Axial Acoustic Fields Research Articles

Miniaturized acoustic focus tunable photoacoustic transmitter probe

Aiming to provide dynamically adjustable acoustic field for in vivo ultrasound application scenarios, a miniaturized optical fiber photoacoustic (PA) transmitter is presented. Different from the existing laser generated focused ultrasound probes with fixed acoustic focal length, its focus can be continuously adjusted from infinity to 2.166 mm by deforming the PA conversion film using pneumatic actuation method. The PA film is designed into a circular suspending elastic membrane structure with clamped boundary, which consists of a PDMS/candle soot nanoparticles (CSNPs) mixture layer sandwiched between two pure polydimethylsiloxane (PDMS) layers. Using soft lithography strategy, the PA film together with a structured PDMS substrate and a multimode fiber (MMF) are assembled together to construct the final PA probe with 2.68 mm aperture. For a proof-of-concept demonstration, three different focusing statuses (with 5.196 mm, 3.246 mm and 2.166 mm focal lengths) are provided and their axial acoustic field distributions are characterized as well. Under the excitation condition with the laser fluence of 13.26 mJ/cm2, acoustic signal with 9 MHz center frequency, 92.91% fractional bandwidth at − 6 dB and peak negative pressure (PNP) amplitude up to 8.91 MPa at the 2.166 mm focal point can be achieved, resulting in mechanical index (MI) of 2.97. Referring to the reported data, the current PA transmitter probe demonstrates excellent application potential.

Variable Aperture Method of Ultrasonic Annular Array for the Detection of Addictive Manufacturing Titanium Alloy

The ultrasonic annular array transducer usually has a stronger focusing acoustic field than the linear array and matrix transducer with the same number of array elements, and is more suitable for the detection of large thickness and high attenuation components. However, due to the special arrangement of array elements, the focusing beam cannot be deflected and has a large near field, which limits its application in practical detection. The element parameters of annular array transducer are often designed and analyzed according to the 2-D acoustic field model of a linear array transducer. Therefore, the 3-D acoustic field distribution model of the annular array transducer is established, and the influence of the element parameters on its spatial acoustic field focusing characteristics is analyzed. The design criteria of the array element division mode and element size are proposed, which can avoid the generation of high-energy side lobe and grating lobe, and have good axial acoustic field. Then, the influence of excitation aperture on the energy and size of focal spot at different depths is discussed. The dynamic focusing method with variable aperture of annular array is established, and the C-scan detection experiment is carried out on the addictive manufacturing titanium alloy specimen. The detection results show that the variable aperture method has better central amplitude consistency and imaging accuracy for different depth defects, and has better near surface detection ability than the fixed aperture method.

Axial acoustic field barrier for fluidic microparticle separation at high-throughput

An acoustic field barrier integrated within a flow tubing system to achieve high-throughput separation of particles in fluid will be reported. We investigate the axial acoustic field of a piezo-tube with an inside diameter of 34 mm, a length of 25 mm, and an operating frequency of 1.15 MHz. Energy concentrates within the tube, and leakage at the ends provides a sharp monotonic acoustic pressure field within a fluidic circuit. This process is not the conventional standing wave mechanism; instead, the geometry produces a spatially stable filtering action without fouling. This powerful filtering action is confirmed theoretically via a COMSOL simulation and demonstrated experimentally by concentrating suspensions of 5 μm proteoglycan tracer particles at a flow rate of 20 ml/min: The corresponding acoustic contrast factor is 0.243, and the trapping force is 11 pN. This tube geometry tackles the limitations of microfluidic standing wave based acoustic concentrators, namely, complex extraction, low-throughput, and distributed focus, by harnessing a stable monotonic field profile.

