Page 49 - January 2006

Scanning the journals
made to “walk” at constant horizontal velocity on the liquid surface by increasing this acceleration. This transition, which occurs slightly below the Faraday instability (where the surface becomes spontaneously wavy), yields a new type of localized state with particle-wave duality. When two walkers come close, they interact through their waves and this “collision” causes the two walkers to orbit around each other.
􏰀 An optical vibration sensor which uses a 1550-nm laser diode as a source and an InGaAs photodiode as a receiver, is sensitive to accelerations as low as 0.01 m/s2, according to a note in the September issue of Photonics Spectra. The device, which is fabricated by creating a waveguide in glass using fem- tosecond pulses from a Ti:sapphire laser, has a linear response over frequencies from 20 to 2000 Hz.
􏰀 A new backpack device makes use of an oscillating mass and spring to harvest energy from walking according to a brief paper in the 9 September issue of Science. The resulting elec- trical energy could be used to recharge cell phone batteries or power a global positioning system while the subject walks. A 20-kg unit can generate more than 2 W of electrical power, while a 38-kg unit can produce more than 7 W. The oscillating device results in a considerable saving in metabolic energy as compared to a device with a fixed payload.
􏰀 Ultrasound can be used to control the viscosity of gelatinous organ fluids, according to a paper in J. Am. Chemical Soc. 176, 9324 (2005). Molecules containing platinum are dissolved in acetone, resulting in an oily liquid that turns into a white gel when zapped with ultrasound at 40 kHz. The gel can be reversed back into a liquid by heating or by another burst of ultrasound. This is believed to be the first demonstration of the instant and remote control of stable so-gel phases.
􏰀 The recent Sumatra tsunami that produced devastation around the Indian Ocean traveled several times around the globe before dissipating, according to a report in the 23 September issue of Science. The history is recorded in a global tide-gauge network and an ocean model is used to understand the global propagation of this tsunami. Large waves were recorded in such places as Peru, Antarctica, and Nova Scotia, probably guided by Earth’s mid-ocean ridge system.
􏰀 According to a letter in the 29 September issue of Nature,
crescent-shaped dunes called barchans behave more like plane propagating waves than like solitary waves. The nucleation and propagation of such waves on a sand bed is governed by the interaction between the bed profile and the sand transport. The instability mechanism is directly related to the asymmetry of the wind flow which originates in the nonlinear inertial term of the Navier-Stokes equations.
􏰀 “Heavenly” is the title of an article on cosmology in the August issue of Scientific American. When scientists “listen” to the music of the cosmos played in the cosmic microwave back- ground (CMB), the strains first sound harmonious but then as they listen more carefully they note that something is off key. As with a sound wave, CMB fluctuations can be analyzed by split- ting them into their component harmonics. The temperature fluctuations are analyzed in terms of spherical harmonics.
􏰀 “Coherent signal amplification in bistable nanomechanical oscillators by stochastic resonance” is the title of a paper in the 13 October issue of Nature. Stochastic resonance is the addi- tion of noise to a noisy system to induce coherent amplification of its response. It runs rather counter to intuition. This paper describes its observation in nanomechanical silicon oscillators consisting of beams that are clamped at each end and driven into transverse oscillation with the use of a radiofrequency source. The “Euler instability” in buckled beams dictates that a slender elastic object, clamped at either end, will move trans- versely if subjected to a longitudinal compressive force beyond a critical value.
􏰀 “Modal Analysis Versus Finite-Element Analysis” is the title of an informative editorial in the September issue of Sound and Vibration. Experimental modal analysis (EMA) or modal testing is used to validate finite element analysis (FEA), but it is also used for trouble shooting noise and vibration problems in the field. Each mode is defined by three different kinds of parame- ters: its modal frequency, its modal damping, and its mode shape. There are a number of reasons why experimental mode shapes don’t match analytical shapes. A significant one is that the boundary conditions may be different between the EMA and FEA. Damping can’t be easily modeled, so it’s not included in most FEA models. Also, experimental mode shapes tend to have fewer degrees of freedom (DOFs) than analytical shapes.
ASA 2006 Meetings–Plan Now to Attend!
http://asa.aip.org/meetings.html
5–9 June • Providence, RI (Abstract submission deadline has passed)
28 November–2 December • Honolulu, HI • 4th joint meeting–Acoustical Society of America and Acoustical Society of Japan (Abstract submission deadline: 30 June 2006)
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