Acoustic Excitation Research Articles

Experimental and numerical modal analysis of the Turkish traditional instrument 'Bendir'

This study carried out an experimental and numerical modal analysis of the Turkish traditional instrument 'Bendir' by an acoustical excitation source. In experimental modal analysis, Bendir was excited acoustically for specified frequencies and elastic deformations of different areas of Bendir due to the excitation were observed with a laser vibrometer. Using the Fourier series handled gained data, modal analysis was done. The first six natural frequencies obtained experimentally and numerically were compared. In addition, other hits in Bendir, symbolizing three fundamental voices such as 'dum', 'te', 'ke' were used to excite the instrument. As a result, time dependent deformation & velocity data were determined and different responses of Bendir were compared.

Sensitivity of acoustic nonlinearity and loss to residual porosity in additively manufactured aluminum

Acoustic nonlinearity and loss are found to be positively correlated with porosity at industrially relevant levels of less than half a percent in commercially pure aluminum produced by laser powder bed fusion (L-PBF) with several different power levels. The technique employed for acoustic measurements involves nonlinear reverberation spectroscopy (NRS) with noncontacting electromagnetic-acoustic transduction, which offers advantages of adaptability to complex part geometries and short inspection times for industrial qualification of additively manufactured (AM) parts of arbitrary size. Porosity and microstructure are characterized with the Archimedes technique, X-ray computed tomography, and scanning electron microscopy. Fit parameters of nonlinearity and loss vs. porosity are found to vary significantly with the height of material in the build, consistent with an hypothesis that the correlations are indirect and involve dislocations as the principal nonlinear/anelastic elements. Nonlinearity and loss decrease with time under acoustic excitation, while being relatively insensitive to pauses in excitation of similar duration, indicating that acoustic excitation at inspection levels induces changes in nonlinear/anelastic defects without predominant involvement of thermal excitation. This remarkable behavior is not seen as a fundamental impediment for the application of the technique to nondestructive AM part qualification because of the brief time required for a measurement.

Phonon dispersion curves in the type-I crystalline and molten clathrate compound Eu8Ga16Ge30

The dynamic structure factor , where Q and E are momentum and energy transfer, respectively, has been measured for liquid Eu8Ga16Ge30 (EGG), using inelastic x-ray scattering. The excitation energy of the longitudinal acoustic mode in the liquid was scaled to that in liquid Ba8Ga16Sn30 (BGS) with the effective mass. This result means that the local structure in both liquids are similar. The longitudinal acoustic excitation energy of type-I clathrate compound EGG disperses faster than that in the liquid, suggesting that the interatomic force is weakened on melting. The lower energy excitation was observed in both liquid EGG and liquid BGS. In comparison with the longitudinal phonon dispersion in crystalline clathrate compound EGG obtained by density functional theory-based calculations, the lower energy in the liquid was found to be near the optical mode energy. The result indicates that the lower energy mode arises from the relative motion between Eu and (Ga, Ge) atoms.

Acoustic excitation of an n-heptane droplet: Evaporation, ignition and combustion characteristics

In aerospace propulsion systems, the coupling between droplet combustion and sound waves can be a key mechanism for driving thermoacoustic instability. In this experimental study, we examine the evaporation, ignition and combustion characteristics of an n-heptane droplet subjected to time-periodic acoustic excitation of different amplitudes (12.5 to 37.5 Pa) and frequencies (100 to 500 Hz). On applying acoustic excitation, we find that the ignition delay time of the droplet decreases, while both the evaporation rate and the combustion rate increase. All three effects strengthen as the excitation amplitude increases, while the first and third effects strengthen as the excitation frequency decreases. These trends are corroborated by analysis of time-resolved CH* chemiluminescence images, which reveal that the heat-release-rate oscillations of the burning droplet grow in amplitude either as the excitation amplitude increases or as the excitation frequency decreases. The pronounced effects on droplet ignition and combustion observed during low-frequency acoustic excitation can be attributed to the time scale of chemical reaction being significantly shorter than that of the acoustic excitation. This study contributes to a better understanding of droplet combustion in the presence of time-periodic acoustic oscillations, facilitating the development of aerospace propulsion systems that can potentially be more resistant to thermoacoustic instability.

