Decrease In Phase Velocity Research Articles

Numerical study of the effect of periodic jet excitation on cylinder aerodynamic instability

Numerical simulations based on the ALE finite element method are carried out to examine the aerodynamics of an oscillating circular cylinder when the separated shear flows around the cylinder are stimulated by periodic jet excitation with a shear layer instability frequency. The excitation is applied to the flows from two points on the cylinder surface. The numerical results showed that the excitation with a shear layer instability frequency can reduce the negative damping and thereby stabilize the aerodynamics of the oscillating cylinder. The change of the lift phase seems important in stabilizing the cylinder aerodynamics. The change of lift phase is caused by the merger of the vortices induced by the periodic excitation with a shear layer instability frequency, and the vortex merging comes from the high growth rate, the rapid increase of wave number and decrease of phase velocity for the periodic excitation in the separated shear flows.

Evaluation of the Interface Between Two Plates by the Lamb Wave

A new expression of the dispersion equation of the Lamb wave in anadhesive two-layered plate is presented. The bond rigidity of theadhesive plate is accounted for in terms of a spring model. Theinfluence of the variations of the compliance constants of the springon the dispersion feature of Lamb modes is numerically studied.The numerical results indicate that the deterioration of the bondrigidity may cause the phase velocity decrease and thefrequency-shift for a given Lamb mode, thus having a possibility forthe evaluation of the bonding state of the adhesive plate by usingultrasonic wave velocity measurements. The phase-tracing method is used tomeasure the phase velocities experimentally and the results for anadhesive plate of aluminium-aluminium are given to verify thetheoretical predictions.

Nonlinear analysis of microwave superconductor devices using full-wave electromagnetic model

This paper presents a full electromagnetic wave analysis for modeling the nonlinearity in high temperature superconductor (HTS) microwave and millimeter-wave devices. The HTS nonlinear model is based on the Ginzburg-Landau theory. The electromagnetic fields associated with the currents on the superconducting structure are obtained using a three-dimensional full wave solution of Maxwell's equations. A three-dimensional finite-difference time-domain algorithm simultaneously solves the resulting equations. The entire solution is performed in time domain, which is a must for this type of nonlinearity analysis. The macroscopic parameters of the HTS, the super fluid penetration depth and the normal fluid conductivity, are calculated as functions of the applied magnetic field. The nonlinear propagation characteristics for HTS transmission line, including the effective dielectric constant and the attenuation constant, are calculated, As the power on the transmission line increases, the phase velocity decreases and the line losses increase. The nonlinearity effects on the current distributions inside the HTS, the electromagnetic field distributions, and the frequency spectrum are also analyzed.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Lamb waves in microstructured plates

The velocity dispersion of guided plate waves has been derived and studied in a plate composed of a coherent microstructured material. The calculated ultrasonic velocity dispersion arises from two sources: the conventional geometric Lamb wave dispersion due to the vanishing of tractions on the plate boundary, and microstructural dispersion due to the comparability of the ultrasonic wavelength and the microstructural dimension. The microstructure is assumed to take the form of laminations oriented vertically to the plate surfaces. The guided sound wave propagates in a direction parallel to the layer interfaces. Microstructural dispersion is treated by the continuum mixture approach, where the spatial dependence of field variables normal to the lamina interfaces has been approximated by averaging these quantities across the lamina thickness. This procedure reduces the dimensionality of the wave equation, but yields additional frequency dependent terms which account for the dispersive nature of the original material system. It is found that the fundamental Lamb waves undergo significant modification as the ultrasonic wavelength approaches the microstructural dimension. For the symmetric mode, this effect consists of a much more rapid decrease in the Lamb wave phase velocity as it approaches the Rayleigh velocity of the slower medium, instead of the mixture value. The antisymmetric mode displays a broad maximum, whose frequency of occurence is dependent on the ratio of plate thickness to microstructural dimension. A specific case is analysed numerically and physically, and limitations of the model are discussed in the context of real composite material systems.

