Excitation Of Acoustic Vibrations Research Articles

Observation of nonlinear dynamics in an optical levitation system

Optical levitation of mechanical oscillators has been suggested as a promising way to decouple the environmental noise and increase the mechanical quality factor. Here, we investigate the dynamics of a free-standing mirror acting as the top reflector of a vertical optical cavity, designed as a testbed for a tripod cavity optical levitation setup. To reach the regime of levitation for a milligram-scale mirror, the optical intensity of the intracavity optical field approaches 3 MW cm−2. We identify three distinct optomechanical effects: excitation of acoustic vibrations, expansion due to photothermal absorption, and partial lift-off of the mirror due to radiation pressure force. These effects are intercoupled via the intracavity optical field and induce complex system dynamics inclusive of high-order sideband generation, optical bistability, parametric amplification, and the optical spring effect. We modify the response of the mirror with active feedback control to improve the overall stability of the system.

Detection of resonance frequency of both the internal defects of concrete and the laser head of a laser Doppler vibrometer by spatial spectral entropy for noncontact acoustic inspection

In order to detect internal defects of concrete, we have studied noncontact non-destructive acoustic inspection and have detected and visualized internal defects using acoustic vibration excitation on the target surface by aerial sound waves and two-dimensional vibration velocity distribution by a laser Doppler vibrometer. Because a high sensitive scanning laser Doppler vibrometer (SLDV) is used, as the sound pressure increases, the reverberation of sound waves from the surroundings may cause resonance in the galvano mirror system of the laser head of the SLDV. For real concrete structures, for example a tunnel inner wall (distance about 5–10 m), there was no problem because the sound pressure was not so large. In the case of a viaduct, it was measured remotely (about 30 m), but because it is surrounded released space, the influence was not so large. However, in a closed space surrounded by concrete, the sound pressure needs to be increased as the distance increases, and the influence of the reverberation from the surroundings on a laser head cannot be ignored. Therefore, we propose a method to solve them, and at the same time, detect the resonance frequency of internal defects and improve the visualization of the defect.

Acoustic vibrations in integrated electro-optic modulators on substrates of lithium niobate

The processes of the excitation of acoustic vibrations and their influence on the optical signals in integrated electro-optic modulators on substrates of lithium niobate have been investigated. The resonant frequencies of the excitation of substrate vibrational modes have been determined. It has been shown that acoustic vibrations can make a significant contribution to the modulator transfer characteristic at frequencies up to several hundred megahertz, which should be taken into account when using the modulators at low frequencies, for example, as a part of fiber-optic sensors.

Studies of acoustic fields excited by a turn-type transmitter in a constant magnetic field

Rigorous mathematical calculations are used to construct graphical dependences that characterize excitation of acoustic vibrations via the electrodynamic effect, which is one of the effects of the electromagnetic-acoustic (EMA) transformation. The calculations were performed using fundamental mathematical expressions, which were obtained in [1], and the plots were constructed for the diametrical plane with respect to the transmitter. A turn above the half-space in a constant magnetic field was chosen as the transmitter. Materials with sharply different electrophysical properties were considered. Calculations were performed for transducers of different dimensions (two limiting cases) and different values of the working gaps. It is shown that the displacement value consists of three summands: a force that is directed along the x axis, a force that is directed along the z axis, and a socalled electromagnetic-like wave. On the basis of the analysis of the obtained plots, it is shown that in all the considered cases, acoustic vibrations propagate as a narrow beam along the normal and the beam width increases as the gap increases.

Acoustic characteristics of a barrier-discharge XeCl excilamp

The generation of an acoustic signal by means of voltage pulses (f = 15 kHz) applied to the electrodes of a barrier-discharge excilamp based on a Xe/Cl2 = (50–500)/1 mixture kept at a pressure of 5–500 Torr is studied. It is shown that, from the time variation of the acoustic signal intensity, one can judge the time instant the excilamp starts operating in a steady mode. Optimal (in power and efficiency) operating conditions of the excilamp are found (Xe/Cl2 = 240/1, p = 98 Torr, η ≈ 9.5%). It is experimentally demonstrated that the discharge energy at a low pressure is spent largely on heating the gas. This is indicative of the volume heat release and volume glow discharge (as the pressure grows, the efficiency of this source of energy consumption drops and more and more energy is spent on acoustic vibration excitation). Under higher pressures, the Fourier spectrum of the acoustic signal becomes richer, the intensity of the spectrum rises, and the dispersion of the signal grows. At very high pressures, the intensity of the acoustic signal drops to a level corresponding to the natural vibrations of the excilamp envelope without the discharge (when the discharge is quenched, the Fourier spectrum of the signal becomes depleted and contains only harmonics corresponding to the carrier frequency of voltage pulses from the power source).

Excitation of acoustic vibrations upon combustion of droplets of liquid fuel at supercritical pressure

Theoretical investigation of the excitation of acoustic vibrations upon combustion of liquid fuel droplets at a supercritical pressure of a gaseous mixture is carried out. The droplets are assumed to be spherical and mono-dispersed. The combustion is assumed to run in the diffusion regime. The binary diffusion coefficients are taken equal and independent of the concentration of the components of the gaseous mixture. The Lewis number is assumed to be unity. The flow is considered to be one-dimensional; the mixing in the transverse direction is assumed to be complete, with no mixing in the longitudinal direction. The rate of combustion at supercritical pressure of the gaseous mixture is calculated on the basis of the modified quasi-stationary theory. Expressions for the frequency and excitation increment of acoustic vibrations are obtained in this paper; the properties of the fuel and the oxidizer are explicitly taken into account.

