Different Modes Of Oscillation Research Articles

Oscillating plasma bubbles. III. Internal electron sources and sinks

An internal electron source has been used to neutralize ions injected from an ambient plasma into a spherical grid. The resultant plasma is termed a plasma “bubble.” When the electron supply from the filament is reduced, the sheath inside the bubble becomes unstable. The plasma potential of the bubble oscillates near but below the ion plasma frequency. Different modes of oscillations have been observed as well as a subharmonic and multiple harmonics. The frequency increases with ion density and decreases with electron density. The peak amplitude occurs for an optimum current and the instability is quenched at large electron densities. The frequency also increases if Langmuir probes inside the bubble draw electrons. Allowing electrons from the ambient plasma to enter, the bubble changes the frequency dependence on grid voltage. It is concluded that the net space charge density in the sheath determines the oscillation frequency. It is suggested that the sheath instability is caused by ion inertia in an oscillating sheath electric field which is created by ion bunching.

Modal dependence of dissipation in silicon nitride drum resonators

We have fabricated large (≤400 μm diameter) high tensile stress SiN membrane mechanical resonators and measured the resonant frequency and quality factors (Q) of different modes of oscillation using optical interferometric detection. We observe that the dissipation (Q−1) is limited by clamping loss for pure radial modes, but higher order azimuthal modes are limited by a mechanism which appears to be intrinsic to the material. The observed dissipation is strongly dependent on size of the membrane and mode type. Appropriate choice of size and operating mode allows the selection of optimum resonator designs for applications in mass sensing and optomechanical experiments.

Similarities between the tropical Atlantic seasonal cycle and ENSO: An energetics perspective

The tropical Pacific and Atlantic Oceans have similar mean states ‐ easterly winds and a zonally sloping thermocline which shoals in the east ‐ but strikingly different sea surface temperature variability. Seasonal and interannual variations have comparable amplitudes, and correspond to different modes of oscillation in the Pacific. In the Atlantic, the seasonal cycle is dominant and has properties of both those modes. The Bjerknes feedback between the wind and SST, and its associated delayed, negative feedback constitute a free (interannual) mode of the Pacific, but in the Atlantic influence the seasonal cycle. This difference between the two oceans is attributable to the smaller dimensions of the Atlantic. An energetic analysis shows a circular relationship between available potential energy and the work done by the wind from April until September in the Atlantic. This energetics perspective suggests that a seasonally excited thermocline mode of coupled variability plays an active role in the seasonal cycle during these months, after which point seasonal forcing regains control.

Spatial instability analysis in pressurized water reactors

A simple methodology has been developed to assess the spatial dynamic behavior of large PWRs against xenon spatial instability in different modes. Method of analysis aims to analyze xenon dynamic behavior against anticipated reactivity perturbations. Reactivity perturbations in different modes have been evaluated based on reactivity device movements as well as localized thermal variations in the core. Effect of individual core design and operating parameters on xenon spatial instability has been studied. Behavior of spatial stability index (SI) with core size is investigated. Based on SI-core size curve, a threshold core size has been determined beyond which a PWR core tends to become spatially unstable. Methodology has been used to assess the spatial xenon dynamic behavior of different modes of oscillations in VVER1000 and AP1000 reactor cores.

Improvement of Plasma Spray Torch Stability by Controlling Pressure and Voltage Dynamic Coupling

The development of coating formation processes involving electric arcs depends on process stability and the capacity to ensure a constant reproducibility of coating properties. This is particularly important when considering suspension plasma spraying or solution precursor plasma spraying. Submicron particles closely follow plasma instabilities and have nonhomogeneous plasma treatment. Recently, it has been shown that arc voltage fluctuations in direct-current (dc) plasma torches, showing dominant fluctuation frequencies between 4 and 6 kHz, are linked to pressure oscillations in the cathode cavity of the plasma torch. In this study, first, a method to isolate the different oscillation modes in arc voltage and pressure signals using signal processing methods is presented. Second, correlations between the different modes of oscillations are analyzed following the plasma torch operating parameters. Lastly, it is shown that the use of an acoustic stub, mounted on the torch body, decreases the amplitude of arc voltage fluctuations and slightly increases the mean voltage.

