Frequencies Of Acoustic Modes Research Articles

Experimental Research on Excitation Condition and Performance of Airflow-Induced Acoustic Piezoelectric Generator.

This paper aims to present a novel airflow-induced acoustic piezoelectric generator that can be used to solve the problem of insufficient power supply of modern intelligent fuzes. The sound waves induced by airflow are the key to power generation performance. It is proposed that an edge tone frequency equal to the acoustic mode frequency is a sufficient condition for evoked acoustic waves, and a design idea and scheme for a universal fuze power supply is provided. We establish the vibration model of the airflow-induced acoustic piezoelectric generator. According to the model, the experimental research on the power generation performance shows that the sound pressure frequency, vibration displacement frequency, and output voltage frequency are consistent. The model provides a design idea for a vibration sensor. At the flow rate of 100.8 m/s, the output power is 45.3 mW, which is much higher than the fuze power sources such as the magnetic backseat generator. Therefore, the airflow-induced piezoelectric generator can effectively solve the problem of the modern fuze less types of power supply and low output energy.

Multipoint acoustic impedance sensing based on frequency-division multiplexed forward stimulated Brillouin scattering.

Acoustic impedance sensing based on forward stimulated Brillouin scattering (FSBS) in fiber has attracted the attention of researchers in recent years. Here, we propose a novel, to the best of our knowledge, multipoint sensing scheme based on FSBS, which explores frequency-division multiplexing. Since the resonance center frequencies of the FSBS-invoked acoustic modes are dependent on the fiber diameter, different fiber sections can be differentiated using carefully selected representative peaks in the FSBS spectrum. With a Sagnac loop structure, continuous-wave light is adopted for FSBS excitation, reducing the required fiber length for efficient opto-acoustic interaction. By evaluating the linewidths of representative peaks, simultaneous acoustic impedance sensing at multiple locations is realized with 0.8 m sensor size.

On using dipolar modes to constrain the helium glitch in red giant stars

ABSTRACT The space-borne missions CoRoT and Kepler have revealed numerous mixed modes in red giant stars. These modes carry a wealth of information about red giant cores, but are of limited use when constraining rapid structural variations in their envelopes. This limitation can be circumvented if we have access to the frequencies of the pure acoustic dipolar modes in red giants, i.e. the dipole modes that would exist in the absence of coupling between gravity and acoustic waves. We present a pilot study aimed at evaluating the implications of using these pure acoustic mode frequencies in seismic studies of the helium structural variation in red giants. The study is based on artificial seismic data for a red giant branch stellar model, bracketing seven acoustic dipole radial orders around νmax. The pure acoustic dipole-mode frequencies are derived from a fit to the mixed-mode period spacings and then used to compute the pure acoustic dipole-mode second differences. The pure acoustic dipole-mode second differences inferred through this procedure follow the same oscillatory function as the radial-mode second differences. The additional constraints brought by the dipolar modes allow us to adopt a more complete description of the glitch signature when performing the fit to the second differences. The amplitude of the glitch retrieved from this fit is 15${{\ \rm per\ cent}}$ smaller than that from the fit based on the radial modes alone. Also, we find that thanks to the additional constraints, a bias in the inferred glitch location, found when adopting the simpler description of the glitch, is avoided.

Effects of Inert Fuel Diluents on Thermoacoustic Instabilities in Gas Turbine Combustion

The effects of inert diluents in the fuel mixture of a model swirl stabilized gas turbine combustor, under thermoacoustically unstable limit cycle operation, were studied experimentally. The measurements included particle image velocimetry (PIV), high-speed CH* chemiluminescent imaging, and dynamic pressure signals. The paper focuses on the dynamic phenomenon of the period doubling bifurcation, which came about when the equivalence ratio of undiluted flames was enriched from 0.55 to 0.60 under constant Reynolds number (). The bifurcation featured an emergence of an aerodynamically related timescale in addition to the fundamental timescale that was induced from an unstable acoustic eigenmode. The aerodynamic timescale is introduced by azimuthal convection of a high-heat-release-rate region and is linked in the literature with a precessing motion of the recirculation zone. It was found that on increasing the diluent molar fraction up to 50%, the amplitude and the frequency of the limit cycle fundamental acoustic mode decreased. A mechanism to interpret this suppression is that increasing the diluent molar fraction of the fuel makes the flame more susceptible to quenching because the extinction strain rate of the mixture is decreased. The paper argues that the existence of inert diluents in the fuel can significantly alter the dynamic state of the combustor, because the anchoring locations of the flame greatly depend on the composition-sensitive extinction strain rate of the mixture.