Full‐wave synthetic acoustic logs in radially semiinfinite saturated porous media

The wave field generated by a point source in an axisymmetric fluid‐filled borehole embedded in a saturated porous formation is studied in both the spectral domain and time domain. The formation is modeled following Biot theory modified in accordance with homogenization theory. When the borehole wall is permeable, guided waves can be significantly affected by the permeability of the formation. Whatever the formation, fast or slow, Stoneley‐wave phase velocity and energy decrease and attenuation (in the sense of [Formula: see text]) increases with increasing permeability. These effects are more important in the very low‐frequency range, where Darcy’s law governs the fluid motion and the wave energy at the interface is maximum, than at higher frequencies. The effects increase and persist over a larger frequency range with decreasing viscosity and increasing compressibility of the saturant fluid, with increasing pore‐fluid volume, and with decreasing borehole radius. In contrast, the effects decrease with decreasing stiffness of the formation because of more efficient coupling of the interface wave to the surrounding medium. When present, the first pseudo‐Rayleigh mode also carries useful information. Fluid flow affects only the attenuation of the pseudo‐Rayleigh mode’s Airy phase; an increase in attenuation may be used to detect permeable zones and to infer the saturant fluid properties. However, the most reliable types of information are the formation shear‐wave velocity and attenuation from the low‐frequency part of the mode. In the time domain, all the modes overlap. Any signal processing should then be performed in the frequency domain, where mode spectra are more easily separable. The frequency band of the actual logging tool has to be large enough to ensure significant amplitude for each mode. Finally, the larger the number of receivers and the offset range, the better.

Control of human optokinetic nystagmus by the central and peripheral retina: Effects of partial visual field masking, scotopic vision and central retinal scotomata

Optokinetic nystagmus (OKN) was elicited in humans by a horizontally moving grating covering the whole visual field. Selective stimulation of central or peripheral parts of the retina was achieved by partial masking or scotopic viewing conditions in normals; three patients with a unilateral central retinal scotoma were studied in addition. In all cases, the elimination of foveal stimulation was accompanied by a decrease in OKN slow phase velocity compared to whole field stimulation. Vertical masks with retinally stabilized edges were used to selectively occlude or stimulate central or peripheral sectors with a fixed retinal location. A central stimulus was always more effective than the complementary peripheral stimulus, until the central zone was narrowed down to a width of 5–10°. This central dominance was found throughout the range of velocities (6–180°/s) and apatial frequencies (0.05–0.5 cycles/deg) used. A horizontal central band of occlusion caused a smaller decrease of OKN than a vertical occlusion with the same width. Scotopic vision caused a uniform mild decrease in OKN gain throughout the velocity range, provided that the spatio-temporal frequency of the stimulus remained within the scotopic resolution range. The patients had a slightly lower OKN gain when viewing with the scotomatous eye than with the contralateral, normal eye. The normal slight preference for temporal-to-nasal motion was not accentuated by masking or scotopic vision in normal eyes, but was enhanced in the eyes with the pathological scotomata in two of the three patients. All responses were immediate; no slow build-up was seen under any condition.

Propagation and attenuation of intense tones in air‐filled porous materials

In a previous presentation [J. Acoust. Soc. Am. Suppl. 1 74, S59 (1983)] it was demonstrated that the nonlinear mechanism for propagation in bulk porous material is an amplitude‐dependent resistivity of the form r = r1 + r2u sgn(u), where u is the instantaneous particle velocity and r1 and r2 are constants that depend on porosity and material geometry. An approximate Helmholtz equation was derived that governs the change in signal harmonies with distance. Recently obtained numerical solutions of this equation compare favorably with experimental data. By assuming that harmonic interaction is weak, we have obtained analytical solutions for the amplitude and phase of the fundamental. It is found that the attenuation increases at high intensity, while at the same time the phase velocity decreases. Experiments have been done with Kevlar® 29 in the porosity range 0.94 to 0.98, in the frequency range 500–1500 Hz, at SPLs up to 165 dB. The experimental data confirm the predictions. [Work supported by NASA and ONR.]

