2,467 research outputs found
Enhanced nonlinear imaging through scattering media using transmission matrix based wavefront shaping
Despite the tremendous progresses in wavefront control through or inside
complex scattering media, several limitations prevent reaching practical
feasibility for nonlinear imaging in biological tissues. While the optimization
of nonlinear signals might suffer from low signal to noise conditions and from
possible artifacts at large penetration depths, it has nevertheless been
largely used in the multiple scattering regime since it provides a guide star
mechanism as well as an intrinsic compensation for spatiotemporal distortions.
Here, we demonstrate the benefit of Transmission Matrix (TM) based approaches
under broadband illumination conditions, to perform nonlinear imaging. Using
ultrashort pulse illumination with spectral bandwidth comparable but still
lower than the spectral width of the scattering medium, we show strong
nonlinear enhancements of several orders of magnitude, through thicknesses of a
few transport mean free paths, which corresponds to millimeters in biological
tissues. Linear TM refocusing is moreover compatible with fast scanning
nonlinear imaging and potentially with acoustic based methods, which paves the
way for nonlinear microscopy deep inside scattering media
Lab and Field Tests of a Low-Cost 3-Component Seismometer for Shallow Passive Seismic Applications
We performed laboratory tests and field surveys to evaluate the performance of a low-cost 3-component seismometer, consisting of three passive electromagnetic spring-mass sensors, whose 4.5 Hz natural frequency is extended down to 0.5 Hz thanks to hyper damping. Both lab and field datasets show that the −3 dB band of the seismometer ranges approximately from 0.7 to 39 Hz, in agreement with the nominal specifications. Median magnitude frequency response curves obtained from processing field data indicate that lower corner of the −3 dB band could be extended down to 0.55 Hz and the nominal sensitivity may be overestimated. Lab results confirm the non-linear behavior of the passive spring-mass sensor expected for high-level input signals (a few to tens of mm/s) and field data confirm relative timing accuracy is ±10 ms (1 sample). We found that absolute timing of data collected with USB GPS antennas can be affected by lag as large as +0.5 s. By testing two identical units, we noticed that there could be differences around 0.5 dB (i.e., about 6%) between the components of the same unit as well as between the same component of the two units. Considering shallow passive seismic applications and mainly focusing on unstable slope monitoring, our findings show that the tested seismometer is able to identify resonance frequencies of unstable rock pillars and to generate interferograms that can be processed to estimate subsurface velocity variations
Nonlinear hydrodynamic modelling of wave energy converters under controlled conditions
One of the major challenges facing modern industrialized countries is the provision of energy:
traditional sources, mainly based on fossil fuels, are not only growing scarcer and
more expensive, but are also irremediably damaging the environment. Renewable and
sustainable energy sources are attractive alternatives that can substantially diversify the
energy mix, cut down pollution, and reduce the human footprint on the environment.
Ocean energy, including energy generated from the motion of wave, is a tremendous untapped
energy resource that could make a decisive contribution to the future supply of
clean energy. However, numerous obstacles must be overcome for ocean energy to reach
economic viability and compete with other energy sources. Energy can be generated from
ocean waves by wave energy converters (WECs). The amount of energy extracted from
ocean waves, and therefore the profitability of the extraction, can be increased by optimizing
the geometry and the control strategy of the wave energy converter, both of which
require mathematical hydrodynamic models that are able to correctly describe the WEC-
uid interaction. On the one hand, the accuracy and representativeness of such models
have a major in
uence on the effectiveness of the WEC design. On the other hand, the
computational time required by a model limits its applicability, since many iterations or
real-time calculations may be required. Critically, computational time and accuracy are
often mutually contrasting features of a mathematical model, so an appropriate compromise
should be defined in accordance with the purpose of the model, the device type, and
the operational conditions. Linear models, often chosen due to their computational convenience,
are likely to be imprecise when a control strategy is implemented in a WEC: under
controlled conditions, the motion of the device is exaggerated in order to maximize power
absorption, which invalidates the assumption of linearity. The inclusion of nonlinearities
in a model is likely to improve the model's accuracy, but increases the computational
burden. Therefore, the objective is to define a parsimonious model, in which only relevant
nonlinearities are modelled in order to obtain an appropriate compromise between accuracy
and computational time. In addition to presenting a wider discussion of nonlinear
hydrodynamic modelling for WECs, this thesis contributes the development of a computationally
efficient nonlinear hydrodynamic model for axisymmetric WEC devices, from
