Quasi-standing Waves Research Articles

Numerical and experimental analysis of strongly nonlinear standing acoustic waves in axisymmetric cavities

In this paper the behaviour of strongly nonlinear waves in axisymmetric resonators is experimentally and numerically studied. Experiments are carried out in a cylindrical cavity, which transversal dimension is bigger than the longitudinal one, excited by a narrow band transducer. The quality factor and displacement amplitudes are experimentally quantified. A finite difference numerical model is developed to solve, in the time domain, a proposed set of full nonlinear differential equations written in Lagrangian coordinates. Pressure field is obtained for complicated modes. Good agreement between numerical and experimental results is found. New nonlinear properties of quasi-standing waves in axisymmetric resonators are described. Results are compared to linear approximation and show the importance of three-dimensional analysis.

Theoretical calculation of the acoustic radiation force acting on elastic and viscoelastic cylinders placed in a plane standing or quasistanding wave field

In this work, a new expression of the acoustic radiation force function Yst for solid cylindrical targets, suspended in inviscid fluids in a plane standing wave field, is presented. The case of a plane quasistanding wave field is also considered. Numerical calculations of the radiation force function Yst are performed in a wide range of frequencies for elastic and viscoelastic cylinders and compared to those of rigid cylinders. The fluid-loading effect on the radiation force function curves is also analysed. The results show several features quite different from the rigid cylinder solution.

Full Quantum Mechanical Simulation of Ultra-Small Silicon Devices in Three-Dimensions: Physics and Issues

The combination of the need for alternative devices and the improvement in process technology has led to the examination of silicon quantum wires for future MOS technology. However, in order to properly model these devices, a full three-dimensional quantum mechanical treatment is required. In this paper, we present the results of a three-dimensional, fully quantum mechanical, self-consistent simulation of a silicon quantum wire MOSFET (Metal Oxide Field Effect Transistor) with a narrow channel (8 nm). A quasi-standing wave is formed in the narrow channel at certain gate voltages as the electron density is trapped in narrow channel. These effects are the result of two competing effects: (1) the interaction of the propagating electrons with the channel dopants, as well as with the dopants in the source and drain of the device. (2) the reflections from the boundaries that form the narrow channel both on the source side and the drain side.

Seasonal, interannual and short‐term variability of planetary waves in Met Office stratospheric assimilated fields

AbstractMet Office stratospheric assimilated data have been used to study the variability of travelling planetary waves in the upper stratosphere during the last decade (1992–2001). To distinguish the various travelling and quasi‐standing waves, a new space‐time filtering method has been developed. The results of this analysis show that longer‐period planetary waves (10<T<20 days) have significant amplitudes mainly during winter in both hemispheres. Westward‐propagating waves are dominant in the northern hemisphere, but in the southern hemisphere eastward‐ and westward‐propagating waves have comparable amplitudes and sometimes eastward‐propagating waves are dominant. It is suggested that the main forcing of longer‐period travelling waves is an internal variability of the quasi‐stationary planetary waves in the tropo/stratosphere, and that the substantial activity of eastward‐propagating waves in the southern hemisphere is associated with seasonal changes of the general circulation of the middle atmosphere. In the shorter‐period part of the wavelet spectra (T<10 days), westward‐propagating waves are dominant and the observed maxima in amplitude spectra can be identified with the first symmetric normal atmospheric modes, the so‐called 5‐ and 4‐day waves. These waves, present in the stratosphere during most of the year, increase in amplitude during seasonal transitions. Analysis of the behaviour of wavelet amplitudes at different pressure levels shows that these normal‐mode waves propagate upward from the lower atmosphere and their amplitudes increase with altitude. There exists also a substantial long‐period modulation of the 5‐day wave amplitude with quasi‐period of about 30 days. Copyright © 2004 Royal Meteorological Society

Matching design and mismatching analysis towards radar absorbing coatings based on conducting plate

Abstract Through a detailed investigation on the interaction of electromagnetic waves with the radar absorbing coating (RAC), we propose two design rules for single layer RAC. According to the rules, we may find the suitable electromagnetic parameters so as to satisfy the practical requirements of applied background of the coating. On the basis of the rules, a significant deduction has been gained, which represents the selecting law of the electromagnetic parameters of the coating when no reflecting wave has occurred at the interface between coating and free space to the incident wave. Combining our previous researches about quasi-standing wave and RAM design results, the applications of the rules has been presented and the interaction characteristic of a series of related parameters has been studied. It shows that the design rules have important guiding effects both in the material research and the single layer RAC design. Especially, the effect of various mismatching conditions for RAC performance has been given.

