Excitation Of Acoustic Modes Research Articles

Acoustic Resonance in a Model Ducted-Jet System

Jet engines are tested in so-called test cells, in which the desired environmental conditions are controlled. The jet plume is exhausted out of the test cell, usually through a diffuser. Under certain testing conditions, high-intensity pressure fluctuations (170 dB) can arise in such facilities. Their frequency is close to that of an acoustic normal mode of the exhaust-diffuser, but the mechanisms driving the resonance are unclear. In the present work the resonance phenomenon is studied using numerical simulations of a model configuration, which shares the key features of actual facilities: an underexpanded supersonic jet, a finite-length solid-wall shroud surrounding the jet, and the receptivity of acoustic disturbances to excite jet instabilities at the nozzle. For this configuration a high-amplitude resonance, qualitatively similar to that of experiments in actual facilities, is observed for a symmetric overexpanded M jet = 1.2 jet. This strongly resonant case is contrasted with a nonresonant ducted M jet = 1.5 jet in the same geometry, as well as with a free jet at M jet = 1.2. The hydrodynamic mechanism of this resonance is studied using linear stability analysis. The presence of excited acoustic modes of the duct is revealed by the Fourier analysis of the data. A numerical experiment shows that slight artificial damping of just the most excited acoustic mode suppresses the resonance.

Acoustic mode scattering from a heat source

The scattering of an incident acoustic wave by a non-uniform mean flow resulting from a heat source is investigated. The heat source produces gradients in the mean flow and the speed of sound that scatter the incident duct acoustic mode into vortical, entropic, and higher-order acoustic modes. Linear solutions utilizing the compact source limit and nonlinear solutions to the Euler equations are computed to understand how variations in the amplitude and axial extent of the heat source as well as the incident acoustic wave propagation angle and amplitude modify the scattered solution. For plane wave excitation, significant entropy waves are produced as the net heat addition increases at the expense of the transmitted acoustic energy. When the net heat addition is held constant, increasing the axial extent of the heat source results in a reduction of the entropy waves produced downstream and a corresponding increase in the downstream scattered acoustic energy. For circumferential acoustic mode excitations the incident acoustic wave angle, characterized by the cutoff ratio, significantly modifies the scattered acoustic energy. As the propagating mode cutoff ratio approaches unity, a rise in the scattered vortical disturbance and a decrease in the entropic disturbance amplitude is observed. As the cutoff ratio increases, the scattered solution approaches the plane wave results. Moreover, incident acoustic waves with different frequencies and circumferential mode orders but similar cutoff ratios yield similar scattered wave coefficients. Finally, for large amplitude incident acoustic waves the scattered solution is modified by nonlinear effects. The pressure field exhibits nonlinear steepening of the wavefront and the nonlinear interactions produce higher harmonic frequency content which distorts the sinusoidal variation of the outgoing scattered acoustic waves.

The CoRoT target HD 49933

From the seismic data obtained by CoRoT for the star HD 49933 it is possible, as for the Sun, to constrain models of the excitation of acoustic modes by turbulent convection. We compare a stochastic excitation model described in Paper I (arXiv:0910.4027) with the asteroseismology data for HD 49933, a star that is rather metal poor and significantly hotter than the Sun. Using the mode linewidths measured by CoRoT for HD 49933 and the theoretical mode excitation rates computed in Paper I, we derive the expected surface velocity amplitudes of the acoustic modes detected in HD 49933. Using a calibrated quasi-adiabatic approximation relating the mode amplitudes in intensity to those in velocity, we derive the expected values of the mode amplitude in intensity. Our amplitude calculations are within 1-sigma error bars of the mode surface velocity spectrum derived with the HARPS spectrograph. The same is found with the mode amplitudes in intensity derived for HD 49933 from the CoRoT data. On the other hand, at high frequency, our calculations significantly depart from the CoRoT and HARPS measurements. We show that assuming a solar metal abundance rather than the actual metal abundance of the star would result in a larger discrepancy with the seismic data. Furthermore, calculations that assume the ``new'' solar chemical mixture are in better agreement with the seismic data than those that assume the ``old'' solar chemical mixture. These results validate, in the case of a star significantly hotter than the Sun and Alpha Cen A, the main assumptions in the model of stochastic excitation. However, the discrepancies seen at high frequency highlight some deficiencies of the modelling, whose origin remains to be understood.