one to six degrees of freedom, based on a novel approach to the nonlinear computation of
static and dynamic Froude-Krylov forces
Advanced Engineering Laboratory project summaries : 1995-1996
The Advanced Engineering Laboratory of the Woods Hole Oceanographic Institution is a development laboratory within
the Applied Ocean Physics and Engineering Department. Its function is the development of oceanographic instrumentation
to test developing theories in oceanography and to enhance current research projects in other disciplines within the
community. This report summarizes recent and ongoing projects performed by members of this laboratory
Controlling waves in space and time for imaging and focusing in complex media
In complex media such as white paint and biological tissue, light encounters nanoscale refractive-index inhomogeneities that cause multiple scattering. Such scattering is usually seen as an impediment to focusing and imaging. However, scientists have recently used strongly scattering materials to focus, shape and compress waves by controlling the many degrees of freedom in the incident waves. This was first demonstrated in the acoustic and microwave domains using time reversal, and is now being performed in the optical realm using spatial light modulators to address the many thousands of spatial degrees of freedom of light. This approach is being used to investigate phenomena such as optical super-resolution and the time reversal of light, thus opening many new avenues for imaging and focusing in turbid medi
Development, Analysis and Comparison of two Concepts for Wave Energy Conversion in the Mediterranean Sea
The investigations carried out during this Ph.D. programme relate to the marine
engineering field, more specifically to wave energy conversion. Two different wave energy
converters have been modelled numerically with the aim to assess the feasibility of wave
energy conversion in the Mediterranean Sea.
The first studied wave energy converter, named HPA-LG, is a heaving point absorber
with a linear generator placed at the seabed. Two variants of the WEC have
been examined, paying particular attention to the floater dimensions and to the geometrical
design of the PTO. Initially, only the heave mode has been modelled and the
performance of both devices has been analysed. Subsequently, the surge mode has been
added to the model and its effect in prevalent heaving point absorbers
has been studied.
For the two-body device, although the dynamic behaviour changes when the surge
is included, no relevant differences are observed regarding the power production. When
studying the three-body device, results show two clear trends, for
steep waves the surge leads to a decrease in the production, for flatter waves it affects positively the power absorption. However, the overall the negative contribution is more relevant.
The MoonWEC is the second studied wave energy converter, it merges several working
principles with the aim to benefit from the assets of each single principle. It consists
in a hollow floating structure, where water fills a central whole creating a moonpool. Device optimization has also been carried out, six different CALM mooring configurations and three Wells turbines have been tested.
Both device show similarities in their performance, their production is maximised
for a specific range of wave conditions. The HPA-LG has a broader optimal range.
However, the MoonWEC is more efficient for mild wave conditions, its
annual energy production is [50-100] % higher, depending on the location and HPA-LG
variant
First electrostatic probe results from Explorer 17
Electrostatic probe results from ionospheric sounding by Explorer XVII satellit
Digital Signal Processing Research Program
Contains table of contents for Section 2, an introduction, reports on twenty-one research projects and a list of publications.U.S. Navy - Office of Naval Research Grant N00014-93-1-0686Lockheed Sanders, Inc. Contract P.O. BY5561U.S. Air Force - Office of Scientific Research Grant AFOSR 91-0034National Science Foundation Grant MIP 95-02885U.S. Navy - Office of Naval Research Grant N00014-95-1-0834MIT-WHOI Joint Graduate Program in Oceanographic EngineeringAT&T Laboratories Doctoral Support ProgramDefense Advanced Research Projects Agency/U.S. Navy - Office of Naval Research Grant N00014-89-J-1489Lockheed Sanders/U.S. Navy - Office of Naval Research Grant N00014-91-C-0125U.S. Navy - Office of Naval Research Grant N00014-89-J-1489National Science Foundation Grant MIP 95-02885Defense Advanced Research Projects Agency/U.S. Navy Contract DAAH04-95-1-0473U.S. Navy - Office of Naval Research Grant N00014-91-J-1628University of California/Scripps Institute of Oceanography Contract 1003-73-5
Formation of matter-wave soliton trains by modulational instability
Nonlinear systems can exhibit a rich set of dynamics that are inherently
sensitive to their initial conditions. One such example is modulational
instability, which is believed to be one of the most prevalent instabilities in
nature. By exploiting a shallow zero-crossing of a Feshbach resonance, we
characterize modulational instability and its role in the formation of
matter-wave soliton trains from a Bose-Einstein condensate. We examine the
universal scaling laws exhibited by the system, and through real-time imaging,
address a long-standing question of whether the solitons in trains are created
with effectively repulsive nearest neighbor interactions, or rather, evolve
into such a structure
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