Production Of Acoustic Waves Research Articles

Theory and application of reverberated direct and indirect noise

The generation of a temperature disturbance in a flow is accompanied by the production of acoustic waves (direct noise) and of an entropy perturbation. If this entropy perturbation is accelerated or decelerated (as is the case through a nozzle or flow restriction), additional acoustic waves are generated (indirect noise). Several studies have demonstrated this mechanism in controlled conditions by using entropy wave generators, in which entropy waves are generated and convected through a nozzle, leading to direct and indirect noise. An analytical analysis of the direct and indirect noise produced by the generation and acceleration of entropy waves in a reflective environment is presented. The effect of reverberation (repeated acoustic reflections) on low-frequency perturbations (characteristic of entropy wave generators) is determined analytically. These results are then implemented in a set of limit cases, showing the limit behaviours of such systems. The analytical model is applied to the case of the Cambridge entropy wave generator experiment, in which entropy waves are generated by an electric heater and accelerated through a subsonic orifice plate. Due to the clear time separation of direct and indirect noise in the experimental results, direct and indirect noise transfer functions can be extracted from the experimental data for the first time and compared directly with existing theoretical models. The backward-propagating indirect noise generated at an orifice plate is shown to be significantly higher than predicted by existing theoretical models for isentropic nozzles.

Production of acoustic waves from pulsed thermal radiation

Generation of acoustic waves from a pulsed thermal radiation beam was experimentally investigated in this paper. Unlike other TACs (thermal acoustic converters), acoustic wave frequencies and phases of this type of TACs can be easily adjusted and controlled—a feature essential to the synchronization of TACs in an array. The TACs we developed were made up of a cylindrical aluminum housing, a glass cover, and metallic wool for effective thermal-to-acoustic energy conversion. Radiation emitted from an infrared heater was chopped at a constant frequency prior to entering the converter through the glass cover. The metallic wool was periodically heated by the pulsed radiation beam and cooled because of heat losses to the surroundings. Experiments were conducted for different converter designs and metallic wools, as well as different chopper speeds and radiation intensities. The amplitudes of generated sound waves were found to be dependent on the radiation intensity, the grade and properties of the metallic wool, and the cooling rate of the wool. Higher radiation intensities and thinner wools of lower thermal conductivity produced sounds of higher amplitude. More effective cooling of the metallic wool also helped. Acoustic energy output can also be enhanced by reducing the amount of air in the converter. Copyright © 2015 John Wiley & Sons, Ltd.

Temporal investigation of acoustic wave pressure spectrum of high repetition rate TEA-CO2 lasers by using a three-dimensional mathematical model

The production of acoustic wave is one of the most important factors on the action of High Repetition Rate TEA-CO2 lasers. In this paper a three-dimensional mathematical modeling has been considered for investigation of temporal and spatial variations of acoustic waves in the laser cavity and its effect on the output of HRR lasers. By calculation of equations obtained from this model and plotting the pressure spectrum in different states, the effect of electrodes dimensions, cavity dimensions, gas flow velocity and repetition rate of laser in different times have been acquired. At last, optimum conditions for performance of laser action and having a good output have been arrived.

Production and detection theory of pulsed photoacoustic wave with maximum amplitude and minimum distortion in absorbing liquid

The photoacoustic effect produced by laser-induced stress waves in liquids has found a variety of scientific and practical applications in industry, medicine, and environmental protection. The photoacoustic technique has the natural advantage of being less sensitive to scattering than traditional optical tools, thus offering increased accuracy and sensitivity. This paper aims to give a systematic outline of photoacoustic techniques, starting with the production and extending to the propagation and detection of photoacoustic waves. The focal point is the production of acoustic waves with maximum amplitude and minimum distortion. Maximum amplitude plays an essential role in photoacoustic spectroscopy and minimum distortion is the key to the determination of optical distribution and imaging in turbid media.

The interaction of a shock with a vortex: Shock distortion and the production of acoustic waves

Numerical simulations of a shock interacting with a compressible vortex are presented for shocks and vortices of various relative strengths. The simulations show the effects of the vortex on the shock structure and the structure of the acoustic field generated by the shock–vortex interaction. A relatively weak vortex perturbs the transmitted shock only slightly, whereas a strong vortex leaves the transmitted shock with a structure corresponding to either a regular or Mach reflection. The acoustic wave generated by the interaction consists of two components: a ‘‘quadrupolar’’ component produced by the initial shock–vortex interaction and the complex reflected shock system. When these waves merge, they form the asymmetric structure seen in experiments.

Investigation of viscosity and heat conduction effects on the evolution of a transient picosecond photoacoustic grating

The production of acoustic waves by the absorption of radiation, known as the photoacoustic effect, is described by two coupled differential equations for pressure and temperature. Here, solutions to the coupled equations are given for sinusoidal deposition of heat in space as is typically generated using two picosecond laser beams to form a transient grating. The solutions to the coupled equations, along with the equation of state of the fluid, gives the pressure, temperature, and density of the fluid following rapid deposition of heat. The time dependence of the grating diffraction efficiency is readily calculated from the modulation of the density and temperature. For small values of the viscosity and heat conduction parameters, closed form expressions for the state variables are given which describe thermal and acoustic modes of wave motion in the fluid. The method is applied for computation of the state variables using a frequency-dependent viscosity. Experiments with ethylene glycol are reported where the viscosity and sound speed are measured as functions of temperature.

A review of the processes by which ultrasound is generated through the interaction of ionizing radiation and irradiated materials: some possible applications.

The production of acoustic waves following the absorption of energy deposited by ionizing radiation, with a consequent production of localized thermal spikes has been confirmed by a number of papers published in the physics literature. This paper reviews the basic theory and presents most of the supporting experimental data. Some of the experimental methods used and the results obtained are summarized. In addition to the rather straightforward and routine use of acoustic phenomena produced by ionizing radiation for the detection and measurements of such radiation, there are some special applications that appear to be especially attractive for medical physics. Some of these are unique to ionizing radiation in that the amplitude of the ultrasound wave is proportional to the energy deposited in small volumes at localized sites of these interactions, while others derive from methodologies already in use with nonionizing radiations. The detection and measurement of this ultrasonic radiation could possibly lead to methods for the study of such fundamental phenomenon as track structure, precision localization of therapeutic treatment beams, and even the possible imaging of internal anatomic structures to provide on-line portal images.

Afterglow gain measurements in CO2–He–N2 mixtures at pressures up to 1 atmosphere

A CW 10.6 μm CO2 laser has been used to measure the small-signal gain as a function of time in transversely excited capacitive discharges in CO2–He–N2 mixtures at pressures up to 1 atm. The gain of the amplifier and the average gain coefficient were measured. Different gas mixtures were investigated and the general effect of N2 and He on the gain was observed. The gain invariably increased with reduction of pressure and amplifications of greater than six were measured in an amplifier length of 2.45 m. The gain increased with the voltage applied to the discharge condenser. Deflection of the laser beam by the discharge and the production of acoustic waves in the gas were attributed to gas heating.

LXXVII.On the production of acoustic waves by means of an air-jet of a velocity exceeding that of sound

LXXVII.<i>On the production of acoustic waves by means of an air-jet of a velocity exceeding that of sound</i>