Frequencies Of Acoustic Modes Research Articles

Electronic and Lattice Vibrational Properties of BaSi2 from Density Functional Theory Calculations

BaSi2 is a potential thermoelectric material because of its very low thermal conductivity. Using the full-potential linearized augmented plane-wave method and semiclassical Boltzmann theory, thermoelectric transport properties of BaSi2 have been investigated. The calculations show that the thermoelectric properties can be remarkably improved by optimizing the carrier concentration. The linear response method within the framework of density functional theory was employed to investigate the underlying physics of heat transport. There are rather flat optical dispersion curves and low frequency of acoustic phonon modes in the phonon band structure of BaSi2. The low-lying optical phonon branch at the Γ point of the Brillouin zone (BZ) corresponds to rigid-unit vibration of the Si tetrahedron. The rigid-unit vibration mode confines the acoustic phonon modes and scatters the heat-carrying acoustic modes, leading to the low lattice thermal conductivity.

Characterization of solar-cycle induced frequency shift of medium- and high-degree acoustic modes

Although it is well known that the solar acoustic mode frequency increases as the solar activity increases, the mechanism behind it is still unknown. Mode frequencies with 20 < l < 900 obtained by applying spherical harmonic decomposition to MDI full-disk observations were used. First, the dependence of solar acoustic mode frequency with solar activity was examined and evidence of a quadratic relation was found indicating a saturation effect at high solar activity. Then, the frequency dependence of frequency differences between the activity minimum and maximum was analyzed. The frequency shift scaled by the normalized mode inertia follows a simple power law where the exponent for the p modes decreases by 37% for modes with frequency larger than 2.5 mHz.

Dependence of Frequency Shift of Depolarized Guided Acoustic Wave Brillouin Scattering in Photonic Crystal Fibers

We study through experiment, calculation and simulation the frequency dependence of depolarized GAWBS observed in four different photonic crystal fibers and extend the results to others through simulation. In two of the fibers studied, high frequency modes (>; 1 GHz) are observed in addition to the usual lower frequency torsional radial (TR) modes. The acoustic modes responsible for GAWBS are characterized through a combination of simulations and experiments. The calculations are done using the silica rod approximation, which is valid for PCFs with a low to intermediate air-fill fraction or with a very high air-filling fraction. Further simulations extend the range of photonic crystal fibers in which acoustic modes can be characterized. These are used to study the dependence of the acoustic mode frequency on key fiber parameters such as the ratio of air to silica across the whole cross section (κ) and the lattice pitch (Λ) .

Acoustic oscillations in stars near the tip of the red giant branch

Small amplitude oscillations are observed in red giant branch (RGB) stars. Data on such oscillations are a source of information about the objects, notably about properties of convection in their envelopes and about the systems these objects inhabit. The OGLE-III catalog contains data for about 80 thousand small amplitude variable red giants (OSARGs) in the Large Magellanic Cloud. We want to explain variability in OSARGs as the solar-like oscillation and to associate the peaks in power spectra with frequencies of acoustic modes. We use data on reddening-free magnitudes of the objects and interpret them in terms of stellar physical parameters using tabulated isochrones calculated for ages and composition parameters corresponding to the upper RGB of the LMC. Massive data on the peak frequencies and amplitudes are compared with expectations for stochastically excited oscillations. The frequencies are also compared with those calculated for radial modes in envelope models with parameters taken from the isochrones. In stars close to the tip of the RGB, the peaks in power spectra are found in the 0.1-1.0 $\mu$Hz range, which is consistent with extrapolation of the frequency-luminosity relation for the solar-like oscillation. The dominant peaks occur close to the first two radial overtones. The increase in amplitude with luminosity is slower than linear. The exponent s=0.9 is similar to what is found from recent analysis of CoRoT data on less luminous red giants. Frequency separations between dominant peaks are found to be smaller by about 20% than calculated separations between these modes. After examining various possibilities, we left this discrepancy unexplained. The small amplitude variability of stars at the RGB tip is likely to be caused by a stochastic excitation of acoustic oscillations, but interpreting of individual peaks in power spectra presents a problem.

