Interaction Of Acoustic Fields Research Articles

Application of the catastrophe theory to acoustoplasma research

In the paper the possibility of applying the mathematical theory of catastrophes to the processes occurring during the interaction of acoustic fields with plasma is considering. In particular, it is proposed to apply the theory to find out possible jumps in parameters and phase transitions in an acoustoplasmic medium. Based on the experimentally obtained database (in this case, for acoustoplasmic processes), a mathematical model of the process under study was constructed. And then, applying the theory of catastrophes, it is determined whether there are sudden changes in the values of control parameters in the area under consideration, critical points of unstable equilibrium and phase transitions, i.e. catastrophic parameter jumps. The choice of the equation describing the process is made based on the number of control parameters for this process. It is also proposed to use the methodology in various fields of science and technology, where similar conditions may arise.

On the Importance of Consistent Insonation Conditions During Hydrophone Calibration and Use.

Hydrophones are generally calibrated in acoustic fields with temporally localized (short pulse) or long duration (tone burst) signals. Free-field conditions are achieved by time gating any reflections from the hydrophone body, mounting structures, and surrounding water tank boundaries arriving at the active sensing element. Consequently, the sensitivity response of the hydrophone is a result of direct waves incident on its active element, free from any contaminating effects of reflections. However, when using tone bursts below 400 kHz to calibrate hydrophones, it may not be possible to isolate the direct wave from reflection artifacts. This means that the sensitivity responses derived at these frequencies using short pulse and tone burst signals might not be comparable as they can be characteristic of the acoustic field interaction with either/both the hydrophone active element alone or the hydrophone active element and body. Therefore, there is a need to consider an appropriate calibration method for a given hydrophone type, depending on whether the eventual application employs short pulse or tone burst acoustic fields. This article presents the findings from a short study comprising four needle-type hydrophones of active element diameters in the range of 1-4 mm. These hydrophones were calibrated from 30 kHz to 1.6 MHz using established calibration methodologies within the underwater acoustics (UWA) and ultrasound (US) areas employed at the National Physical Laboratory (NPL), Teddington, U.K. In UWA tone, burst acoustic fields are used, while in US, it is short pulses. The 2- and 4-mm-diameter needle hydrophones showed the largest variation at the overlapping frequencies, in which the maximum disagreement of UWA calibration was 30% relative to US calibration. For the 4-mm hydrophone, UWA calibration exhibited resonant sensitivity structure between 100 and 450 kHz, but which was absent in US calibration. This observed behavior was further investigated theoretically by using a validated acoustic wave solver to confirm the resonant sensitivity structure seen in the case of UWA calibration. The work contained within illustrates the need to ensure that the method of calibration is carefully considered in the context of the duration of the acoustic signals for which the hydrophone is intended.

Evaluation of the damping capacity according to the geometric and the number of resonator with thermal environment using a Rijke tube

Combustion instabilities are often-observed phenomena which take place in acoustically closed spaces such as combustion chambers in propulsion systems. These phenomena should be studied consistently because they can make considerable damage on the combustion chambers and even entire engine systems. Acoustic cavity has been widely used as a passive stabilization device to suppress these combustion instabilities. To elucidate damping capacity of acoustic cavity according to various geometric shapes, a well-known Rijke tube facility has been used to simply include the interaction of heat and acoustic field. The damping capacity has been evaluated quantitatively in terms of bandwidth and amplitude ratio. Present results showed that the acoustic cavities having large orifice area showed better damping capacity. But, in case of orifice length, the length above a certain value makes cavity volume and resultant acoustic stiffness too small, so the damping capacity was decreased above a threshold-like specific length. As for the number of cavities, the damping capacity increases with the number, but, in a certain number or more, the damping capacity has been attenuated and the significant increase of the capacity was not observed. In case of the acoustic cavities which were thought to have sufficient damping capacity, decay time has been measured to quantify the damping capability in time domain. The effect of orifice area on decay time was much higher than those of orifice length. These results from heat and acoustic field interaction through Rijke tube showed that the pure acoustic approach would be insufficient for the fine tuning of acoustic cavity, and further combustion tests would also be necessary to optimize the shapes of the cavity.

Errors in Measurements of the Vibration Parameters of Structural Units by Wave Methods

Conditions under which a quasistatic approximation may be employed in the process of interaction of acoustic fields with vibration fields are considered. Criteria are found based on which the method error may be estimated in the case of a wideband range of a particular vibration field. The errors that arise in modulation of a reflected acoustic signal by phase and amplitude are estimated.

