Experimental evidence for partial waves in the optical investigation of Lamb waves

An optical probing technique has been used to determine the propagation parameters and the acoustic field distribution of the longitudinal and shear partial waves coupled in Lamb modes. The partial wave momenta were determined from measurements of the light diffraction spectrum and partial acoustic fields analysed by combining spatial light filtering and polarization discrimination techniques. A method for the identification of the Lamb mode is presented.     

Singular filaments organize chemical waves in three dimensions: III. Knotted waves

This is the third of a series of papers on the anatomy of three-dimensional organizing centers in excitable media. We here ask whether all self-consistent waves in excitable media are topologically equivalent to the experimentally-verified scroll ring, whose axis lies in a plane. As a test case we examine a scroll ring whose axis contains a knot. It proves to be incompatible with the requirements of physical chemistry unless simultaneously twisted by an amount equal to the “writhing number” of its axis (which is zero for planar closed curves). Appropriate initial conditions are suggested for experimentally creating a wave whose source is a scroll ring knotted and twisted in this way.     

Experimental evidence of the effect of heat flux on thomson scattering off ion acoustic waves

Thomson self-scattering measurements are performed in a preionized helium gas jet plasma at different locations along the laser propagation direction. A systematic and important variation of the intensity ratio between the blue and the red ion spectral components is observed, depending on whether the location of the probed region is in front of or behind the focal plane. A simple theoretical calculation of Thomson scattering shows that this behavior can be qualitatively understood in terms of a deformation of the electron distribution function due to the return current correlated with the classical thermal heat flux.     

Universal scaling in BCS superconductivity in three dimensions in non-s waves

The solutions of a renormalized BCS equation are studied in three space dimensions in s, p and d waves for finite-range separable potentials in the weak to medium coupling region. In the weak-coupling limit, the present BCS model yields a small coherence length and a large critical temperature, , appropriate for some high- materials. The BCS gap, , and specific heat as a function of zero-temperature condensation energy are found to exhibit potential-independent universal scalings. The entropy, specific heat, spin susceptibility and penetration depth as a function of temperature exhibit universal scaling below in p and d waves. Received: 18 July 1997 / Revised: 8 September 1997 / Accepted: 29 September 1997     

A fully discrete Galerkin method for high frequency exterior acoustic scattering in three dimensions

Standard Galerkin discretization techniques (with locally- or globally-supported basis functions) for boundary integral equations are inefficient for high frequency three dimensional exterior scattering simulations because they require a fixed number of unknowns per wavelength in each dimension, leading to large CPU time and memory requirements to set up the dense Galerkin matrix, with each entry requiring evaluation of multi-dimensional highly oscillatory integrals. In this work, using globally-supported basis functions, we describe an efficient fully discrete Galerkin surface integral equation algorithm for simulating high frequency acoustic scattering by three dimensional convex obstacles that includes a powerful integration scheme for evaluation of four dimensional Galerkin integrals with high-order accuracy. Such high-order order accuracy for various practically relevant frequencies (k ∈ [1, 100,000]) substantially improves on approximations based on standard asymptotic techniques. We demonstrate the efficiency of our algorithm for spherical and non-spherical convex scattering for several wavenumbers 1 ? k ? 100,000 for low to high order prescribed tolerance. Our fully discrete algorithm requires only mild growth in the number of unknowns and CPU time as the frequency increases.     

Experimental evidence of rotational elastic waves in granular phononic crystals

A generalized theory of elasticity, taking into account the rotational degrees of freedom of point bodies constituting a continuum, was proposed at the beginning of the twentieth century by the Cosserat brothers. We report the experimental observation of coupled rotational-translational modes in a noncohesive granular phononic crystal. While absent in the classical theory of elasticity, these elastic wave modes are predicted by the Cosserat theory. However the Cosserat theory fails to predict correctly the dispersion of the elastic modes in granular crystals even in the long-wavelength limit.     

Experimental study of the formation of nonlinear acoustic waves in focused beams

The shock wave formation in focused beams produced by spherical hydroacoustic transducers with different apertures and an operating frequency of 3 MHz, as well as in weakly divergent high-intensity beams of the same frequency, is studied experimentally. The profiles of the received signals are analyzed for different receiving points in the acoustic beam and for different combinations of nonlinear and diffraction effects. It is found that the distortion of the initial waveform (i.e., of the compression and rarefaction phases) is asymmetric. The asymmetry of the wave profile in a focused beam is more pronounced than that in a quasi-plane wave while the asymmetric distortion of the high-frequency carrier causes an asymmetric distortion of the pulse envelope. The angular characteristics of the difference-frequency waves produced by parametric sound radiators are compared using both focused and weakly divergent beams of pump waves. The experiments also show that the appearance of a bubbly phase screen in the region before the point of the shock formation either shifts this point to greater distances or makes the discontinuity formation impossible. Results illustrating the changes that occur in the shock wave characteristics when the bubbly phase screen is placed in the region of the fully developed shock are presented.     

Experimental study of nonlinear dust acoustic solitary waves in a dusty plasma

The excitation and propagation of finite-amplitude low-frequency solitary waves are investigated in an argon plasma impregnated with kaolin dust particles. A nonlinear longitudinal dust acoustic solitary wave is excited by pulse modulating the discharge voltage with a negative potential. It is found that the velocity of the solitary wave increases and the width decreases with the increase of the modulating voltage, but the product of the solitary wave amplitude and the square of the width remains nearly constant. The experimental findings are compared with analytic soliton solutions of a model Korteveg-de Vries equation.     

Experimental study on subharmonic and ultraharmonic acoustic waves in water-saturated sandy sediment

Experimental observations of the subharmonic and ultraharmonic acoustic waves in water-saturated sandy sediment are reported in this paper. Acoustic pressures of both nonlinear acoustic waves strongly depend on the driving acoustic pressure at a transducer. The first ultraharmonic wave reaches a saturation value as the driving acoustic pressure increases. The acoustic pressure levels of both nonlinear acoustic waves exhibit some fluctuations in comparison with that of the primary acoustic wave as the receiving distance of hydrophone increases in sediment. The subharmonic and the ultraharmonic phenomena in this study show close resemblance to those produced in bubbly water.     

Potential wells for classical acoustic waves

The acceleration theorem of Bloch waves is utilized to construct random potential wells for classical acoustic waves in systems composed of alternating‘cavities’and‘couplers’.One prominent advantage of this method is these‘cavities’and‘couplers’are all monolayer structures.It allows forming more compact classical potential wells,which leads to the miniaturization of acoustic devices.We systematically investigate properties of harmonic,tangent,hyperbolic function,and square classical potential wells in quasi-periodic superlattices.Results show these classical potential wells are analogues of quantum potential wells.Thus some technologies and concepts in quantum potential well fields may be generalized to classical acoustic wave fields.In addition,some abnormal cases regarding forming classical potential wells are also found.     

Broadband acoustic cloak for ultrasound waves

Invisibility devices based on coordinate transformation have opened up a new field of considerable interest. We present here the first practical realization of a low-loss and broadband acoustic cloak for underwater ultrasound. This metamaterial cloak is constructed with a network of acoustic circuit elements, namely, serial inductors and shunt capacitors. Our experiment clearly shows that the acoustic cloak can effectively bend the ultrasound waves around the hidden object, with reduced scattering and shadow. Because of the nonresonant nature of the building elements, this low-loss (～6 dB/m) cylindrical cloak exhibits invisibility over a broad frequency range from 52 to 64?kHz. Furthermore, our experimental study indicates that this design approach should be scalable to different acoustic frequencies and offers the possibility for a variety of devices based on coordinate transformation.