Coherent Velocity Research Articles

Frequency-modulation detection of particle diffusion from heterodyne quasi-elastic light scattering

The Brownian/diffusive motion of micrometer-sized, suspended particles has been observed by using heterodyne light scattering with frequency-modulation (FM) detection and signal processing. The FM output is linear in the coherent velocity of the particles, with the random diffusive velocity component giving rise to background noise in the FM output. Experimental measurements of polystyrene latex spheres in water corroborate the predicted result for the signal-to-noise ratio. The noise component of the FM output was found to be independent of the particle number and, within the instrumental bandwidth, to be flat. Good quantitative agreement between the predicted and measured noise backgrounds was obtained for particle diameters from 0.5 to 50 μm.

Scattering of acoustic waves by piezoelectric composites

Piezoelectic composite materials can be designed so that they effectively alter the reflection and transmission characteristics of acoustic and electromagnetic waves that are incident on them. This can be achieved by controlling their microstructure so that their performance characteristics match desired goals. With this end in mind basic research has been performed on the propagation of elastic P, SV, and SH waves and electromagnetic waves propagating through a composite containing a random distribution of parallelly aligned, infinitely long cylinders made of a piezoelectric material that are embedded in a host material such as rubber. If elastic waves are incident on such a medium, both elastic and electromagnetic waves are produced due to the piezoelectric effect. We use a self-consistent multiple scattering theory that takes into account the interacting elastodynamic and electromagnetic fields as well as statistical correlations among the cylinders that enter the expressions for the average fields in this medium. The response of a single piezoelectric cylinder to applied elastic and electromagnetic fields is characterized by a T matrix. From the configurationally averaged fields, the effective elastic, electromagnetic, and piezoelectric properties can be obtained. In our computations, BaTiO3 fibers embedded in a rubberlike host material with different rigidities are considered. Numerical results are presented for both coherent phase velocity and attenuation as a function of frequency and concentration of the piezoelectric cylinders. To evaluate the piezoelectric effect in altering the propagation characteristics of the waves, the numerical results obtained are compared to those obtained suppressing this effect.

Discrimination of coherent features in turbulent boundary layers by the entropy method

Entropy in information theory is defined as the expected or mean value of the measure of the amount of self-information contained in the ith point of a distribution series x sub i, based on its probability of occurrence p(x sub i). If p(x sub i) is the probability of the ith state of the system in probability space, then the entropy, E(X) = - sigma p(x sub i) logp (x sub i), is a measure of the disorder in the system. Based on this concept, a method was devised which sought to minimize the entropy in a time series in order to construct the signature of the most coherent motions. The constrained minimization was performed using a Lagrange multiplier approach which resulted in the solution of a simultaneous set of non-linear coupled equations to obtain the coherent time series. The application of the method to space-time data taken by a rake of sensors in the near-wall region of a turbulent boundary layer was presented. The results yielded coherent velocity motions made up of locally decelerated or accelerated fluid having a streamwise scale of approximately 100 nu/u(tau), which is in qualitative agreement with the results from other less objective discrimination methods.

Sea Surface Flow Estimation with Infrared and Visible Imagery

Abstract Sequential Nimbus-7 CZCS infrared and visible images obtained on orbits 3157 and 3171 during 9–10 June 1979 have been used to derive sea surface flow from advective sea surface pattern displacements and elapsed time. Individual analyses with infrared (11 microns) and visible blue/yellow ratio (0.443 and 0.550 microns) pairs of images yielded coherent velocity distributions over an oceanic region near Georges Bank. A composite of eighty flow vectors illustrates a seaward diversion of cold surface water off Northeast Channel, Gulf of Maine by a northeastward intrusion of Gulf Stream water along the continental slope. These results demonstrate that instances arise when infrared and visible surface pattern changes can be used jointly to compose flow regimes. A sea surface topography map derived from the composite vector distribution has a range of 20 cm and an expected repeatability of 0.39 cm.

Potential for coherent Doppler wind velocity lidar using neodymium lasers

Existing techniques for the frequency stabilization of Nd: YAG lasers operating at 1.06 μm, and the high-gain amplification of radiation at that wavelength, make possible the construction of a coherent Doppler wind velocity lidar using Nd:YAG. Velocity accuracy and range resolution are better at 1.06 μm than at 10.6 μm at the same level of the SNR. Backscatter from the atmosphere at 1.06 μm is greater than that at 10.6 μm by ~2 orders of magnitude, but the quantum-limited noise is higher by 100 also. Near-field attenuation and turbulent effects are more severe at 1.06 μm. In some configurations and environments, the 1.06-μm wavelength may be the better choice, and there may be technological advantages favoring the use of solid-state lasers in satellite systems.