Efficiency enhancement of coupled-cavity TWT's through cavity resonance tapering

Beam-wave resynchronization through circuit velocity reduction is a well-known method of traveling-wave tube (TWT) efficiency enhancement. Circuit-velocity reduction in coupled-cavity TWT's is usually accomplished through period tapering, that is, the periodic length is reduced with the circuit resonant frequencies kept more or less unchanged. However, the amount of period tapering permitted is limited by stability considerations. Beyond a certain critical value of velocity reduction, the tube may be subject to zero drive oscillations originating in the velocity taper region at frequencies near the low-frequency cutoff of the circuit (lower band-edge oscillations). An alternate approach to circuit-velocity reduction in coupled-cavity TWT's is suggested. This approach, referred to as cavity resonance tapering, allows the velocity reduction to continue beyond the limit of stable period tapering. Cavity resonance tapering is accomplished by a gradual reduction in the cavity resonant frequency with the period kept unchanged and the circuit bandwidth kept more or less unchanged. For any frequency that remains in the circuit passband, there is a gradual increase in phase shift per period and, hence, a gradual decrease in phase velocity. The advantages and disadvantages of cavity resonance tapering vis-a-vis period tapering are discussed. Cold test data and the results of large-signal computer calculations are presented for illustrative purposes. Results indicate that, when applicable, cavity resonance tapering can produce efficiencies as high as can period tapering without incurring the same risk of lower band-edge oscillations.

Change in Rayleigh wave phase velocity due to decrease of P-wave velocity before an earthquake

20per cent decrease of P-wave velocity has often been noted by many workers before an earthquake. Change in Rayleigh wave phase velocity due to such a decrease is studied so as to note the possibility of using the phase velocity change as one of the methods to predict earthquake. For this purpose theoretical phase velocity and its change due to the decrease of P-wave velocity have been obtained for a two-layered crustal model over a homogeneous mantle. 20 per cent decrease of P-wave velocity in the first layer gives a decrease of phase velocity by 4 per cent over a wide range of period with a maximum decrease of 4.8 per cent. While 20 per cent decrease of P-wave velocity in the both first and second layers of the crust gives a phase velocity decrease of 5 per cent over a wide range of period with a maximum decrease of 7.0 per cent.

On the Interaction of Vestibular and Optokinetic Nystagmus in Man

The interaction of vestibular and optokinetic nystagmus (OKN) was investigated by applying periods of ipsi- and contradirectional optokinetic stimulation during perand postrotatory vestibular nystagmus in healthy humans. Slow phase velocity and cumulative eye displacement of OKN was enhanced by vestibular nystagmus beating in the same direction of decreased by contradirectional vestibular nystagmus. A distinct correlation between the intensity of vestibular stimulation and the modification of OKN could be demonstrated by averaging the responses obtained in different subjects. On the other hand there was also a modification (enhancement or decrease) of vestibular nystagmus slow phase velocity by preceding OKN. Considering the intensity of these modifications, large interindividual differences were observed.

The free shear layer tone phenomenon and probe interference

Free shear layer stability measurements with a hot wire revealed that the probe itself can trigger and sustain upstream instability modes like the slit jet-wedge edge tones. The flow fields associated with the free shear layer tones induced in axisym-metric and plane air shear layers by a hot-wire probe and by a plane wedge were then explored experimentally, and found to be different in many ways from the widely investigated jet edge tone phenomenon.As many as four frequency stages have been identified, there being a fifth stage associated with the subharmonic attributed to vortex pairing in the free shear layer. No evidence of hysteresis could be found in the shear layer tone. In the interstage jump (i.e. bimodal) regions, the tone occurred in only one mode at a time while intermittently switching from one to the other. Frequency variations in each stage are shown to collapse on a single curve when non-dimensionalized with the initial momentum thickness θeor with the lip-wedge distanceh, and plotted as a function ofh/θe.Phase average measurements locked onto the tone fundamental show that the phase velocity and wavelength of the tone-induced velocity fluctuation are essentially independent of the stage of tone generation; in each stage, both phase velocity and wavelength decrease with increasing frequency but undergo jumps at starts of new stages. The measured amplitude and phase profiles, as well as the variations of the shear tone wavenumber and phase velocity with the Strouhal number, show reasonable agreement with the predictions of the spatial stability theory. The wavelength λ bears a unique relation toh, thish, δ relation being different from the Brown-Curle equation for the jet edge tone.Shear layer tones would be typically induced in near-field shear layer measurements involving invasive probes, and can produce misleading results. A method for determining the true free shear layer natural instability frequency is recommended.

Ion-acoustic waves in large radio-frequency electric fields

The propagation of ion-acoustic waves in high frequency fields is experimentally studied. The resulting phase-velocity decrease and enhanced damping are observed for radio-frequency field amplitudes up to αe≃0.5, where αe=eE/(meωovc) is the normalized amplitude. Good agreement with the theory is found.