Parametric Amplification of Magnons in Synthetic Antiferromagnets

We demonstrate the parametric amplification of an acoustic magnon mode induced by an optical magnon mode in synthetic antiferromagnets, which was achieved by using the all-optical pump-probe time-resolved magneto-optical Kerr effect. The acoustic and optical modes with low and high frequencies, respectively, are clearly observed under different field directions and pump-laser powers. For a relatively high laser power, the acoustic mode shows a temporal increase in amplitude in the time domain; this is observed when the acoustic mode frequency is approximately half the frequency of the optical mode. Correspondingly, we also observe a large enhancement in the spectral density of the acoustic mode in the frequency domain. These data are supported by a numerical simulation using a macrospin model; furthermore, the optical mode amplitude threshold for achieving the acoustic mode amplification is also discussed. The parametric effect in synthetic antiferromagnets demonstrated here can be applied to nanoscale parametric amplifiers and oscillators of magnons, which are the building blocks for spintronic and magnonic computing beyond von Neumann architectures.

F-Orbital based Dirac states in a two-dimensional uranium compound

Theoretical evidence of the existence of Dirac cones in two-dimensional UB_4 is provided. Dirac cones are created due to the interaction of strongly localized U anisotropic f-orbitals with the delocalized network of B p-orbitals in a bilayer honeycombed lattice. Spin–orbit coupling splits the relativistic electronic states in the vicinity of the Fermi level creating cone-shaped gaped bands. The contribution of f-orbitals to the formation of dispersive Dirac states is clearly determined with several theoretical approximations. U atom provides the exact amount of charge to stabilize the B sublattices creating a heavy-electron based material with reminiscent properties of graphene. The interplay between f- and p-orbitals of U and B atoms, respectively, is revealed as the origin of the itinerant electronic states, defying the paradox of delocalized electrons in a heavy-electron based material. Computed phonon diagram exhibits decoupled acoustic and optic modes arising from U and B atom vibrations, respectively, with frequencies of acoustic modes rather small as compared to optic modes. The dynamical properties of isoelectronic UAl4 and UGa4 are also analyzed.

Nonlinear frequency shift of acoustic waves in semiconductor plasmas

The evolution of nonlinear instability of an electron acoustic wave from the coupling of pair modes of slightly different frequencies in semiconductor plasmas is studied. The linear dispersive properties of pair modes, viz., pump wave and sideband wave, are analyzed on employing the quantum hydrodynamic model. The linear fields of pair modes couple through a second order convective term to unfold the nonlinear beat frequency of the electron acoustic mode. The nonlinear dispersion relation leads to the derivation of the growth rate of three-wave parametric instability. The quantum effects of semiconductor electrons are taken through the exchange-correlation potential, the Bohm potential, and statistical Fermi pressure. The growth rate is also analyzed graphically. This study seeks its applications in semiconductor devices.