Sub-Nanometer Acoustic Vibration Sensing Using a Tapered-Tip Optical Fiber Microcantilever.

We demonstrate a highly sensitive acoustic vibration sensor based on a tapered-tip optical fiber acting as a microcantilever. The tapered-tip fiber has a unique output profile that exhibits a circular fringe pattern, whose distribution is highly sensitive to the vibration of the fiber tip. A piezo transducer is used for the acoustic excitation of the fiber microcantilever, which results in a periodic bending of the tip and thereby a significant output power modulation. Using a multimode readout fiber connected to an electric spectrum analyzer, we measured the amplitude of these power modulations over the 10-50 kHz range and observed resonances over certain frequency ranges. Two types of tapered-tip fibers were fabricated with diameter values of 1.5 µm and 1.8 µm and their frequency responses were compared with a non-tapered fiber tip. Thanks to the resonance effect as well as the sensitive fringe pattern of the tapered-tip fibers, the limit of detection and the sensitivity of the fiber sensor were obtained as 0.1 nm and 15.7 V/nm, respectively, which were significantly better than the values obtained with the non-tapered fiber tip (i.e., 1.1 nm and 0.12 V/nm, respectively). The sensor is highly sensitive, easy to fabricate, low-cost, and can detect sub-nanometer displacements, which makes it a promising tool for vibration sensing, particularly in the photoacoustic sensing of greenhouse gases.

Quantum coherent control in pulsed waveguide optomechanics

Coherent control of traveling acoustic excitations in a waveguide system is an interesting way to manipulate and transduce classical and quantum information. So far, these interactions, often based on optomechanical resonators or Brillouin scattering, have been studied in the steady-state regime using continuous waves. However, waveguide experiments are often based on optical pump pulses, which require treatment in a dynamic framework. In this paper, we present an effective Hamiltonian formalism in the dynamic regime using optical pulses that links waveguide optomechanics and cavity optomechanics, which can be used in the classical and quantum regime including quantum noise. Based on our formalism, a closed solution for coupled-mode equation under the undepleted assumption is provided and we found that the strong coupling regime is already accessible in current Brillouin waveguides by using pulses. We further investigate several possible experiments within waveguide optomechanics, including Brillouin-based coherent transfer, Brillouin cooling, and optoacoustic entanglement.

Multi-mechanism collaboration enhanced photoacoustic analyzer for trace H2S detection

To realize the real-time highly sensitive detection of SF6 decomposition product H2S, a multi-mechanism collaboration enhancement photoacoustic spectroscopy analyzer (MCEPA) based on acoustic resonance enhancement, cantilever enhancement and excitation light enhancement is proposed. An SF6 background gas-induced photoacoustic cell (PAC) was used for acoustic resonance (AR) enhancement of the photoacoustic signals. A fiber-optic acoustic sensor based on a silicon cantilever is optimized and fabricated. The narrow-band acoustic signal enhancement based on cantilever mechanical resonance (MR) is realized in the optimal working frequency band of the PAC. A fiber-coupled DFB cascaded an Erbium-doped fiber amplifier (EDFA) realized the light power enhancement (LPE) of the photoacoustic signals excitation source. Experimental results show that the MR of the fiber-optic silicon cantilever acoustic sensor (FSCAS) is matched with the AR of the PAC and combined with the LPE, which realizes the multi-mechanism collaboration enhancement of weak photoacoustic signals. The Allan-Werle deviation evaluation showed that the minimum detection limit of H2S in the SF6 background is 10.96 ppb when the average time is 200 s. Benefiting from the all-optimization of photoacoustic excitation and detection, the MCEPA has near-field high-sensitivity gas detection capability immune to electromagnetic interference.