Axial acoustic field along a solid-liquid fluidized bed under power ultrasound

This work investigates the ultrasound propagation within a liquid-solid fluidized bed. The acoustic mapping of the reactor is achieved by means of a hydrophone. A spectral analysis is carried out on the measured signals to quantify the cavitation activity. The effects of several parameters on the spectral power distribution is appraised – including emitted ultrasound power, liquid superficial velocity and solid hold-up. Results show that increasing US power promotes a higher energy transfer from the driving frequency toward the broad-band noise – which is the signature of transient cavitation – and yields a stronger acoustic shielding. The presence of a flow opposite to the acoustic streaming may affect the sonoreactor behavior by sweeping the cavitation bubbles away from the ultrasonic horn. Finally the presence of millimeter sized particles significantly increases wave attenuation, presumably due to viscous losses on the one hand, and through the contribution of their surface defects to bubble nucleation on the other hand. Moreover, the influence of the solid hold-up appears to depend upon the particle material (glass or polyamide).

Axial acoustic field barrier for fluidic particle manipulation

An acoustic field barrier integrated within a flow tubing system to achieve high-throughput separation of particles in fluid is reported in this work. We investigate the axial acoustic field of a piezo-tube with an inside diameter of 34 mm, a length of 25 mm, and an operating frequency of 1.15 MHz. Energy concentrates within the tube, and leakage at the ends provides a sharp monotonic acoustic pressure field within a fluidic circuit. This process is not the conventional standing wave mechanism; instead, the geometry produces a spatially stable filtering action without fouling. This powerful filtering action is confirmed theoretically via a COMSOL simulation and demonstrated experimentally by concentrating suspensions of 5 μm proteoglycan tracer particles at a flow rate of 20 ml/min: The corresponding acoustic contrast factor is 0.243, and the trapping force is 11pN. This tube geometry tackles the limitations of microfluidic standing wave based acoustic concentrators, namely, complex extraction, low-throughput, and distributed focus, by harnessing a stable monotonic field profile.

Role of induced axial acoustics in transverse acoustic flame response

This paper addresses the mechanisms through which transverse acoustic oscillations excite unsteady heat release. Forced and self-excited transverse acoustic instability studies to date have strong coupling between the transverse and axial acoustic fields near the flame. This is significant, as studies suggest that it is not the transverse disturbances themselves, but rather the induced axial acoustic disturbances, that control the bulk of the heat release response. This paper presents results from an experiment that controls the relative amplitudes of transverse and axial disturbances and measures the flow field and heat release response for an acoustically compact, swirling flame. 5 kHz, simultaneous sPIV and OH-PLIF measured the flow field and flame edge, and OH* chemiluminescence measured the relative heat release. Experiments performed with essentially the same transverse acoustic wave field, but with and without axial acoustics, show that significant heat release oscillations are only excited in the former case. The results show that the axial disturbances are the dominant cause of the heat release oscillations. These observations support the theory that the key role of the transverse motions is to act as the “clock” for the instability, setting the frequency of the oscillations while having a negligible direct effect on the actual heat release fluctuations. They also show that transverse instabilities can be damped by either actively canceling the induced axial acoustics in the nozzle (rather than the much larger energy transverse combustor disturbances), or by passively tuning the nozzle impedance to drive an axial acoustic velocity node at the nozzle outlet.

Influence of the liquid viscosity on the formation of bubble structures in a 20 kHz field

The cavitation field in a cylindrical vessel bottom-insonified by a 19.7kHz large area transducer is studied experimentally. By adding controlled amounts of Poly-Ethylene Glycol (PEG) to water, the viscosity of the liquid is varied between one- and nine-fold the viscosity of pure water. For each liquid, and for various displacement amplitudes of the transducer, the liquid is imaged by a high-speed camera and the acoustic field is measured along the symmetry axis. For low driving amplitudes, only a spherical cap bubble structure appears on the transducer, growing with amplitude, and the axial acoustic pressure field displays a standing-wave shape. Above some threshold amplitude of the transducer, a flare-like structure starts to build up, involving bubbles strongly expelled from the transducer surface, and the axial pressure profile becomes almost monotonic. Increasing more the driving amplitude, the structure extends in height, and the pressure profile remains monotonic but decreases its global amplitude. This behavior is similar for all the water–PEG mixtures used, but the threshold for structure formation increases with the viscosity of the liquid. The images of the bubble structures are interpreted and correlated to the measured acoustic pressure profiles. The appearance of traveling waves near the transducer, produced by the strong energy dissipated by inertial bubbles, is conjectured to be a key mechanism accompanying the sudden appearance of the flare-like structure.