Effect of impurity ions on the geodesic acoustic mode

A dispersion relation of the geodesic acoustic mode with the effect of impurity ions is systematically derived. It is found that the frequency of the geodesic acoustic mode for a plasma with impurity ions is lower than that without impurity ions, which are mainly due to the polarization of impurity ions. It is also found that the damping rate of the mode increases with the increase in effective charge in the small effective charge limit due to the polarization currents of impurity ions, and decreases in the large effective charge limit mainly due to the effect of the curvature drift of impurity ions. A maximum damping rate is found in the intermediate effective charge regime.

The acoustics of lip-excited animal horns.

For thousands of years the thrilling sound of the lip-excited animal horn has been a central feature of religious ceremonies, and has also played an important role in signaling and communication. It can be argued that all the brass instruments now in use in Western orchestras and bands have evolved from this common source. Modern brass instruments, however, have bore profiles which have been developed and refined over centuries of trial and error to serve specific musical functions. The internal bore profile of the animal horn is largely determined by the choice of horn: the maker has therefore little control over the frequencies of the air column acoustic modes. This paper discusses the different types of horn which have been commonly used to make musical instruments and explores the effect of the natural bore profile on the sounding properties of the instrument.

Further Evidence for Acoustic Resonance in Full Size Steam Generator and Tubular Heat Exchanger Tube Banks

Acoustic resonance or acoustic vibration, which develops in flow channels containing a tube bank, is caused by vortex shedding generated by crossflow over the tube bank. Transverse acoustic modes are excited, which are perpendicular to the direction of flow and of the tube axes. For the excitation of the acoustic modes resulting in acoustic resonance, two conditions must be met: (a) The frequency of vortex shedding must coincide with the frequency of the particular acoustic mode to be excited, and (b) there must be sufficient energy available to initiate the vibration. If the frequency coincidence is not satisfied or if the excitation energy is insufficient, the acoustic resonance will not be possible. It is important to define the criteria, which need to be met for the initiation of the acoustic resonance. In this paper, new criteria are developed on the basis of the acoustic particle velocity for the onset of acoustic resonance in steam generator and tubular heat exchanger tube banks.

The effects of μ gradients on pulsations of rapidly rotating stars

AbstractRecently, Reese et al. (2008), Lignières &amp; Georgeot (2008) and Lignières &amp; Georgeot (2009) showed that the frequencies of low-degree acoustic modes in rapidly rotating stars, also known as “island modes”, follow an asymptotic formula, the coefficients of which can be deduced from ray dynamics. We investigate how this asymptotic behaviour is affected by μ gradients by comparing pulsation spectra from models with and without such a discontinuity.

Benchmark test of drift-kinetic and gyrokinetic codes through neoclassical transport simulations

Two simulation codes that solve the drift-kinetic or gyrokinetic equation in toroidal plasmas are benchmarked by comparing the simulation results of neoclassical transport. The two codes are the drift-kinetic δf Monte Carlo code (FORTEC-3D) and the gyrokinetic full- f Vlasov code (GT5D), both of which solve radially-global, five-dimensional kinetic equation with including the linear Fokker–Planck collision operator. In a tokamak configuration, neoclassical radial heat flux and the parallel flow relation, which relates the parallel mean flow with radial electric field and temperature gradient, are compared between these two codes, and their results are also compared with the local neoclassical transport theory. It is found that the simulation results of the two codes coincide very well in a wide rage of plasma collisionality parameter ν ∗ = 0.01 ∼ 10 and also agree with the theoretical estimations. The time evolution of radial electric field and particle flux, and the radial profile of the geodesic acoustic mode frequency also coincide very well. These facts guarantee the capability of GT5D to simulate plasma turbulence transport with including proper neoclassical effects of collisional diffusion and equilibrium radial electric field.

Low‐Frequency Raman Scattering from Nanocrystals Caused by Coherent Excitation of Phonons

Since the first observation of surface acoustic modes from silicon nanocrystals (NCs) embedded in silica byDuval et al., low-frequency Raman scattering from NCs has become an important researcharea inmanyfields including semiconductor technologies, medical therapeutics, and biophysics. Recent research has indicated that low-frequency Raman spectroscopy is a feasible non-destructive technique for investigating virus functionalization, for example, introducing viruses on different materials, attaching viruses to quantum dots and carbon nanotubes, and forming multiple superstructures. These superstructures are expected to have important applications in biological science and medicine. However, the current assignment of the low-frequency Raman modes is based on Lamb’s theory that mainly focuses on the mode frequencies of an elastic vibration of a free isotropic sphere. Many studies have demonstrated that the surface acoustic-mode frequencies observed from free NCs and NCembedded matrix systems are consistent with the theoretical prediction. This is understandable because a large number of frequency values can be selected to match the experimental results. At the same time, other studies have unequivocally