Acoustofluidics 15: streaming with sound waves interacting with solid particles

In Part 15 of the tutorial series "Acoustofluidics-exploiting ultrasonic standing waves forces and acoustic streaming in microfluidic systems for cell and particle manipulation," we examine the interaction of acoustic fields with solid particles. The main focus here is the interaction of standing waves with spherical particles leading to streaming, together with some discussion on one non-spherical case. We begin with the classical problem of a particle at the velocity antinode of a standing wave, and then treat the problem of a sphere at the velocity node, followed by the intermediate situation of a particle between nodes. Finally, we discuss the effect of deviation from sphericity which brings about interesting fluid mechanics. The entire Focus article is devoted to the analysis of the nonlinear fluid mechanics by singular perturbation methods, and the study of the streaming phenomenon that ensues from the nonlinear interaction. With the intention of being instructive material, this tutorial cannot by any means be considered 'complete and comprehensive' owing to the complexity of the class of problems being covered herein.

Visualising scattering underwater acoustic fields using laser Doppler vibrometry

Analysis of acoustic wavefronts are important for a number of engineering design, communication and health-related reasons, and it is very desirable to be able to understand the interaction of acoustic fields and energy with obstructions. Experimental analysis of acoustic wavefronts in water has traditionally been completed with single or arrays of piezoelectric or magnetostrictive transducers or hydrophones. These have been very successful, but the presence of transducers within the acoustic region can in some circumstances be undesirable. The research reported here, describes the novel application of scanning laser Doppler vibrometry to the analysis of underwater acoustic wavefronts, impinging on circular cross section obstructions. The results demonstrate that this new non-invasive acoustics measurement technique can successfully visualise and measure reflected acoustic fields, diffraction and refraction effects.

Acoustic field interaction with a boiling system under terrestrial gravity and microgravity.

Pool boiling experiments from a platinum wire heater in FC-72 liquid were conducted under terrestrial and microgravity conditions, both with and without the presence of a high-intensity acoustic standing wave within the fluid. The purpose of this research was to study the interaction between an acoustic field and a pool boiling system in normal gravity and microgravity. The absence of buoyancy in microgravity complicates the process of boiling. The acoustic force on a vapor bubble generated from a heated wire in a standing wave was shown to be able to play the role of buoyancy in microgravity. The microgravity environment was achieved with 0.6 and 2.1-s drop towers. The sound was transmitted through the fluid medium by means of a half wavelength sonic transducer driven at 10.18 kHz. At high enough acoustic pressure amplitudes cavitation and streaming began playing an important role in vapor bubble dynamics and heat transfer. Several different fixed heat fluxes were chosen for the microgravity experiment and the effects of acoustics on the surface temperature of the heater were recorded and the vapor bubble movement was filmed. Video images of the pool boiling processes and heat transfer data are presented.

Transient effects in droplet ignition phenomenon

We discuss the transient effects of mass and energy accumulation processes and of an applied acoustic field interaction on the droplet ignition phenomenon. In order to observe simultaneously the influence of those processes on the ignition, we assume that the chemical reaction occurs in the droplet unsteady far field, where the transient accumulation processes and the acoustic field are as important as the other processes (i.e., the conduction process and the chemical reaction). We also consider the Lewis number as being constant and nonunity. The results show that the ignition time is modified by the acoustic field.

Nonlinear resonant interaction of acoustic fields with Tollmen-Schlichting waves

Nonlinear resonant interaction of acoustic fields with Tollmen-Schlichting waves

Schlieren Visualization of Acoustic Fields

The schlieren method has been used for visualization of the interaction of acoustic fields and solid surfaces. The reradiated waves caused by an incident pulse are observed at different positions by strobe schlieren systems. Individual cycles can be displayed in an acoustic wave, and thus phase information is available in a visualizationsystem that utilizes a very short optical illumination by means of a strobed high-power laser. Color schlieren pictures permit visualization of small-amplitude effects in the presence of larger-amplitude (brighter) fields. A movie is shown demonstrating the usefulness of observing wave motion and changing effects with changing experimental conditions. Applications of schlieren visualization to basic studies of wave phenomena, nondestructive testing, and radiation pressures are shown.