Acoustic field characteristics of ultrasonic linear phased array for an interface condition

Acoustic field characteristics of an ultrasonic linear phased array are studied under the condition of an interface. The issue about a phased array transducer with a wedge is reasonably simplified into a liquid-solid interface problem. According to ray acoustics theory, the acoustic field produced by a single element on the liquid-solid interface is computed. Then focus laws are derived and the expressions of the sound field and the displacement of the ultrasonic linear array with a liquid-solid interface are obtained. Simulations are carried out for a phased array transducer mounted on a wedge. The focusing-generated effects on axial and horizontal acoustic field of transducer are discussed. It is shown that the focusing can enhance sensitivity and resolution, but degrades the performance of beams beyond the focal region. So focusing should be reasonably used in actual testing.

An experimental investigation of interaction of sprays with acoustic fields

This paper presents the effect of axial acoustic fields on an air-atomized spray. In these experiments, a spray of ethanol is established in a transparent test section in which an acoustic field is set up using acoustic drivers. Visual observation shows that a high-amplitude acoustic field (160 dB) reduces the length of the spray. The spray velocity field was characterized by particle image velocimetry (PIV) measurements. The spray velocities are considerably reduced in the presence of the acoustic oscillations, indicating the presence of smaller droplets. The presence of acoustic oscillations increases the spray cone angle. The flow field in the near field of the spray was characterized. There is considerable entrainment of air into the spray in the presence of acoustic oscillations. It is seen that the acoustic velocity rather than the acoustic pressure primarily affects the spray.

Experimental Investigation of the Evaporation of Droplets in Axial Acoustic Fields

The effect of axialacoustic Ž elds on the evaporationof individualdroplets is investigatedboth experimentallyand theoretically. A setup was developed in which images of droplets moving through an acoustic Ž eld were acquired using a state-of-the-art, high-speed, intensiŽ ed video system. The evaporation rates of droplets were determined from the droplet diameters measured from these images. Experimental investigations using methanol droplets showed that the presence of an acoustic Ž eld signiŽ cantly enhances the evaporation of droplets. The evaporation rate is a strong function of the acoustic amplitude, but increases only slightlywith frequency. The effect of acoustic oscillations on water and methanol droplet evaporation was theoretically modeled by numerically integrating the differential equations for droplet mass, momentum,and heat transfer. The model uses quasisteady correlations for momentumheat and mass transfer. The model predicted the measured trends correctly. However, the quantitative agreement of these predictions with the experimental data depends strongly on the correlations for Nussult and Sherwood numbers used to model the energy and mass transfer, respectively.

A Theoretical Investigation of the Behavior of Droplets in Axial Acoustic Fields

This paper describes a theoretical investigation of the behavior of small droplets in an acoustic field. It was motivated by the increasing interest in the use of pulsations to improve the performance of energy intensive, industrial processes which are controlled by rates of mass momentum and heat transfer. The acoustic field is expected to enhance heat and mass transfer to and from the droplets, probably because of the relative motion between the droplets and the gas phase. Relative motion is traditionally quantified by an entrainment factor which is defined as the ratio between the amplitude of the droplet and the gas phase oscillations, and a phase delay. In an alternate approach, these two quantities are combined into a single quantity called the “degree of opposition” (DOP), which is defined as the ratio of the amplitude of the relative velocity between the droplet and the gas phase to the amplitude of the acoustic velocity. The equation for the droplet motion is solved using two methods; by numerical integration and by using a spectral method. Despite the nonlinear nature of the problem, the results were found not to be sensitive to initial conditions. The DOP was predicted to increase with increasing droplet diameter and frequency. In other words, larger diameters and higher acoustic frequencies reduce the ability of the droplets to follow the gas phase oscillations. The DOP also decreases with increasing acoustic velocity. It was shown that the amplitude of the higher harmonics are very small and that the droplet mean terminal velocity decreases with increasing acoustic velocity. Theoretical predictions were compared with experimental data and good agreement was observed.