Development of a Heavy Ion Beam Probe for Measuring Electrostatic Potential Profile and Its Fluctuation in LHD

A heavy ion beam probe (HIBP) using a 3-MV tandem accelerator has been installed on large helical device (LHD). Electrostatic potential in core plasma can be measured under the toroidal magnetic field strength of up to 3 T. By using the HIBP, the transition of potential profiles from electron-root to ion-root is observed in core plasmas during ramp-up of the electron density. Potential fluctuations are also measured electron cyclotron current drive (ECCD). Two kind of characteristic fluctuations are observed. One is a reversed-shear-induced Alfvén eigenmode (RSAE), whose frequency varies during the evolution of the rotational transform profile, and the other is with a constant geodeisc acoustic mode (GAM) frequency.

Passive Control of the Flow Around the Stratospheric Observatory for Infrared Astronomy

The unsteady flowfield around the Stratospheric Observatory for Infrared Astronomy, a reflecting telescope carried inside an open port of a Boeing 747SP, has been simulated by means of detached eddy simulations and unsteady Reynolds-averaged Navier–Stokes simulations. Vortex generators have been placed upstream of the cavity to control the shear layer spanning the opening. The influence of baffles and displacement bodies placed inside the telescope port on the frequencies of acoustic resonant modes was investigated by acoustic simulations, based on the solution of the homogenous Helmholtz equation. The objective of this study is to improve the telescope’s performance by mitigating the amplitudes and changing the characteristic frequencies of the pressure fluctuations inside the cavity. The present simulations show that the investigated measures have a high potential to increase the telescope’s pointing stability.

Asynchronous control of vortex-induced acoustic cavity resonance using imbedded piezo-electric actuators

This paper presents an experimental investigation of the control of a vortex-induced acoustic cavity resonance from flow over a bluff body using embedded piezo-ceramic actuators in order to alter the resonant flow-acoustic interactions. The action of the actuators was asynchronous. Experiments were mainly conducted at the flow velocity of acoustic resonance, where the vortex shedding frequency from the upstream bluff body approached the frequency of the first acoustic mode of two downstream cavities. The fluctuating acoustic pressure was measured using a microphone. The perturbed flow field around the bluff body was monitored using two single hot wire anemometers and one X-wire. It was found that the induced transverse vibrations were effective to reduce the acoustic resonance. The cavity sound pressure level at resonance was reduced by 8.2 dB in presence of actuation. The physics behind the control mechanism is discussed.

Quantum ground-state cooling and tripartite entanglement with three-mode optoacoustic interactions

We present a quantum analysis of three-mode optoacoustic parametric interactions in an optical cavity, in which two orthogonal transverse optical-cavity modes are coupled to one acoustic mode through radiation pressure. Due to the optimal frequency matching -- the frequency separation of two cavity modes is equal to the acoustic-mode frequency -- the carrier and sideband fields simultaneously resonate and coherently build up. This mechanism significantly enhances the optoacoustic couplings in the quantum regime. It allows exploration of quantum behavior of optoacoustic interactions in small-scale table-top experiments. We show explicitly that given an experimentally achievable parameter, three-mode scheme can realize quantum ground-state cooling of milligram scale mechanical oscillators and create robust stationary tripartite optoacoustic quantum entanglements.

Plasma elongation effects on temperature gradient driven instabilities and geodesic acoustic modes

Plasma shaping effects on temperature gradient driven instabilities and geodesic acoustic oscillations are investigated with gyrokinetic theory and a local magnetohydrodynamic equilibrium model. In particular, we focus on the effect of the elongation κ, including its radial derivative sκ = (r/κ)(∂κ/∂r), in the large aspect ratio limit. An analytical formula of the dependence of the geodesic acoustic mode (GAM) frequency on the elongation is given. It is found that the GAM frequency sharply decreases with increasing elongation by the dependence of [(2 − αsκ)/(κ2 + 1)]1/2 with α = 0.5–1, which comes from the modification of ion classical polarization balanced by that of curvature drift polarization. The dependence of the critical threshold of the ETG/ITG instability on the elongation is numerically studied and a semi-analytical formula is given as (R0/LTc)/(R0/LTc)sκ=0,κ=1 = (1 + 0.36sκ)[1 + 0.11(κ − 1)], where R0 is the major radius and LTc is the critical scale length of the temperature gradient.