Analytical modelling of period spacings across the HR diagram

ABSTRACT The characterization of stellar cores may be accomplished through the modelling of asteroseismic data from stars exhibiting either gravity-mode or mixed-mode pulsations, potentially shedding light on the physical processes responsible for the production, mixing, and segregation of chemical elements. In this work, we validate against model data an analytical expression for the period spacing that will facilitate the inference of the properties of stellar cores, including the detection and characterization of buoyancy glitches (strong chemical gradients). This asymptotically based analytical expression is tested both in models with and without buoyancy glitches. It does not assume that glitches are small and, consequently, predicts non-sinusoidal glitch-induced period-spacing variations, as often seen in model and real data. We show that the glitch position and width inferred from the fitting of the analytical expression to model data consisting of pure gravity modes are in close agreement (typically better than 7 ${{\ \rm per\ cent}}$ relative difference) with the properties measured directly from the stellar models. In the case of fitting mixed-mode model data, the same expression is shown to reproduce well the numerical results, when the glitch properties are known a priori. In addition, the fits performed to mixed-mode model data reveal a frequency dependence of the coupling coefficient, q, for a moderate-luminosity red-giant-branch model star. Finally, we find that fitting the analytical expression to the mixed-mode period spacings may provide a way to infer the frequencies of the pure acoustic dipole modes that would exist if no coupling took place between acoustic and gravity waves.

Asymmetric viscothermal acoustic propagation and implication on flow measurement for SOFC

In the applications of heating generation based on the solid oxide fuel cells (SOFC) technology, a high-accuracy performance of flow measurement is of great importance. Due to the advantage of non-invasive no-moving-parts construction and bi-direction measurement, ultrasonic flow meter, where viscothermal dissipation and asymmetric acoustic modes cannot be overlooked, may be a promising method in the SOFC-based applications. The present paper mathematically formulates asymmetric linear disturbance dynamics in terms of velocity and temperature disturbances based on the conservations of mass, momentum and energy. An iterative calculation procedure, which is similar to Galerkin method, is presented. Numerical analysis of asymmetric acoustic features (phase velocity and attenuation coefficient) are comprehensively given under the effects of viscothermal dissipation and shear flow convection. In the end, flow measurement performance of asymmetric acoustic modes is literally discussed. Numerical study shows that viscothermal dissipation affects the cut-on frequency of acoustic modes and couples nonlinearly with shear convection when the flow Mach number is large. These parameters impose significant influences on measurement performance. Each acoustic mode has inherent measurement derivation which can be theoretically used to compensate the acoustic flow measurement error. Apparent prediction error may occur if the viscothermal dissipation is taken out of consideration.

Acoustic phonon dynamics of Pb(Sc1/2Ta1/2)O3 ceramics studied by Brillouin scattering spectroscopy

Brillouin spectra of highly-ordered Pb(Sc1/2Ta1/2)O3 (PST-O) and highly-disordered PST (PST-D) were measured over a wide temperature range. The frequency of the longitudinal acoustic (LA) phonon mode of PST-O showed a sharp anomaly corresponding to the sharp maxima of the dielectric permittivity, whereas that of PST-D exhibits a broad anomaly. The remarkable softening of LA phonon was observed below T* = 500 K, which indicates the occurrence of a piezoelectric coupling and/or electrostrictive coupling due to the appearance of non-centrosymmetric polar nanoregions below T*. The relaxation time of the LA phonon of PST-O shows a critical slowing down for order-disorder phase transition, while that of PST-D exhibits a stretched slowing down due to random electric fields related to B-site randomness.

On the saturation of acoustic mode frequencies at high solar activity

ABSTRACT Acoustic mode frequencies obtained by applying spherical harmonic decomposition to HMI, MDI, and GONG observations were analysed throughout the solar cycle. Evidence of a deviation from a linear relation with solar radio flux was found indicating a saturation effect at high solar activity. The Gompertz model, which is one of the most frequently used sigmoid functions to fit growth data, is used. It is shown that its fitting to MDI and GONG data are statistically significant and a median saturation of 400 sfu is estimated. This saturation level is 50 per cent larger than any obtained in the last century, hence the small effect observed in the minimum-to-maximum frequency shift. However, as shown here, it should not be disregarded.