Flow-Acoustic Coupling in T-Junctions: Effect of T-Junction Geometry

The flow-acoustic coupling mechanism in a T-junction, which combines flows from two branches, forming the “cross-bar” of the T-junction, into one pipe, forming the “stem” of the T-junction, is investigated experimentally. The T-junction has a step pipe expansion at its inlets. The shear layer separating from this step expansion is found to excite intense acoustic resonances over multiple ranges of flow velocity. The excited acoustic mode is confined to the branch pipes and has an acoustic pressure node at the centerline of the T-junction. The length of the expansion section of the T-junction is found to control the frequency of the shear layer oscillation and therefore determines the ranges of flow velocity over which acoustic resonances are excited. Introducing asymmetry in the T-junction expansion length has shown little influence on the excitation of acoustic resonance. An additional T-junction arrangement made of rectangular cross-sectional ducts is also investigated to facilitate a flow visualization study of unsteady flow structures in the T-junction during acoustic resonance, and thereby improve understanding of the acoustic resonance mechanism and the nature of the aero-acoustic sources in the T-junction.

Modelling a surface acoustic wave based remotely actuated microvalve

We present a normally closed, remotely actuated, secure coded, electrostaticallydriven active microvalve using passive components. This is carried outby utilizing the complex signal processing capabilities of two identical5 × 2-bit Barker sequence encoded acoustic wave correlators. An electrostatically drivenmicrochannel, comprising two conducting diaphragms as the top and bottom walls, isplaced in between the compressor interdigital transducers of the two correlators. Secureinterrogability of the microvalve is demonstrated by finite element modelling of thecomplete structure and the quantitative deduction of the code dependent microchannelactuation. Furthermore, the influence of the excited acoustic modes of the correlator onthe microchannel deflection is investigated to optimize the microvalve design.

Photoinduced Structural Dynamics in Laser-Heated Nanomaterials of Various Shapes and Sizes

We examined photoinduced ultrafast structural dynamics such as coherent acoustic waves in many shapes of fcc metallic nanomaterials. Experimental data of nanosized thin films, prisms, spheres, rods, disks and pyramids from transient optical absorption/reflectance measurements were analyzed based on a combined Fermi-Pasta-Ulam model and two-temperature model. This work elucidates the structural dynamics induced by femtosecond laser heating, its size and shape effects on the period and phase of the excited acoustic phonon modes.

A Transient Grating approach to elastic wave and thermal propagation in a 2D free-standing micrometric phononic crystal

We investigate the phonon propagation in a 2D phononic crystal (with typical dimensions on the micrometric scale) by means of a transient grating (TG) heterodyne detected experiment. In a TG experiment both a temperature and a density grating are induced by means of optical techniques. The relaxation dynamics of the induced grating are then monitored live-time over 6 temporal decades with a probe beam. Our sample is a freestanding 100 micrometer thick polymer matrix with empty rods (filled with gas) arranged in a triangular lattice. Evidence of the presence of two different bulk wave acoustic modes are experimentally found. The excited acoustic modes show a correlation with the orientation of the sample with respect to the induced grating wave vector, while the thermal properties show a significant dependence on the magnitude of the induced grating wave vector.

Nonlinear excitations in Debye bi-crystals

The nonlinear dynamics of longitudinal dust lattice waves propagating in a dusty plasma bi-crystal is investigated. A “diatomic”-like one-dimensional dust lattice configuration is considered, consisting of two distinct dust grain species with different charges and masses. Two different frequency dispersion modes are obtained in the linear limit, namely, an optical and an acoustic wave dispersion branch. Nonlinear solitary wave solutions are shown to exist in both branches, by considering the continuum limit for lattice excitations in different nonlinear potential regimes. For this purpose, a generalized Boussinesq and an extended Korteweg de Vries equation is derived, for the acoustic mode excitations, and their exact soliton solutions are provided and compared. For the optic mode, a nonlinear Schrödinger-type equation is obtained, which is shown to possess bright- (dark-) type envelope soliton solutions in the long (short, respectively) wavelength range. Optic-type longitudinal wavepackets are shown to be generally unstable in the continuum limit, though this is shown not to be the rule in the general (discrete) case.

Resonant excitation of microwave acoustic modes in n-GaAs film

Resonant excitation of coupled space charge-acoustic waves in thin n-GaAs film is theoretically investigated. Possible mechanisms of electroacoustic coupling are piezoeffect and deformation potential. The film is placed onto a substrate i-GaAs without an acoustic contact. The film includes two-dimensional electron gas with a high negative differential conductivity. A possibility of resonant excitation of acoustic modes of the film at the first and the second harmonic of input microwave signal has been demonstrated.

