14,627 research outputs found
Time-Reversal of Nonlinear Waves - Applicability and Limitations
Time-reversal (TR) refocusing of waves is one of fundamental principles in
wave physics. Using the TR approach, "Time-reversal mirrors" can physically
create a time-reversed wave that exactly refocus back, in space and time, to
its original source regardless of the complexity of the medium as if time were
going backwards. Lately, laboratory experiments proved that this approach can
be applied not only in acoustics and electromagnetism but also in the field of
linear and nonlinear water waves. Studying the range of validity and
limitations of the TR approach may determine and quantify its range of
applicability in hydrodynamics. In this context, we report a numerical study of
hydrodynamic TR using a uni-directional numerical wave tank, implemented by the
nonlinear high-order spectral method, known to accurately model the physical
processes at play, beyond physical laboratory restrictions. The applicability
of the TR approach is assessed over a variety of hydrodynamic localized and
pulsating structures' configurations, pointing out the importance of high-order
dispersive and particularly nonlinear effects in the refocusing of hydrodynamic
stationary envelope solitons and breathers. We expect that the results may
motivate similar experiments in other nonlinear dispersive media and encourage
several applications with particular emphasis on the field of ocean
engineering.Comment: 14 pages, 17 figures ; accepted for publication in Phys. Rev. Fluid
Airframe Noise Reduction Studies and Clean-Airframe Noise Investigation
Acoustic wind tunnel tests were conducted of a wing model with modified leading edge slat and trailing edge flap. The modifications were intended to reduce the surface pressure response to convected turbulence and thereby reduce the airframe noise without changing the lift at constant incidence. Tests were conducted at 70.7 and 100 m/sec airspeeds, with Reynolds numbers 1.5 x 10 to the 6th power and 2.1 x 10 to the 6th power. Considerable reduction of noise radiation from the side edges of a 40 deflection single slotted flap was achieved by modification to the side edge regions or the leading edge region of the flap panel. Total far field noise was reduced 2 to 3 dB over several octaves of frequency. When these panels were installed as the aft panel of a 40 deg deflection double slotted flap, 2 dB noise reduction was achieved
In vivo laser Doppler holography of the human retina
The eye offers a unique opportunity for non-invasive exploration of
cardiovascular diseases. Optical angiography in the retina requires sensitive
measurements, which hinders conventional full-field laser Doppler imaging
schemes. To overcome this limitation, we used digital holography to perform
laser Doppler perfusion imaging of the human retina in vivo with near-infrared
light. Wideband measurements of the beat frequency spectrum of optical
interferograms recorded with a 39 kHz CMOS camera are analyzed by short-time
Fourier transformation. Power Doppler images and movies drawn from the zeroth
moment of the power spectrum density reveal blood flows in retinal and
choroidal vessels over 512 512 pixels covering 2.4 2.4 mm
on the retina with a 13 ms temporal resolution.Comment: 5 pages, 5 figure
Spectroscopic Observations of New Oort Cloud Comet 2006 VZ13 and Four Other Comets
Spectral data are presented for comets 2006 VZ13 (LINEAR), 2006 K4 (NEAT),
2006 OF2 (Broughton), 2P/Encke, and 93P/Lovas I, obtained with the Cerro-Tololo
Inter-American Observatory 1.5-m telescope in August 2007. Comet 2006 VZ13 is a
new Oort cloud comet and shows strong lines of CN (3880 angstroms), the Swan
band sequence for C_2 (4740, 5160, and 5630 angstroms), C_3 (4056 angstroms),
and other faint species. Lines are also identified in the spectra of the other
comets. Flux measurements of the CN, C_2 (Delta v = +1,0), and C_3 lines are
recorded for each comet and production rates and ratios are derived. When
considering the comets as a group, there is a correlation of C_2 and C_3
production with CN, but there is no conclusive evidence that the production
rate ratios depend on heliocentric distance. The continuum is also measured,
and the dust production and dust-to-gas ratios are calculated. There is a
general trend, for the group of comets, between the dust-to-gas ratio and
heliocentric distance, but it does not depend on dynamical age or class. Comet
2006 VZ13 is determined to be in the carbon-depleted (or Tempel 1 type) class.Comment: 8 pages, 6 figures, 6 tables; Accepted by MNRA
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