Excitation of acoustic vibrations in the case of combustion in a channel of alternating cross section

Excitation of acoustic vibrations in the case of combustion in a channel of alternating cross section

Excitation of acoustic vibrations under conditions of condensation of moist steam in a heated channel

Excitation of acoustic vibrations under conditions of condensation of moist steam in a heated channel

Acoustic instability in combustion of droplets of liquid fuel

This study deals with the excitation of acoustic vibrations under conditions of combustion of droplets of liquid fuel in a direct-flow combustor. In so doing, a distributed model of combustion is employed: it is assumed that the zone of burn-up of fuel droplets is approximately equal to the combustor length. The fuel droplets were taken to be spherical and monodisperse. The coefficients of binary diffusion were taken to be identical and independent of the concentration of mixture components. The Lewis number was taken to be unity. The Soret and Dufour effects were ignored. The flow was assumed to be one-dimensional, with thorough stirring in the transverse direction and with the absence of stirring in the longitudinal direction. In considering acoustic vibrations, the gas mixture was taken to be a perfect gas, and the presence of the dispersed phase in the mixture was ignored. Such assumptions are standard in calculations of the working processes in combustors [2, 3]. Then the equations of continuity, motion, and energy, as well as the equation of continuity for droplets, may be written as

Excitation of acoustic vibrations in the postcritical mode of boiling

Excitation of acoustic vibrations in the postcritical mode of boiling

Microwave radiation force on a parallel-plate resonator

A simulation method is proposed and tested in order to determine the radiation force on metal targets whose size is comparable to wavelength. The method is based on the method of moments solution of the electric-field integral equation, accurate calculation of the near field, and removal of the self-interaction terms responsible for the pinch effect. The method is used to determine the local force distribution for a parallel-plate metal resonator. It is observed that, at the resonance, the individual metal plates may experience large force densities, despite the fact that the net radiation force on the resonator still remains very small. A potential use of this observation is discussed, which is directed toward possible excitation of acoustic vibrations.

Excitation of acoustic vibrations by heating a liquid in a supercritical state

Excitation of acoustic vibrations by heating a liquid in a supercritical state

Acoustic instability while combusting aluminum particles in MHD facilities

In this paper, the theoretical study of acoustic-vibration excitation while combusting aluminum particles in a gas mixture is carried out. At long distances, aluminum particles are considered to be monodisperse and immobile with respect to the gas. In determining the excitation conditions for acoustic vibrations, the gas mixture used was considered as a perfect gas. Expressions for the frequency and excitation increment of acoustic vibrations are obtained, characteristics of a fuel and of an oxidizer explicitly entering into these expressions. In [1], powders prepared on the basis of carbon and metals were suggested as a fuel for pulsed MHD generators. Air or oxygen are oxidizers in this case. Characteristics of MHD generators while operating with these fuels, in fact, do not differ from those obtained using solid rocket propellants. In this case, the powder-like fuel presents the possibility for profound control of the device power when changing the mass flow-rate of components and provides for long-term operation of powerful MHD generators. It was shown in [2] that aluminum fuel is optimal for pulsed MHD generators, provided that the air oxidizer is employed. It is known [3] that in the process of burning a disperse fuel the acoustic vibrations can be excited, which results in violation of the normal operation of combustion chambers and even in their destruction. In the practice of rocket-engine development [4], engines having pressure-vibration amplitudes of not more than 5% from the nominal value are related to those exhibiting stable combustion. It was shown in [5] that the requirements for stability of the combustion process in the MHD-generator combustion chamber should be more rigorous than in rocket engines. This is caused by the fact that for such pressure vibrations electric-power pulsation phenomena hinder the normal operation of an MHD generator. Therefore, consideration of acousticvibration excitations in the case of combusting a powder-like fuel in the combustion camber of an MHD generator is an urgent problem. Usually, in studies of acoustic-vibration excitation, it is suggested that, in the case of combusting a disperse fuel, the burning zone is substantially smaller than the length of the combustion chamber [3, 4]. In actual conditions, the burning zone has a certain extension along the longitudinal axis of the combustion chamber, which is comparable with its length [6]. Therefore, in the case of acoustic vibrations of pressure, disturbances of the velocity and temperature of the gas mixture along the length of the combustion chamber are different, which affects the stability of the combustion process [3, 4]. In this paper, in studies of the excitation of acoustic vibrations, a distributed combustion model is used. In other words, we assume that the zone of burning aluminum particles is approximately equal to the length of the combustion chamber.

Excitation of acoustic vibrations during combustion of droplets of liquid fuel

A theoretical investigation is performed of the excitation of acoustic vibrations during combustion of droplets of liquid fuel. The droplets are taken to be monodisperse, spherical, and stationary with respect to gas away from the droplets. It is assumed that the combustion of droplets proceeds in the diffusion mode. The binary diffusion coefficients are taken to be the same and independent of the concentration of the gas mixture components, and the Lewis number is taken to be unity. In determining the conditions of excitation of acoustic vibrations, the gas mixture is treated as perfect gas. Expressions are derived for the frequency and increment of excitation of acoustic vibrations, which explicitly include the characteristics of fuel and oxidizer.

Excitation of Acoustic Vibrations during Volume Condensation in a Steam–Gas Mixture

A theoretical investigation is performed of the acoustic instability under conditions of volume condensation in a steam–gas mixture. The steam–gas mixture is assumed to be stationary and consist of a gas and moist steam. It is assumed that gas and steam are ideal gases and that the condensation process is nonequilibrium. Condensate droplets are assumed to be spherical and monodisperse, and their volume concentration is assumed to be minor compared to unity. It is demonstrated that acoustic vibrations are excited during volume condensation of steam. Formulas are derived for the calculation of the increment and frequency of vibrations.