Acoustic platforms meet MXenes - a new paradigm shift in the palette of biomedical applications.

The wide applicability of acoustics in the life of mankind spread over health, energy, environment, and others. These acoustic technologies rely on the properties of the materials with which they are made of. However, traditional devices have failed to develop into low-cost, portable devices and need to overcome issues like sensitivity, tunability, and applicability in biological in vivo studies. Nanomaterials, especially 2D materials, have already been proven to produce high optical contrast in photoacoustic applications. One such wonder kid in the materials family is MXenes, which are transition metal carbides, that are nowadays flourishing in the materials world. Recently, it has been demonstrated that MXene nanosheets and quantum dots can be synthesized by acoustic excitations. In addition, MXene can be used as a mechanical sensing material for building piezoresistive sensors to realize sound detection as it produces a sensitive response to pressure and vibration. It has also been demonstrated that MXene nanosheets show high photothermal conversion capability, which can be utilized in cancer treatment and photoacoustic imaging (PAI). In this review, we have rendered the role of acoustics in the palette of MXene, including acoustic synthetic strategies of MXenes, applications such as acoustic sensors, PAI, thermoacoustic devices, sonodynamic therapy, artificial ear drum, and others. The review also discusses the challenges and future prospects of using MXene in acoustic platforms in detail. To the best of our knowledge, this is the first review combining acoustic science in MXene research.

A Survey of Antenna Miniaturization Technology Based on the New Mechanism of Acoustic Excitation

Antenna miniaturization technologies have always been the focus of antenna research. In recent years, mechanical antennas whose size does not depend on the electromagnetic (EM) wavelength have attracted researchers’ attention. Acoustically excited antennas (AEAs), as a kind of mechanical antenna, can achieve reciprocal transmission and reception. AEAs utilize internal charge and magnetization oscillations of piezoelectric materials and magnetoelectric (ME) composite materials, respectively, to radiate EM waves, which enables the size of the antenna to be related to the wavelength of the acoustic wave, reducing the size of the antenna by 4–5 orders of magnitude over conventional antennas. Additionally, it is expected to achieve breakthrough application changes in the fields of mobile low-frequency antennas, portable shortwave antennas, and chip-based microwave antennas. In this article, the theory of AEAs is summarized and different modeling and measurement studies of AEAs are described in detail. Finally, the challenges of AEAs research are elaborated and future research on AEAs is prospected.

Shell viscosity estimation of lipid-coated microbubbles.

Understanding the shell rheology of ultrasound contrast agent microbubbles is vital for anticipating their bioeffects in clinical practice. Past studies using sophisticated acoustic and optical techniques have made enormous progress in this direction, enabling the development of shell models that adequately reproduce the nonlinear behaviour of the coated microbubble under acoustic excitation. However, there have also been puzzling discrepancies and missing physical explanations for the dependency of shell viscosity on the equilibrium bubble radius, which demands further experimental investigations. In this study, we aim to unravel the cause of such behaviour by performing a refined characterisation of the shell viscosity. We use ultra-high-speed microscopy imaging, optical trapping and wide-field fluorescence to accurately record the individual microbubble response upon ultrasound driving across a range of bubble sizes. An advanced model of bubble dynamics is validated and employed to infer the shell viscosity of single bubbles from their radial time evolution. The resulting values reveal a prominent variability of the shell viscosity of about an order of magnitude and no dependency on the bubble size, which is contrary to previous studies. We find that the method called bubble spectroscopy, which has been used extensively in the past to determine the shell viscosity, is highly sensitive to methodology inaccuracies, and we demonstrate through analytical arguments that the previously reported unphysical trends are an artifact of these biases. We also show the importance of correct bubble sizing, as errors in this aspect can also lead to unphysical trends in shell viscosity, when estimated through a nonlinear fitting from the time response of the bubble.