A simulation of the quasi-standing wave and generalized half-wave loss of electromagnetic wave in non-ideal media

In this paper, the phenomena of reflection towards normal incident electromagnetic wave on double loss-medium are analyzed, and the superposition wave mechanism that is similar to standing wave in transparent medium is exposed. In this case, it is more suitable that wave impedance of media is used to determine the thin or thick medium and half-wave loss. Based on the thought mentioned above, the new concept of quasi-standing wave and generalized half-wave loss are put forward for the first time. Moreover, the basic criteria of generalized half-wave loss are given. Especially, the basic characters and generalized law of quasi-standing wave is simulated by means of computations, and their physical mechanisms are also described.

SST intraseasonal oscillation and atmospheric forcing system of the South China Sea

This study used National Center for Environmental Prediction (NCEP) reanalysis data to confirm that the variance of sea surface temperature (SST) in the South China Sea (SCS) has pronounced intraseasonal oscillations characterized by quasi-standing waves; and was aimed to document how intraseasonal time scale SST formed and developed in the SCS. The results derived from the composite analysis indicated the existence of a local low-level atmospheric dynamic forcing system over the SCS. The main formation mechanism of SST intraseasonal oscillation is the low-level rotational atmospheric circulation forcing over the SCS on intraseasonal time scales and the solar radiation variations caused by cloud amount changes.

An attempt to explain turbulence occurring when yeast cells suspended in water-rich ethanol mixtures are irradiated in a quasistanding ultrasonic plane wave at 2.2 MHz

Ultrasonic resonators are successfully used for the separation of suspended particles or cells, e.g., as cells filters in biotechnology. These systems are based on the radiation forces spatially rearranging the particles in the pressure nodal planes of the quasistanding ultrasonic wave field. When yeast—as this technology is tested to be applied in brewing—is suspended in water this separation/immobilization takes place as predicted. However, suspensions of yeast in water-rich ethanol mixtures show a breakdown of the spatial distribution above certain concentrations of ethanol and the cells are turbulently mixed in the separation system. This has earlier been reported to go along with an impaired viability of yeast in a 12% (v/v) ethanol/water mixture. The scope of this paper is to attempt an explanation based on the acoustic contrast between the cells and the liquid as water-rich ethanol mixtures have unexpected acoustical and physical properties. Data of the spectral behavior of the system at in situ levels of true electrical power input are shown as well as the results of examinations, if (sub) harmonic and/or transient cavitation were taking place. [Work supported in part by the European Commission’s TMR Programme, Contract No. ERBFMRXCT97-0156, EuroUltraSonoSep.]

The influence of microphone vents on measurements of acoustic intensity and impedance

The pressure equalization vents of conventional microphones introduce bias errors in measurements of the sound intensity. Two error terms in the estimated active intensity are derived using a low‐frequency model of microphone. The first error term is associated with the lower‐limiting frequency of the microphones (1 to 2 Hz) and is proportional to the reactive sound intensity. It is shown here that the difference between limiting frequencies of the two microphones causes the second error term to be proportional to the mean‐square pressure and to be of comparable importance at low frequencies. The amplitudes of errors due to the vents are analyzed in a quasistanding wave. Unlike the first error, the second can be corrected at the same time as the phase error between measurement channels, and several correction techniques are examined. For the error proportional to the reactive intensity, a correction method in a standing wave tube is suggested, using the joint measurement of several other energetic quantities. Examples of the influence of these errors on typical parameter values for half‐inch microphones are presented in the case of measuring the acoustic impedance of materials using a two‐microphone probe.

Relation between X-ray absorption near-edge spectra and interatomic distances

Abstract We have performed first-principles calculations for the X-ray absorption near-edge structure (XANES) of PX 3 (X = F, Cl) molecules and AO 4 n − (A = P, S, Cl) cluster ions, using the discrete Xα method. Theoretical X-ray spectra for PX 3 agree with the experimental ones. The theoretical spectra for AO 4 n − correspond to the major peaks in the experimental spectra which have been identified as contributions of the AO 4 n − clusters in the M n AO 4 (M = Li, Na, K, Rb, Cs) compounds. In the theoretical as well as the experimental spectra, the peak energies vary with the P-X and A-O interatomic distances. The XANES spectra and the wavefunctions are examined in terms of potential scattering. The results indicate that the wavefunction in each intra-atomic region is essential to approximate the whole quasi-standing wave of a shape resonance. In consistency with the calculated results, a model of the wavefunction is proposed for the XANES peaks of simple molecules and an equation is derived which expresses a relation among interatomic distance, peak energy and atomic number. The interatomic distances calculated with this equation including three optimized parameters have a notably high correlation with the experimental data for the free molecules involving B, C, N, O and F atoms.