Reduced-order modeling of vortex-driven excitation of acoustic modes

Vortex shedding that occurs in ducts with baffles in the presence of mean flow often leads to excitation of acoustic modes. Resulting flow oscillations may feed back to the process of vortex formation. A simple model is proposed for describing this complex interaction using the hypotheses for a quasi-steadiness of vortex shedding and for a short-period acoustic perturbation at the moment of vortex collision with a downstream baffle. The model is capable of predicting typical real-system phenomena, such as the lock-in of a dominant frequency of the vortex-acoustic instability in some ranges of the mean flow velocity.

A study of thermoacoustic instabilities in a resonator with heat release

Excitation of acoustic modes in systems with heat release is a complex process, involving coupling between the heat transfer rate and the acoustic pressure. The Rijke tube is the simplest device for studying the fundamental principles of thermoacoustic instabilities both experimentally and theoretically. In our investigation, special attention is paid to reproducibility and precision of the measurements. Experimental results for transition to instability are obtained when controlled parameters (flow rate, heater position, and supplied power) are varied in a quasisteady fashion. The stability boundaries exhibit hysteretic behavior not previously reported. Properties of the limit cycles in the region of instability have also been determined. A mathematical model is developed that incorporates heat transfer, acoustics, and thermoacoustic interactions. An energy balance, accounting for all types of heat transfer, allows for the determination of temperature variations and heat flux along the tube. The acoustic modes, generally nonorthogonal, corresponding to particular experimental thermal conditions, are computed. Solutions of the linearized wave equation with both loss and heat source terms determine the stability of acoustic modes. Hysteresis effects at the stability boundary and amplitudes of the limit-cycles are calculated from the nonlinear model.

Application of acoustic resonators in photoacoustic trace gas detection and metrology

The application of different types of acoustic resonators such as pipes, cylinders, and spheres in photoacoustics will be considered. This includes a discussion of the fundamental properties of these resonant cavities. Modulated and pulsed laser excitation of acoustic modes will be discussed. The theoretical and practical aspects of high-Q and low-Q resonators and their integration into complete photoacoustic detection systems for trace gas monitoring and metrology will be covered. The main characteristics of the available laser sources and the performance of the photoacoustic resonators, such as signal amplification will be discussed including setup properties and noise features. Recent results will be presented for a state-of-the-art dual-resonator differential cell suitable for sensitive trace gas detection with low electronic and acoustic noise. With this resonant cell and a near-infrared DFB diode laser radiating at 1.53 μm (42 mW) polar ammonia molecules have been detected with a sensitivity of 200 ppbv under flow conditions to reduce adsorption effects. By excitation of fundamental vibrations in methane using a pulsed optical parametric oscillator (OPO), with 60 mW output power, a sensitivity of 1.2 ppbv has been achieved employing this resonant cell. This setup allows accurate detection of the greenhouse gas methane which has a concentration of about 1.7 ppmv in ambient air.

Mixing Effects Between Self-Sustained and Unstable Hydrodynamic Oscillations Near Injecting Walls

Self-sustained oscillations in a shear layer were studied in a confined chamber with injecting walls. Experiments were carried out over a wide range of Reynolds numbers (0.86 x 10 5 ≤ Re ≤ 1.94 x 10 5 ) and of Mach numbers (0.05 ≤ M ≤ 0.227) in order to underline the effect of dynamic conditions on the pressure amplifications and on the excited acoustic modes. The presence of an obstacle generated a highly sheared flow in which vortex-shedding frequency was set by an acoustic-hydrodynamic coupling. The Mach number controlled the selectivity process of the vortex-shedding frequency, but turbulence dissipation weakened this coupling by a loss of structure coherence. As this did not affect the pressure fluctuation level, a second noise source was detected; vortices near the injecting wall, resulting from a mixing effect between shear-layer oscillations and injection flow through the porous wall, were identified

Application of acoustic resonators in photoacoustic trace gas analysis and metrology

The application of different types of acoustic resonators such as pipes, cylinders, and spheres in photoacoustics is considered. This includes a discussion of the fundamental properties of these resonant cavities. Modulated and pulsed laser excitation of acoustic modes is discussed. The theoretical and practical aspects of high-Q and low-Q resonators and their integration into complete photoacoustic detection systems for trace gas monitoring and metrology are covered in detail. The characteristics of the available laser sources and the performance of the photoacoustic resonators, such as signal amplification, are discussed. Setup properties and noise features are considered in detail. This review is intended to give newcomers the information needed to design and construct state-of-the-art photoacoustic detectors for specific purposes such as trace gas analysis, spectroscopy, and metrology.

