultrasound-driven oscillating micro-bubbles have been used as active
actuators in microfluidic devices to perform manifold tasks such as mixing,
sorting and manipulation of microparticles. A common configuration consists on
side-bubbles, created by trapping air pockets in blind channels perpendicular
to the main channel direction. This configuration consists of acoustically
excited bubbles with a semi-cylindrical shape that generate significant
streaming flow. Due to the geometry of the channels, such flows have been
generally considered as quasi two-dimensional. Similar assumptions are often
made in many other microfluidic systems based on \emph{flat} micro-channels.
However, in this paper we show that microparticle trajectories actually present
a much richer behavior, with particularly strong out-of-plane dynamics in
regions close to the microbubble interface. Using Astigmatism Particle Tracking
Velocimetry, we reveal that the apparent planar streamlines are actually
projections of a \emph{streamsurface} with a pseudo-toroidal shape. We
therefore show that acoustic streaming cannot generally be assumed as a
two-dimensional phenomenon in confined systems. The results have crucial
consequences for most of the applications involving acoustic streaming as
particle trapping, sorting and mixing.Comment: 5 pages, 4 high quality figures. Accepted for Publication in Phys.
Rev. Applied, March 201
Prepontine non-giant neurons drive flexible escape behavior in zebrafish
Many species execute ballistic escape reactions to avoid imminent danger. Despite fast reaction times, responses are often highly regulated, reflecting a trade-off between costly motor actions and perceived threat level. However, how sensory cues are integrated within premotor escape circuits remains poorly understood. Here, we show that in zebrafish, less precipitous threats elicit a delayed escape, characterized by flexible trajectories, which are driven by a cluster of 38 prepontine neurons that are completely separate from the fast escape pathway. Whereas neurons that initiate rapid escapes receive direct auditory input and drive motor neurons, input and output pathways for delayed escapes are indirect, facilitating integration of cross-modal sensory information. These results show that rapid decision-making in the escape system is enabled by parallel pathways for ballistic responses and flexible delayed actions and defines a neuronal substrate for hierarchical choice in the vertebrate nervous system
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