Numerical simulations of a subsonic round jet with transverse acoustic and mechanical forcing

The results of numerical experiments on the bifurcation effects for a free round jet caused by acoustic or vibrational excitations are presented. The three-dimensional distributions of the instantaneous and time-averaged velocity vector components have been obtained and analyzed. The effects of Strouhal and Reynolds numbers on the jet splitting angle, as well as those of the amplitude and type of jet perturbations is discussed in such a comparative parametric study.

AXIAL ACOUSTIC VELOCITY INTERACTION WITH HOLLOW CONE LIQUID SHEET IN A SWIRLING FLOW FIELD

This experimental investigation concerns the effect of imposed axial acoustic velocity fluctuations on the primary atomization of a hollow cone liquid sheet in the presence of a strong air swirl. The atomization dynamics are elucidated by positioning the spray at an acoustic velocity node, antinode, and a mixed point in the standing wave field generated due to the imposed axial acoustic excitation. High-speed shadowgraph images acquired in-sync with dynamic pressure measurements are processed to clarify the unstable behavior observed in the spray dynamics; this was achieved by extracting key parameters such as breakup length, spatial growth rates, phase differences, and by employing Proper Orthogonal Decomposition (POD). A novel method to obtain the breakup length of a hollow cone spray from the position of maximum wave amplitude is presented. The breakup length is the smallest for the mixed point. The phase difference between the left and right half-angle fluctuations shows that the flapping motion of the spray is predominantly observed at the mixed point for different air-to-liquid ratios. Another novel approach is adopted to identify the physical mechanisms corresponding to each POD spatial mode by comparing POD spatial modes obtained from experiments to those generated artificially.

Effect of sound excitation on the flow field and convection heat transfer around a cylinder

The combination of experiment and numerical simulation, sound frequency and sound pressure level on the flow field, and heat transfer around a cylinder with two flow patterns are investigated. Flow patterns are periodic laminar flow (L3, Re0 = 100) and wake transition (TrW2, Re0 = 300), respectively. Sound frequency range f = 100–1000 Hz, and sound pressure level range SPL = 127–145 dB. Based on the influence of acoustic characteristics on the viscous boundary layer and thermal boundary layer of cylindrical wall, constructed a functional relationship between convection heat transfer and sound wave frequency and sound pressure level. Influenced by the acoustic action, the flow field force around the cylinder, the vortex street structure, the vortex size, and the length of the wake area may be changed. Acoustic action flow field excitation fluid particle velocity amplitude increased. As the sound pressure level increased, the amplitude of the fluid mass vibration velocity increased. From the experimental results, it can be seen that for the acoustic frequency effect, the heat transfer coefficient increases with increasing sound frequency. For the particle velocity amplitude effect, as the acoustic Reynolds number increases, the heat transfer coefficient increases and is 2.3 times higher than that without acoustic disturbance in the wake turning flow regime. In comparison with numerical and experimental results, the error is less than 15%, which meets the actual requirements of the project. It has been shown that sound waves can enhance the convective heat transfer around a cylinder in the cross-flow.

Jet Velocity and Acoustic Excitation Characteristics of a Synthetic Jet Actuator

The effect of the excitation frequency of synthetic jet actuators on the mean jet velocity issuing from an array of circular orifices is investigated experimentally, focusing on the acoustic excitation characteristics of the actuator’s cavity. Two cavity configurations are considered. In the first configuration, synthetic jets are generated by exciting a single, large cavity having an array of sixteen orifices via sixteen piezoelectric elements. In the second configuration, the cavity volume of the first configuration is divided into eight isolated compartments, each with two orifices and two piezoelectric elements. Several distinct resonant peaks were observed in the frequency response of the synthetic jet actuator built with a single large-aspect-ratio cavity, whereas the case of compartmentalised cavities exhibited a single resonant peak. Acoustic simulations of the large-aspect-ratio-cavity volume showed that the multiple peaks in its frequency response correspond to the acoustic standing-wave mode shapes of the cavity. Due to its large aspect ratio, several acoustic mode shapes coexist in the excitation frequency range aside from the Helmholtz resonance frequency. When the actuator’s cavity volume is compartmentalised, only the Helmholtz resonance frequency is observed within the excitation frequency range.