An Experimental Investigation of the Behavior of Droplets in Axial Acoustic Fields

This paper describes an experimental investigation of the behavior of water droplets in axial acoustic fields. It was motivated by the increasing interest in the use of pulsations to improve the performance of energy intensive, industrial processes. The presence of an acoustic field is believed to enhance heat and mass transfer to and from the droplets, probably because of the relative motion between the droplets and the gas phase. This relative motion is characterized by the ratio of the amplitude of the oscillatory droplet velocity to that of the acoustic velocity (entrainment factor), and by the phase between the droplet and gas phase oscillations. An experimental set-up was developed to investigate the effect of acoustic oscillations on the motion of individual droplets. In these experiments a droplet produced by a piezo-ceramic droplet generator is allowed to fall through a transparent test section in which an acoustic field has been set up using a pair of acoustic drivers. Images of the droplets in the test section acquired at consecutive instants using a high speed, intensified imaging system were used to determine the time dependent droplet trajectory and velocity. The acoustic velocity was calculated from measured acoustic pressure distributions. The entrainment factor and the phase difference were then determined from these data. The results show how the entrainment factor decreases and the phase difference increases with increasing droplet diameter and frequency, indicating that larger diameters and higher frequencies reduce the “ability” of the droplets to follow the gas phase oscillations. The measured data are in excellent agreement with the prediction of the Hjelmfelt and Mockros model. Both theoretical predictions and measured data were correlated with the Stokes number, which accounts for the effects of droplet diameter and frequency. It was also shown that acoustic oscillations decrease the mean terminal velocity of the droplets.

Flame radiation and acoustic intensity measurements in acoustically excited diffusion flames

This article describes results of an investigation of the mechanisms responsible for the driving of axial instabilities in solid propellant rocket motors. Specifically, the results of two different experimental methods for measuring the driving of axial acoustic fields by sidewall stabilized diffusion flames are compared. These methods utilize laser Doppler velocimetry (LDV) and CH flame radiation measurements, respectively. The results reveal that the interaction between the acoustic field and the flame produces a space-dependent, oscillatory normal velocity component in the flame region, and oscillatory reaction-and heat-release rates, which depend upon the excited acoustic field. The results suggest that the flame either drives or damps the acoustic fields, depending upon the amplitude and frequency of the excited acoustic wave. In addition, these studies show that the flame driving processes vary within the flame region. The results obtained from the flame radiation measurements are in excellent agreement with those obtained using the velocity measurements.

Oscillatory velocity response of premixed flat flames stabilized in axial acoustic fields

This article describes a combined theoretical-experimental investigation of the processes that drive axial instabilities in solid propellant rocket motors. In this study, the solid propellant flame has been simulated by a premixed flat flame that has been stabilized on the porous side-wall of a duct. The driving processes have been investigated by studying the interaction of the premixed flame with an axial acoustic field. Using experimentally determined acoustic pressures, burner surface admittances, and steady-state flame temperature distributions as input data, the developed model was used to determine the characteristics of the velocity field in the flame region under a variety of test conditions. The predicted velocity field was then compared with LDV velocity measurements to check the validity of the model and determine the flame driving. These studies reveal that the investigated flame responds to the presence of an axial acoustic field by producing an oscillatory velocity component, ν′, normal to the duct wall, that can drive or damp the acoustic field. Comparison of the measured data with the model predictions reveal satisfactory agreement. These studies also showed that driving and damping of the acoustic field occur simultaneously in different regions of the flame. The net effect of the flame upon the acoustic field depends upon the relative magnitudes of these opposing tendencies. It is also shown that the flame driving depends upon the acoustic admittance of the side-wall surface and the frequency of the acoustic field.

Acoustic radiation of rotary positive displacement machinery

A theoretical model for the mechanism by which rotary positive displacement machines generate noise is developed. As a first approximation, the machine is regarded acoustically as a rigid, flat and circular piston pulsating inside an infinite baffle. An original expression for the source strength is exploited to give expressions for the axial acoustic pressure field and the acoustic impedance of the piston source. The acoustic pressure field of the piston source when placed in a duct is also derived. Linear plane wave theory is used throughout and no correction is attempted for the effects of flow and friction. Comparison with experiment shows that the simple mathematical model predicts the qualities of the observed sound field rather well; quantitative agreement on the whole is fair. Suggestions for future improvement are made.