Phase-resolved PIV measurements of flow over three unevenly spaced cylinders and its coupling with acoustic resonance

The characteristics of acoustic resonance excitation by flow over the arrangement of three unevenly spaced inline cylinders in cross flow were experimentally investigated. Phase-resolved particle image velocimetry (PIV) measurements were conducted to demonstrate the role of the separated shear layers around the cylinders in the excitation mechanism of acoustic resonance. The Strouhal number of self-sustained flow oscillations around the investigated arrangement is presented. Before the onset of acoustic resonance excitation, the location of the middle cylinder has a significant effect on the shear layer separation and impingement mechanism. At flow velocities that caused coincidence between an acoustic mode frequency and the intrinsic vortex shedding frequency that would occur under free-field condition, severe acoustic resonance corresponding to acoustic particle velocities of up to one-tenth of the main flow velocity was observed. For certain arrangements, acoustic resonance was detected at lower flow velocities than necessary for frequency coincidence. The Strouhal number of these pre-coincidence oscillations corresponds to that of a cavity formed between two successive cylinders. Phase-resolved PIV measurements show significant differences between flow field during and in the absence of acoustic resonance. Most importantly, acoustic resonance is excited when vortices roll up and impinge on the middle cylinder and the downstream cylinder, and no flow passes through the two gaps. The acoustic mode frequency and the Strouhal number of the Rossiter-like modes are decreased when resonance takes place at a higher Mach number. The aerodynamic interference between the two successive gaps formed by the three cylinders seems essential to explain the variation in the amplitude of resonance excitation.

Global turbulence features across marginality and non-local pedestal-core interactions

Spatially non-local aspects of turbulent transport in tokamak plasmas are examined with global gyrokinetic simulations using the ORB5 code. Inspired by very accurate measurements in the TCV tokamak in L-mode, we initialise plasma profiles with constant logarithmic gradients in the core and constant linear gradients in the ‘pedestal’ (). The main finding is that transport in the core is strongly affected by the presence of pedestal gradients. This non-local pedestal-core coupling appears to be correlated with the appearance of repetitive avalanches that propagate across both pedestal and core regions. Below a certain threshold value in pedestal gradient, no well-defined frequency is found for avalanches. Above this threshold, a well-defined frequency shows up, which roughly matches that of the local geodesic acoustic mode (GAM) frequency near the plasma edge and is thus well below the local GAM frequency in the core: this behaviour is very similar to the global coherent mode structure observed in TCV. Above this threshold in pedestal gradient, the core transport increases sharply: there is therefore a non-locality in marginality. The probability density functions (PDFs) of density, temperature, temperature gradient and potential are found to have nearly Gaussian distributions, whereas the heat flux can have, in the presence of avalanches, a more or less strongly positively skewed PDF, which could be fitted by a log-normal distribution. The skewness of the heat flux is found to be radially and non-locally dependent: its value in the plasma core critically depends on the presence of gradients in the pedestal. The relation flux versus gradient is examined in detail. The local instantaneous flux versus gradient relation shows a hysteresis behaviour during an avalanche but no clear correlation, unlike the flux and zonal flow (ZF) shearing rate, which are anti-correlated: flux is higher when shearing rate is lower. This leads to corrugated time-averaged radial profiles of transport, heat and temperature gradient, with heat diffusivity having local maxima where the ZF shearing rate goes to zero and temperature gradient has local minima. Finally, we show how the flux versus gradient relation can be analysed locally for series of simulations with different averaged gradients.

Analytical modeling of the one-dimensional acoustic field in hybrid rocket combustion chamber

An analytical model to predict the longitudinal acoustic modes of hybrid rocket combustion chambers is presented. In the classical motor configuration, a dump plenum and an aft-mixing chamber are mated to the solid fuel grain; this arrangement produces cross-sectional discontinuities that vary over the motor firing because of the widening of the fuel port. This feature mainly, in conjunction with mixture-ratio shift in the motor operation to a much lesser degree, usually induces a change of the frequency of longitudinal modes during combustion. Furthermore, in the combustion chamber there is an axial gradient of the speed of sound, which has an influence on the mode characteristics. The model developed here accounts for both the cross-sectional area discontinuities and the axial variations of temperature and gas composition, assuming a linear trend of the squared speed of sound from the injection head to the exhaust nozzle entrance, and zero mean flow. Comparison with the results computed with a comprehensive one-dimensional model, which considers also the effects of the vaporizing fuel mass injection and of the actual temperature variation in the chamber, reveals maximum deviations around 3%. Finally, the model has been employed to predict the first longitudinal acoustic mode frequency measured versus time in a couple of firing tests of a laboratory-scale rocket showing fairly good agreement.