Double-diffusive Marangoni convection in a rectangular cavity: Onset of convection

Double-diffusive Marangoni convection in a rectangular cavity with horizontal temperature and concentration gradients is considered. Attention is restricted to the case where the opposing thermal and solutal Marangoni effects are of equal magnitude (solutal to thermal Marangoni number ratio Rσ=−1). In this case a no-flow equilibrium solution exists and can remain stable up to a critical thermal Marangoni number. Linear stability analysis and direct numerical simulation show that this critical value corresponds to a supercritical Hopf bifurcation point, which leads the quiescent fluid directly into the oscillatory flow regime. Influences of the Lewis number Le, Prandtl number Pr, and the cavity aspect ratio A (height/length) on the onset of instability are systematically investigated and different modes of oscillation are obtained. The first mode is first destabilized and then stabilized. Sometimes it never gets onset. A physical illustration is provided to demonstrate the instability mechanism and to explain why the oscillatory flow after the onset of instability corresponds to countersense rotating vortices traveling from right to left in the present configuration, as obtained by direct numerical simulation. Finally the simultaneous existence of both steady and oscillatory flow regimes is shown. While the oscillatory flow arises from small disturbances, the steady flow, which has been described in the literature, is induced by finite amplitude disturbances.

Adiabatic perturbations in homologous conventional polytropic core collapses of a spherical star

We perform a non-radial adiabatic perturbation analysis on homologous conventional polytropic stellar core collapses. The core collapse features a polytropic exponent $\Gamma=4/3$ relativistic gas under self-gravity of spherical symmetry while three-dimensional perturbations involve an adiabatic exponent $\gamma$ with $\gamma\neq\Gamma$ such that the Brunt-V$\ddot{\rm a}$is$\ddot{\rm a}$l$\ddot{\rm a}$ buoyancy frequency ${\cal N}$ does not vanish. With proper boundary conditions, we derive eigenvalues and eigenfunctions for different modes of oscillations. In reference to stellar oscillations and earlier results, we examine behaviours of different modes and the criterion for instabilities. The acoustic p$-$modes and surface f$-$modes remain stable. For $\gamma<\Gamma$, convective instabilities appear as unstable internal gravity g$^{-}-$modes. For $\gamma>\Gamma$, sufficiently low-order internal gravity g$^{+}-$modes are stable, whereas sufficiently high-order g$^{+}-$modes, which would have been stable in a static star, become unstable during self-similar core collapses. For supernova explosions, physical consequences of such inevitable g$-$mode instabilities are speculated.

On the Flutter and Drag Forces on Flexible Rectangular Canopies in Normal Flow

The paper presents results of experimental investigations on flexible rectangular canopies fixed at two edges (tapes). Data are presented for the aerodynamic drag as a function of the planform area, the slenderness, and the Reynolds number. The slenderness varies from 3.3 to 30. The Reynolds number is limited to 2×106. The wind speed is in the range 6–19m∕s. The results show that the drag decreases with the increasing aspect ratio. The dominant parameter is the slenderness; a second dominant factor is the planform area. The drag is only weakly dependent on the Reynolds number. The analysis of the drag data indicates that a proper scaling parameter is the crosswise length scale. When normalization is done, the data tend to collapse into a single curve. A further study addressed the drag characteristics of the canopies with perforated planforms. Perforation of up to 6% of the planform area has been considered. Corrections for air permeability of the fabric are introduced. The drag is reduced roughly linearly with perforated area. A detailed study into the flutter characteristics of the canopies in the turbulent wind is presented. The investigation has highlighted different modes of oscillations, including deformations through folding and twisting, vertical flapping, rotations, and cross-type oscillations.

Experimental study of the frequency repulsion effect

We discuss an experiment on coupled RLC circuits with variable coupling strength. The inductive coupling can be easily varied and measured for different configurations. The experiment allows us to explore the variation of the resonance frequencies as a function of the coupling strength between the oscillators. The system illustrates the effect of eigenfrequency repulsion when an interaction couples different modes of oscillation. The experiment is conceptually simple, and its results can be compared quantitatively with theoretical predictions.

Signature of quasi–biennial oscillation and long-period equatorial disturbances in the lower atmosphere over Thiruvananthapuram

An attempt has been made in this paper to examine different modes of oscillation in the wind field during different seasons over Thiruvananthapuram (lat. 8.29° N, long. 76.59° E, located at the extreme southwest coast of India) based on daily upper air observations for the period from January 1997 to December 1999. A power spectral analysis is carried out with the upper air data of the station. The study shows that one and half cycle of Quasi–Biennial Oscillation (QBO) and the power spectra of the meridional wind component exhibit peaks between the period of four days and seven days (corresponding frequency range between 0.25 day−1 and 0.15 day−1) during all seasons. The seasonal variation of these large-scale oscillations over the station depends upon the background mean zonal flow, which in turn closely related to the QBO structure.