Molecular dynamics of liquid lead near its melting point

The molecular dynamics of liquid lead is simulated at T = 613 K using the following three models of an interparticle interaction potential: the Dzugutov pair potential and two multiparticle potentials (the “glue” potential and the Gupta potential). One of the purposes of this work is to determine the optimal model potential of the interatomic interaction in liquid lead. The calculated structural static and dynamic characteristics are compared with the experimental data on X-ray and neutron scattering. On the whole, all three model potentials adequately reproduce the experimental data. The calculations using the Dzugutov pair potential are found to reproduce the structural properties and dynamics of liquid lead on the nanoscale best of all. The role of a multiparticle contribution to the glue and Gupta potentials is studied, and its effect on the dynamic properties of liquid lead in nanoregions is revealed. In particular, the neglect of this contribution is shown to noticeably decrease the acoustic-mode frequency.

Identification of low-pressure compressor blades flutter

The unstable oscillations during coaxial low-pressure compressor blade flutter has been examined. The spectral analysis of strain gage records of blade oscillations and synchronous records of pressure pulsations in the countercurrent air flow shows that the damping of oscillation modes is the most informative parameter defining the flutter’s evolution process. The coincidence of the eigenfrequencies of the blades and the synchronous character of the changes in the time relationships of damping at the eigenfrequencies frequencies of the blade, and at the frequencies of acoustic diametrical modes of pressure pulsations occurring in the air flow shows that the phenomenon of flutter relates to the blades collective oscillation.

Low-frequency surface-plasmon resonances in planar metal–dielectric periodic structures

As a part of nanophotonics, surface-plasmon resonance is examined for a periodic semi-infinite structure with metal‐dielectric unit cells in slab geometry. The electromagnetic waves through this structure are analyzed in the low-frequency limit with respect to the bulk plasma frequency of metal. The lossless Drude model is employed throughout the present paper for mathematical simplicity. As a consequence, a distinguished balance is found to exist, where the similarity parameter is formed from an appropriate combination of the filling fraction of the metallic sublayers and the frequency. Interesting nanophotonic behaviors are thus identified by comparing various characteristics such as the frequency bandgaps, cutoff thicknesses, substrate effects, acoustic and optical modes, and group-velocity dispersions.

Intense Geodesic Acousticlike Modes Driven by Suprathermal Ions in a Tokamak Plasma

Intense axisymmetric oscillations driven by suprathermal ions injected in the direction counter to the toroidal plasma current are observed in the DIII-D tokamak. The modes appear at nearly half the ideal geodesic acoustic mode frequency, in plasmas with comparable electron and ion temperatures and elevated magnetic safety factor (q_{min}>or=2). Strong bursting and frequency chirping are observed, concomitant with large (10%-15%) drops in the neutron emission. Large electron density fluctuations (n[over ]_{e}/n_{e} approximately 1.5%) are observed with no detectable electron temperature fluctuations, confirming a dominant compressional contribution to the pressure perturbation as predicted by kinetic theory. The observed mode frequency is consistent with a recent theoretical prediction for the energetic-particle-driven geodesic acoustic mode.

Analysis of MDI High-Degree Mode Frequencies and their Rotational Splittings

We present a detailed analysis of solar acoustic mode frequencies and their rotational splittings for modes with degree up to 900. They were obtained by applying spherical harmonic decomposition to full-disk solar images observed by the Michelson Doppler Imager onboard the Solar and Heliospheric Observatory spacecraft. Global helioseismology analysis of high-degree modes is complicated by the fact that the individual modes cannot be isolated, which has limited so far the use of high-degree data for structure inversion of the near-surface layers (r>0.97R ⊙). In this work, we took great care to recover the actual mode characteristics using a physically motivated model which included a complete leakage matrix. We included in our analysis the following instrumental characteristics: the correct instantaneous image scale, the radial and non-radial image distortions, the effective position angle of the solar rotation axis, and a correction to the Carrington elements. We also present variations of the mode frequencies caused by the solar activity cycle. We have analyzed seven observational periods from 1999 to 2005 and correlated their frequency shift with four different solar indices. The frequency shift scaled by the relative mode inertia is a function of frequency alone and follows a simple power law, where the exponent obtained for the p modes is twice the value obtained for the f modes. The different solar indices present the same result.