Dynamics of an incommensurate harmonic chain

The dynamics of a one-dimensional harmonic chain in a strong, incommensurate, sinusoidal potential and a uniform force $F$ is investigated. A threshold force ${F}_{t}$ exists, above which the mean velocity $\overline{v}$ of the chain satisifes $\overline{v}\ensuremath{\propto}{(F\ensuremath{-}{F}_{t})}^{\ensuremath{\zeta}}$ where $\ensuremath{\zeta}=0.68\ifmmode\pm\else\textpm\fi{}0.05$. We present evidence that no coherent velocity oscillations occur in the infinite system. The possible relevance of this work to experimental observations of charge-density waves is discussed.

A constrained approximation for nuclear barrier penetration and fission

An approximation to the time-dependent mean-field theory for barrier penetration by a nucleus is obtained in terms of constrained Hartree-Fock wave functions and a coherent velocity field. A discrete approximation to the continuum theory suitable for practical numerical calculations is presented and applied to three illustrative models. Potential application of the theory to the study of nuclear fission is discussed.

Multi-mode analysis of Rayleigh-typeLg. Part 1. Theory and applicability of the method

abstractRayleigh-type Lg propagating in a laterally homogeneous continental crust can be synthesized by adding only a few overtones at periods greater than 2 sec. Under minimal assumptions, we show that wavenumber analysis of Lg recorded on a several hundred kilometers long linear array of 10 stations allow us to isolate the different overtones, providing a tool to study crustal structures and excitation of the overtones at the source. In this first paper, we use synthetic Lg seismograms to investigate the applicability of a time-frequency-wavenumber analysis technique (UC diagram algorithm) to realistic arrays of stations. The behavior of the algorithm in the presence of lateral heterogeneities is studied numerically by introducing either random or coherent phase perturbations. We find that (1) the method is tractable if random phase fluctuations from station to station are spread over less than half a cycle, and (2) coherent velocity changes between two halves of a profile are spatially averaged if they are too small to be resolved by the array.

Coherent structures induced by two simultaneous sparks in an axisymmetric jet

The axisymmetric mixing layer of a 12.7-cm-diam circular air jet at the exit speed of 20 m/s has been excited by two sparks fired simultaneously at diametrically opposite points near the lip. The resulting large-scale coherent structure—buried in the large-amplitude turbulent fluctuations in the mixing layer—has been educed at three streamwise stations and along three azimuthal planes. Spatial distributions of the coherent structure velocity perturbations, coherent Reynolds stress, vorticity, pseudo-streamfunctions, intermittency, as well as phase averages of the background fluctuation intensities and Reynolds stress have been deduced. The coherent Reynolds stress is considerably larger than the background turbulence Reynolds stress and the coherent peak vorticity is noticeably larger than time-mean peak vorticity. The coherent structure becomes progressively weaker with increasing x. The azimuthal variations of the educed contours suggest that the induced structure is a naturally occurring structure triggered by the sparks and is essentially axisymmetric; it undergoes a rapid change in its geometry, dynamics, and convection velocity as it travels downstream.

Coherent electromagnetic wave propagation through randomly distributed dielectric scatterers

We present a vector multiple-scattering analysis of the coherent wave propagation through an inhomogeneous media consisting of a random distribution of identical, oriented, nonspherical, dielectric scatterers. The single-scattering aspect of the problem is dealt with through application of the transition or $T$ matrix. Configurational averaging techniques are employed to determine the hole correction integrals which are subsequently solved to yield the dispersion relations characterizing the bulk or effective properties of the medium. Closed-form solutions in the Rayleigh limit are derived for both spherical and spheroidal scatterer geometries. These solutions, together with the $T$ matrix, form the basis of our computational method for determining the coherent wave phase velocity and attenuation as a function of frequency ($\mathrm{ka}$) and scatterer concentration. Numerical results are presented for spherical and oblate spheroidal geometries over a range of $\mathrm{ka}$ values (0.05-2.0) and scatterer concentrations (0.05-0.20).