Effect of the sediment layer on the excitation of acoustic modes and lateral waves in shallow sea

The simplest model of a shallow sea in the form of an isovelocity water layer and a fluid sediment layer overlying a homogeneous elastic halfspace is used to investigate the effect of the thickness of the sediment layer and the sound velocity in it on the behavior of the frequency dependences of the amplitudes of trapped and leaky modes and shear and longitudinal lateral waves that are excited by an acoustic point source in a shallow-water oceanic waveguide.

Mode-profile dependence of the electrostrictive response in fibers

The transverse optical intensity profile in fibers affects the efficiency of acoustic mode excitation by the optical field and the subsequent response of the field to the excited acoustic modes. The magnitude of the electrostrictive nonlinear coefficient for a square-top intensity profile will exceed that of a Gaussian profile by a fact of 2. For current fiber designs the range of values for n(2str) at zero frequency is expected to vary from 0.43 to 0.71x10(-16)cm(2)W(-1) based on mode profile alone. The relative contribution of electrostriction to the total nonlinear response (electrostrictive+Kerr) in fibers increases proportionately as the mode profile flattens.

Pulsed-laser excitation of acoustic modes in open high-Q photoacoustic resonators for trace gas monitoring: results for C_2H_4

The pulsed excitation of acoustic resonances was studied with a continuously monitoring photoacoustic detector system. Acoustic waves were generated in C(2)H(4)/N(2) gas mixtures by light absorption of the pulses from a transversely excited atmospheric CO(2) laser. The photoacoustic part consisted of high-Q cylindrical resonators (Q factor 820 for the first radial mode in N(2)) and two adjoining variable acoustic filter systems. The time-resolved signal was Fourier transformed to a frequency spectrum of high resolution. For the first radial mode a Lorentzian profile was fitted to the measured data. The outside noise suppression and the signal-to-noise ratio were investigated in a normal laboratory environment in the flow-through mode. The acoustic and electric filter system combined with the averaging of the photoacoustic signal in the time domain suppressed the outside noise by a factor of 4500 (73 dB). The detection limit for trace gas analysis of ethylene in pure N(2) was 2.0 parts in 10(9) by volume (ppbV) (minimal absorption coefficient α(min) = 6.1 × 10(-8) cm(-1), pulse energy 20 mJ, 1-bar N(2)), and in environmental air, in which the absorption of other gas components produces a high background signal, we can detect C(2)H(4) to ~180 ppbV. In addition, an alternative experimental technique, in which the maximum signal of the second azimuthal mode was monitored, was tested. To synchronize the sampling rate at the resonance frequency, a resonance tracking system was applied. The detection limit for ethylene measurements was α(min) = 9.1 × 10(-8) cm(-1) for this system.

Surface effect on the soft acoustic mode of KD3(SeO3)2studied by microprobed Brillouin scattering

Abstract By the use of a microprobed Brillouin scattering system, thermally excited acoustic modes in a ferroelastic KD3(SeO3)2 crystal have been observed as a function of depth from the surface. Both a longitudinal mode c22 and a transverse soft mode c44 propagating along the [010] direction become hard as the observation area closes to the (110) surface just above the transition temperature. Especially, the transition temperature at the surface, determined as the minimum position of the frequency shift of the soft acoustic c44 mode, shifts to lower side by about 0.1 K. It is concluded that the soft acoustic mode associated with the bulk ferroelastic phase transition is modified near the surface. On the other hand, these modes do not show any drastic anomaly near the (001) surface. A configuration effect is suggested between the surface and the propagation direction of the acoustic modes.

Quantum vortex dynamics in granular superconducting films.

We study the quantum (zero-temperature) dynamics of vortices in a granular film, which we model as a square lattice of superconducting grains, weakly coupled by Josephson junctions. The model takes into account the capacitance to the ground and between nearest-neighbor grains, as well as the special features related to the tunneling of quasiparticles. Starting from the general model, we derive and discuss different approximate models. We investigate the dissipation in the motion of the vortices, which results from excitation of acoustic modes and particularly, of quasiparticles.

Thermoelastic coupling at the junction of two bonded solids

Enhanced efficiency for the thermoelastic excitation of acoustic modes was observed experimentally by applying a purely thermal or purely acoustic source onto the surface boundary of two bonded materials. An explanatory mechanism of thermoelastic coupling at the interface of the two materials has been suggested.