The vibration of a rotating disk restrained at a space-fixed point.

The vibration of a rotating disk restrained at a space-fixed point, such as a computer memory disk with a magenetic head, is theoretically analyzed. The frequency equation is derived by using the receptance method. The natural frequencies and the vibratory modes are discussed for the rotation speeds below the critical speed. The analysis indicates that the phenomenon of"curve veering"for a frequency-speed diagram is observed for the restraint with high receptance, and that"quasi-standing waves"are formed for the restraint with low.

The steady state response of a rotating disk to a concentrated harmonic force at a space-fixed point.

The steady-state response of a rotating disk to a concentrated harmonic force at a space-fixed point is analyzed. The eigenfunctions are proved to be orthonormal, and formulation of the response is derived as an eigenfunction series. It is proposed that the eigenfunctions are given in the form of rotating waves, and the resonant frequencies and vibratory modes are discussed. Resonance occurs when. the exciting frequency coincides with one of the natural frequencies observed in the space-fixed coordinates. If two of the natural frequencies coincides with each other, the resonant mode that may be called "a quasi-standing wave" is formed in the space-fixed coordinates.

FOCUSING OF A LASER BEAM BY AN ACTIVE CdS OSCILLATOR

A strong lens effect has been experimentally observed when the electric field inside a CdS plate is so adjusted that strong quasi-standing ultrasonic waves are set up inside the crystal. Physical understanding of the observed phenomena and possible device applications are discussed.

Propagation of Waves in the Solar Atmosphere.

The frequencies and horizontal wavenumbers at which the normal modes (large amplitude quasi-standing waves) of the solar atmosphere occur were calcu- lated for a semi empirical model of the region around the temperature minimum. At high frequencies the waves are compressional, modified by gravity, and can propagate into the upper atmosphere; at low frequencies the waves are gravitational, modified by compressibility, and can also propagate into the upper atmosphere. Between these two passhands is a trap baud where the waves are completely reflected. Three types of fundamental modes were found. The fundamental acoustic mode has ~~8x 10~ ku sec-1 for horizontal wavelengths smaller than 1000 km and goes to a constant frequency with a width 0.05> >0.032 sec-' for horizontal wavelengths greater than 2000 km. The fundamental acoustic-gravity mode be- haves like an acoustic mode for horizontal wavelengths greater than 6000 km, where it is composed of many narrow resonances of nearly constant frequency in the range 0.032>~~>0.009 sec-1. At smaller horizontal wavelengths it narrows and changes its behavior to that of a gravity mode with a frequency c~~0.03 sec-1. The fundamental gravity mode has o~~7.5 x 10 k11 sec-' at horizontal wavelengths greater than 3000 km and approaches a constant frequency ~~0.028 sec-' at small horizontal wavelengths. The calculated fundamental acoustic-gravity mode covers the range of frequencies and horizontal wavelengths (0.03>~~>0.015 sec-', AH>SOOO km) where the spectral density of the observed solar oscillations, as calculated by Pierre Mein (Compt. Rend. 260, 1867, 1965), is large. It was also found that the height of the maximum vertical velocity shifts to greater altitudes as the frequency increases through the wide acoustic-gravity fundamental mode. This might explain the observed increase in the frequency of the oscillations with height.

Leaking modes in the crustal waveguide: 1. The oceanicPLwave

The problem of the seismic signal associated with the earliest P wave is treated by application of normal mode theory, in which the signal is regarded as a quasi-surface wave, coupled both to the motion of the earth's layered surface and to body waves propagating in the underlying media. Predictions made for the particular model assumed are relevant to explosion and earth-quake sources. The oscillations following the initial P motion are explained. The transient solution obtained by Rosenbaum for leaking mode propagation in an acoustic waveguide has been generalized to describe propagation in an elastic halfspace overlain by a liquid layer. The early-arriving PL modes known from earthquake studies have been computed for several theoretical models to test the effect of the elastic constants on their dispersion and attenuation. Physical reasoning, based on harmonic plane wave models, appears inadequate for predicting many features of the exact dispersion and attenuation. The analogy between PL waves and normal modes in the case treated by Pekeris is exploited, and it is also believed that PL waves are related to an attenuated pseudo-surface wave of a free solid halfspace. Late-arriving quasi-standing waves are treated briefly and their relevance to certain seismic phenomena is mentioned.