OSCILLATIONS AND VORTEX-INDUCED VIBRATIONS OF A TETHERED SPHERE IN A FLOW

We have investigated the wake of a tethered sphere in a steady uniform flow, which is a typical and basic example of fluid-structure interaction. Experiments have been conducted in a vertical water channel with a circular “plexiglass” test section of diameter 8 cm and of length 100 cm. The free upper surface of water is kept at a constant level and the current in the channel is created by gravity. The sphere, made of glass of density ?sphere = 2430 kg/m3, and of diameter D = 16.6 mm, is attached with glue to a thread supported by a small horizontal metallic pipe. The velocity of the flow is deduced from flow-rate measurement, after corrections taking into account the development of boundary layers along the wall of the channel and the blockage effect due to the sphere. The length L of the thread has been varied from 15 cm to 35 cm. A mirror has been positioned behind the channel with an angle of 45° to the front direction in order to get on the same image two perpendicular views Images have been recorded thanks a Camescope Sony on a numeric memory and then transferred to Mac for post-treatment. The software i-movie has been used to select the best parts of the continuous movement, which was sequenced by QuickTime at the rate of 5 to 8 images per second. When increasing the velocity of the upstream fluid U above a critical value, we have observed oscillation of the sphere induced by the periodic vortex-shedding. Different modes of oscillation of the sphere along circular, straight or elliptic trajectories are possible. Moreover, the transition from steady to oscillatory movement of the sphere reveals an hysteresis when increasing or decreasing the velocity. Moreover, experiments performed in a horizontal larger channel water have shown the existence of different oscillation modes and the desynchronization of the vortex shedding process.

Complex Oscillatory Behavior of the Mercury Beating Heart System

We show the dynamics of the oscillatory Mercury Beating Heart (MBH) reaction in acid aqueous solution with [CeIV(SO4)3]2− as oxidizing agent. The rich dynamics is appropriate to introduce students to oscillatory chemical reactions and nonlinear concepts. These oscillations display different modes of oscillations with geometric structures similar to heart, circle, and pointed stars. We consider oxidation-reduction reactions to explain the different periods of oscillations due mainly to the formation of mercury(I) species and plot the socalled reconstruction attractors from the variation of the Hg electrode potential in time.

Microwave oscillation from Bi2Sr2CaCu2Oy intrinsic Josephson junction arrays

We report the novel nonequilibrium microwave emission from current-injected Bi2Sr2CaCu2Oy (BSCCO)/I/Au intrinsic Josephson junctions and two-mesa array of intrinsic Josephson junctions. For the single intrinsic junctions, it appeared as three different modes of oscillations. Among them, two modes are Josephson plasma emissions which appeared at the quasiparticle branches and the gap-edges. The mutual phase interaction between two-mesa structures of BSCCO intrinsic Josephson junctions has been studied. The measured output power of the two-mesa structures of intrinsic Josephson junctions was almost four times larger than that of the single junction, indicating an exact phase-locking state between the two intrinsic Josephson junctions. The maximum detected power was about microwatt level at millimetre-wave range.

Study of the Extinction Dynamics of the Mercury Beating Heart Reaction in Acid Solution and in the Presence of γ-Radiation

The dynamics of the oscillatory mercury beating heart (MBH) reaction in acid aqueous solution with CeIV as the oxidizing agent was investigated. The reaction slowly runs down over a period of a few hours, until all oscillatory activity eventually ceases. During this period the dynamics slowly evolves, showing qualitatively different forms of oscillations. These oscillations display different modes of oscillations with geometric structures similar to heart, circle, pentagon, hexagon, and 8- and 16-pointed stars. As time proceeds, limit-cycle oscillations of period-1, period-3, and period-2 appear successively. However, these oscillations are damped and could be interpreted in terms of a subcritical Hopf bifurcation. We propose different oxidation−reduction reactions to explain the appearance of these cycles based on the formation of mercury(I) species, in the form of free Hg22+ ion or, in molecular form, as a soluble mercurous sulfate or a mercurous sulfate film. The theoretical potential values calculated for these reactions agree well with our experimental values. An experiment was performed also without adding the CeIV oxidant, generating in situ species of Hg(I) by γ-irradiating the metallic Hg in the same acid solution as before. It is shown that 60Co γ-rays induce the generation of oscillations. In this way we support our claim that oscillations are due to the formation of chemical species of Hg(I) be it ionic or molecular and that extinction occurs when [Hg(II)] ≫ [Hg(I)].