Fast tracking of acoustic resonance frequencies in collection volumes to measure flow

Previously, we demonstrated accurate determinations of the mass of gas in a large collection vessel of known volume from measurements of the pressure and the resonance frequencies of a few acoustic modes. We achieved accurate mass measurements even when linear temperature gradients persisted after adding or removing gas. The change in mass Δm from before and after flow and the collection time Δt may be used to calibrate mass flowmeters. Because the resonance quality factors Q are very high (up to 30000), measuring the complex-valued frequency response using a lock-in amplifier to determine the resonance frequency is slow. This static measurement of mass requires sufficient time to elapse for transient non-linear gradients to dissipate through convection and diffusion. Here, we demonstrate a method to dynamically track the resonance frequency of a large, gas-filled collection vessel while gas is flowing into or out of the vessel. We use positive feedback to excite sustained oscillations at the desired resonance frequency without a lock-in amplifier or frequency synthesizer. As the speed of sound changes, the sustained oscillations track the changing resonance frequency f with response time ~1/f. Preliminary mass flow measurements using this technique agree with a flow standard within their combined uncertainties.

Effect of a Rare-Earth Ion on the Structural Instability in RFe3(BO3)4 Crystals

The dynamics of the crystal lattice of RFe3(BO3)4 (R = Pr, Nd, Sm, Gd, Tb, Dy, and Ho) compounds in the high-symmetry R32 phase has been calculated. Significant changes in spectra of compounds with various rare-earth ions have been obtained only near the edge Λ point of the Brillouin zone (qΛ = 1/3(−2b1 + b2 + b3, where b1, b2, and b3 are the reciprocal lattice vectors) for acoustic oscillation branches. A decrease in the frequency of an acoustic mode at the point Λ has been revealed in all studied compounds. This frequency depends on the type of rare-earth ion and decreases from a compound with Pr to a compound with Ho down to imaginary values. Such a behavior of the frequency of the unstable acoustic mode is in good agreement with experimental data on the dependence of the temperature of the R32 → P3121 structural phase transition on the type of rare-earth ion in ferroborates.

Acoustic Anomalies and Phase Transition Behaviors of Lead-Free Piezoelectric (Na1/2Bi1/2)TiO₃-xBaTiO₃ Single Crystals as Revealed by Brillouin Light Scattering.

The elastic properties of unpoled and prepoled (Na1/2Bi1/2)TiO3-xBaTiO3 (NBT-xBT) single crystals near the morphotropic phase boundary were investigated as a function of temperature using Brillouin light scattering. The acoustic mode frequency and the related acoustic damping of unpoled NBT-xBT showed very broad minimum and maximum, respectively, consistent with typical relaxor behaviors. The frequency softening of the longitudinal acoustic mode together with the increase in acoustic damping was largest along the <100> direction, indicating that polarization fluctuations were most substantial along this crystallographic direction. The difference in acoustic behaviors between the unpoled NBT-xBTs with x = 0.05 and 0.08 were negligible, which means that the NBT-xBT system exhibits typical relaxor properties over a certain composition range of at least 5~8%. The obtained relaxation time of polar nanoregions in the paraelectric phase showed a gradual slowing-down character without any critical divergent behavior. The prepoling of NBT-xBT along the <100> direction induced drastic changes in both mode frequency and damping at ~110 °C when the poling field was larger than 1.4 kV/mm, corresponding to the depoling process from macroscopic/mesoscopic ferroelectric order to ergodic relaxor state upon heating. Phase coexistence of ferroelectric and relaxor states was observed at the intermediate poling field of 1.4 kV/mm.