A model exciter for type III solar radiobursts

In an earlier paper (Harvey and Aubier, 1973) the large scale radial electron density gradient in the corona and solar wind was shown to cause the phase velocity of plasma waves to decrease as they propagate away from the Sun, thus leading to appreciable Landau damping of the plasma waves. It is proposed here that this same phase velocity decrease creates conditions which facilitate the stabilisation of a beam of exciter electrons of finite duration, provided that three conditions are fulfilled. Two of these conditions concern the velocity-time distribution of the exciter electrons at their point of ejection from the Sun, while the third is simply that, above a certain altitude, the coronal electron density decreases with altitude r faster than r−2. The plasma wave source is then associated with the leading edge of the electron stream. The spatial density of the power converted into plasma waves is calculated as a function of position and time, and is shown to be independent of the nature of the stabilisation mechanism. The maximum of this power density is found to move outwards from the Sun at a uniform speed when a simple electron injection model with a Maxwellian velocity distribution is introduced.

The influence of optokinetic training upon vestibular habituation. Ok-induced modification of vestibular responses elicited by sinusoidal stimuli.

Unidirectional optokinetic training modifies the response pattern of bidirectional sinusoidal vestibular stimuli (pendular rotation), resulting in an initial response decline. The latter affects only slow phase velocity, whereas the former shows a frequency increase. Optokinetic training obviously has a facilitating effect upon vestibular habituation which cannot be induced by repeated sinusoidal vestibular stimuli alone. Peak frequency increase as well as slow phase velocity decrease of vestibular nystagmus, induced by optokinetic stimuli, are interpreted as a typical transfer mechanism across two sensory systems.

Kelvin wave propagation along an irregular coastline

We discuss the theory of Kelvin wave propagation along an infinitely long coast-line which is straight except for small deviations which are treated as a stationary random function of distance along the coast. An operator expansion technique is used to derive the dispersion relation for the coherent Kelvin wave field. For the subinertial case σ = ω/f &lt; 1 (ω = wave frequency, f = Coriolis parameter), it is shown that the wave speed is always decreased by the coastal irregularities. Moreover, while the coherent wave amplitude is unaltered, the energy flux along the coast is decreased by the irregularities. For the case σ &gt; 1, however, we show that in the direction of propagation the wave is attenuated (with the energy being scattered into the random Poincaré and Kelvin wave modes) and that the wave speed is again decreased. Applications of the theory are made to the California coast and North Siberian coast to determine the decrease in phase velocity due to small coastal irregularities. For the California coast the percentage decrease is only about 1%. For the Siberian coast, however, the percentage decrease is about 25% for the K1 tide, and a minimum of 25% for the M2 tide. The attenuation of a Kelvin wave, however, appears to be due to very large scale irregularities. An estimate of the actual attenuation rate is not possible, though, because of the relatively short extent of coastal contours available for spectral analysis.Although attention in this paper has been focused on Kelvin wave propagation, the method developed could readily be used to study the behaviour of other classes of waves trapped against a randomly perturbed boundary.

Effect of a transverse interlayer on stresses in a bar under transient loads

1. Equations of the linear elasticity theory can be used for determining stresses in a bar with a transverse interlayer with sufficient accuracy up to stresses equal to the dynamic elastic limit. In the case of large stresses the calculation gives a considerable overestimate of the stresses relative to the actual. 2. In the case of plastic deformation of interlayers whose length is less than some critical value (2.5 mm under our experimental conditions) there occurs an equalization of the maximum stresses at the boundaries and their sharp increase with a decrease of the length of the interlayer due to the predominant effect of interference of plastic waves. For interlayers whose length is greater than the critical a major role is played by damping of the waves and decrease of phase velocity due to plastic deformation. 3. Plastic deformation of the interlayer does not change the duration of the wave propagating in the composite bar.

地球のねじり振動

A method is given to calculate tortional frequencies of an elastic sphere of variable density and elasticity. By this method, the tortional frequencies are calculated of the earth, the internal constitution of which is inferred from the study of body wave propagations within it. The period of fundamental tortional oscillation of the earth is shown to be 43.4 minutes. The phase velocity of the corresponding surface wave has a maximum value 6.90km/sec for a wavelength of about 10000km. The decrease of phase velocity for longer wave lengths may be due to the existence of the earth core at the depth of 2900km.