Stability analyses of two-temperature radiative shocks: formulation, eigenfunctions, luminosity response and boundary conditions

We present a general formulation for stability analyses of radiative shocks with multiple cooling processes, longitudinal and transverse perturbations, and unequal electron and ion temperatures. Using the accretion shocks of magnetic cataclysmic variables as an illustrative application, we investigate the shock instabilities by examining the eigenfunctions of the perturbed hydrodynamic variables. We also investigate the effects of varying the condition at the lower boundary of the post-shock flow from a zero-velocity fixed wall to several alternative types of boundaries involving the perturbed hydrodynamic variables, and the variations of the emission from the post-shock flow under different modes of oscillations. We found that the stability properties for flow with a stationary-wall lower boundary are not significantly affected by perturbing the lower boundary condition, and they are determined mainly by the energy-transport processes. Moreover, there is no obvious correlation between the amplitude or phase of the luminosity response and the stability properties of the system. Stability of the system can, however, be modified in the presence of transverse perturbation. The luminosity responses are also altered by transverse perturbation.

Temperature Dependence of Phonon Hole Position

The frequency where the phonon hole is detected in piezoelectric powder particles shows negative temperature dependence. When the temperature is decreased or increased from the temperature at which the hole was written into the piezoelectric powder, the half-height width of the hole becomes broader and the hole height decreases, and the hole finally becomes undetectable. Then as the temperature returns to the writing temperature, the hole recovers its form. These phenomena are explained as being due to the specimen particles not being uniform in size and/or the crystallographic angle and the hole being composed of many different modes of oscillation whose temperature dependences differ from each other.

A Spatial Analysis of Local Sources of Oscillation

Recent observational studies of the spatial and temporal relations between impulsive and periodic perturbations (Deubner and Kleineisel, 1997; Espagnet et al., 1996; see also Goode et al., these proceedings) suggest that oscillations in the solar atmosphere are excited locally and stochastically by turbulent convection. Proper characterization of the dynamical processes involved requires careful analysis of the observations in the spatial as well as in the temporal domain. Wavelet transforms (Combes et al., 1984) allow the characterization of waveforms together with the temporal delays that occur between the observed fluctuations. We have employed Morlet wavelets (Daubechies, 1992) in the present analysis. In addition to the phase relation between convective eddies and the atmospheric waves we study in particular the 2-D spatial evolution of the perturbations, to better distinguish different modes of oscillation. The observational material used is the same as in Deubner and Kleineisel (1997).

Oscillations of Charged Helium II Drops

We report on an experimental study of the shape oscillations of charged helium drops levitated with a magnetic field. Shape oscillations are excited with an AC electric field. Many different modes of oscillation of the drop are observable. The resonant frequencies of the drops are found to be a function of amplitude. Quantitative measurements of the damping of shape oscillations are made by using a laser beam focused through the drop. The observed damping of shape oscillations is found to be greater than the damping due to the viscosities of the liquid and the surrounding vapor. Other mechanisms possibly responsible for this damping are discussed. We also report experiments on drops with angular momentum.

Vibronic intensity parameters for Er3+ in Cs2NaErCl6

Optical absorption measurements made at 10K on Cs2NaErCl6 in the range 6000 to 26 000cm−1 are analysed. Absolute line intensities of the transitions are used to predict dipole strengths and intrinsic intensity parameters (Āλt) using 60 of the observed oscillator strengths with data from three different modes of oscillation. Results are consistent with intensity calculations in other systems.

Area and Pressure Profiles for Collapsible-Tube Oscillations of Three Types

A technique for measuring the evolution of tube shape in terms of cross-sectional area as a functio of both time and axial position during the course of self-excited oscillation is described. The conductance catheter method is used to measure instantaneous shape-independent area at incrementally adjusted positions along the length of the tube. Pressure at the downstream end of the tube is also recorded, and this signal is subsequently used to line up all the area recordings. The result is a surface of cross-sectional area versus position and time that shows explicitly how disturbances propagate along the tube, the localization of the oscillations to the downstream end, and the shape signature of a given mode of oscillation. Derived results include profiles of the oscillation maximum, minimum and amplitude along the tube, and the time-course of movement of the area minimum (the tube throat) during the cycle. Similar procedures have been used in conjunction with a fine pressure-transducing catheter to obtain pressure surfaces. The technique is limited to strictly periodic oscillation, and to this end the extent of periodicity of operating points representing different combinations of controlling parameters has been systematically investigated. Three different modes of oscillation of a single collapsible tube have been observed with these techniques. These modes have previously been shown to be distinct, with sharp transitions separating them. Two of the modes have a similar low frequency, and are distinguished by waveform shape: the collapse is either brief relative to cycle length or lasts for approximately half period. The third mode has approximately three times the frequency of the other two. On the basis that the higher-frequency mode is obtained by transition from the brief-collapse mode as the appropriate parameter is varied, it was previously postulated that these two were more closely related dynamically, and the detailed examination of waveforms here confirms this link.