Self-sustained, high-frequency detonation wave generation in a semi-bounded channel

Combustion dynamics in a linear, semi-bounded channel were studied to characterize injection, mixing, and ignition processes in a two dimensional analogue of a rotating detonation wave combustor. The linear channel was developed to operate using natural gas and oxygen and to provide large optical accessibility in support of high-fidelity diagnostics at thermal power density levels representative of rotating detonation wave combustors over a broad range of flow conditions. Measurements of pressure and heat release were performed using an array of high-frequency dynamic pressure probes and OH*-chemiluminescence, respectively. Self-sustained, periodic generation of detonation waves was observed. These traveling combustion waves were initiated from low amplitude deflagrative fronts and steepened as they traveled through the combustion channel. The sensitivity of this instability to changes in acoustic boundary conditions was explored to determine the origin of the high-amplitude dynamics. The change in acoustic boundary at the end-wall affected the transverse acoustic mode frequency as expected, but had no effect on the detonation wave initiation. The detonation wave dynamics were found to be correlated with resonant frequencies in the propellant manifolds. It was determined that the frequency of the self-sustained chamber dynamics depends on which manifold has the strongest injection pressure ratio. OH*-chemiluminescence and pressure measurements indicate that the observed transition to detonation is linked to the auto-ignition of the reactants and that the auto-ignition event is coupled with the fluctuation in reactant mass flow associated with the manifold dynamics.

Electron–Phonon Coupling in Luminescent Europium-Doped Hydride Perovskites Studied by Luminescence Spectroscopy, Inelastic Neutron Scattering, and First-Principles Calculations

We present a case study on the vibrational coupling of lattice phonons to the electronic 4f7 (8S7/2)–4f65d1 (eg) transition of divalent europium in the hydrides and deuterides LiMH3 and LiMD3 (M = Sr and Ba). For low doping concentrations, these compounds show extraordinarily well-resolved vibronic fine structures at low temperatures. Besides luminescence emission spectroscopy of the europium-doped compounds, we carried out inelastic neutron scattering (INS) experiments of the europium-free compounds. The phonons coupling to the electronic transition are identified, and a good agreement between the vibronic and the INS data is found. The frequencies of the low-energy acoustic modes do not significantly change upon replacing hydride by deuteride, whereas a decrease by a factor of approximately 2 can be observed for the higher energy optic modes. Furthermore, we compare these experimental results to density functional calculations performed with the Vienna Ab initio Simulation Package. Knowledge of the phon...

Controlling surface charge and spin density oscillations by Dirac plasmon interaction in thin topological insulators

We study the selective excitation at infrared and THz frequencies of optical and acoustic plasmonic modes supported by thin topological insulators. These modes are characterized by effective net charge or net spin density, respectively, and we study their excitation by combining many-body and electromagnetic calculations. We first show that non-locality can significantly modify the plasmonic response: it changes the energy of propagating plasmons up to tens of percent. We then discuss how, by changing the distance between a dipolar source and a semi-infinite 10 nm thin film, it is possible to control the excitation of acoustic and optical propagating plasmons, which can propagate over a distance of several plasmonic wavelengths. Furthermore, we consider 10 nm thin TI nanodisks and study the excitation of acoustic and optical localized plasmon modes by a point dipole source and plane wave illumination, respectively. The resonant plasmonic modes appear at frequencies that strongly depends on the size of the disk, and that can be potentially tuned by applying electrostatic gating to modify the Fermi Energy of the conductive 2-dimensional layer that forms at the interfaces of the TI. We observe a spectral shift from ~29 $\mu$m to ~34 $\mu$m by changing the Fermi Energy from 250meV to 350meV. Last, the electromagnetic energy of these plasmonics modes can be confined to very small regions, of effective volume ~(120 nm)^3 for the smaller disk considered, much less than the free-space wavelength cubed $\lambda$^3 ~(35000 nm)^3. The strong confinement is desirable for achieving very efficient coupling with nearby systems. Our detailed study thus shows that thin topological insulators are a promising system to control both the spin and charge oscillations associated with the plasmonic resonances, with possible applications to fast, compact and electrically-controlled spintronics devices.