Experimental analysis of shear strength and strain rate on H2/CH4 pinch-off flames

An experiment was conducted under acoustic excitation conditions for the physical understanding of the pinch-off phenomenon in non-premixed flames. For fuel conditions, H2/CH4 has a volume ratio of 75/25 %, and for acoustic excitation conditions, experiments were performed at a forcing frequency of 80 Hz and velocity perturbation intensity of 50 %. In addition, experiments were performed in the range of 1.0–4.5 m/s of air and fuel velocity. OH* chemiluminescence analysis using a high-speed camera was performed for flame structure analysis, while particle image velocimetry was performed for flow-field analysis. Simultaneous measurement of OH* chemiluminescence and particle image velocimetry allowed observing the flame separation. Strain rate and shear strength were introduced for pinch-off and non-pinch-off analysis, and the correlation between the two factors was analyzed. Pinch-off mapping analysis at various velocities confirms the presence of a physical boundary. The direction of vortex roll-up reverses depending on the fuel and air velocities, indicating the formation of different flame structures. When acoustic excitation dominates, the strain rate increases and flame pinch-off occurs. In contrast, when the shear effect dominates, a non-pinch-off flame is obtained and the shear strength decreases. Accordingly, the strain rate and shear strength are correlated, and the approximate pinch-off threshold can be determined at various experimental points. These findings suggest that the shear strength and the strain rate can be used as parameters to distinguish pinch-off.

Flame characteristics of backward-inclined pulsating combusting jet in crossflow

The flame appearances, thermal characteristics, and combustion product concentrations of a backward-inclined pulsating combusting jet in crossflow were experimentally studied. The time-averaged and streak instantaneous flame images were acquired using a still camera and a high-speed camera, respectively. The flame extinguishment processes were captured and analysed on the basis of streak instantaneous images. The flame temperatures and combustion product concentrations were measured using a homemade fine-wire thermocouple and a gas analyser, respectively. Three characteristic flame modes (crossflow dominated, transitional, and jet-dominated) were found when the backward-inclined jet flame in crossflow was subject to acoustic excitation. These characteristic flame modes were observed in different regimes of pulsation intensity and jet backward inclination angle. Discrete flame puffs with a frequency equal to the acoustic excitation frequency appeared in the jet-dominated flame. Periodic flame puffing became unstable when the pulsation intensity was increased to values near the instability limit. When the pulsation intensity was beyond the instability limit, a strong stretching effect caused the recirculation flame, which played the role of a flame ignitor, to almost vanish (misfire). The flame extinguishment then occurred from the area around the lee side of the burner tube tip. The temperature and gas concentration measurement results showed the significant improvement of combustion performance in low inclination angle even at small pulsation intensity in the regime of the crossflow dominated flame. To enhance combustion performance at a large jet backward inclination angle, the pulsation intensity must be large enough to push the flame in the regime of the jet-dominated flame.

Acoustically‐Induced Water Frustration for Enhanced Hydrogen Evolution Reaction in Neutral Electrolytes

AbstractA novel strategy utilizing high‐frequency (10 MHz) hybrid sound waves to dramatically enhance hydrogen evolution reactions (HER) in notoriously difficult neutral electrolytes by modifying their network coordination state is presented. Herein, the practical limitations associated with existing electrolyzer technology is addressed, including the need for highly corrosive electrolytes and expensive electrocatalysts, by redefining conceptually‐poor hydrogen electrocatalysts in neutral electrolytes. The improvement in HER performance is attributed to the unique capability of the intense local electromechanical coupling arising from the acoustic‐forcing to ‘frustrate’ the tetrahedrally‐coordinated hydrogen bond network of water molecules at the electrode–electrolyte interface, resulting in the generation of a high concentration of “free” water molecules that are more readily able to access catalytic sites on the unmodified polycrystalline electrode. Together with the other synergistic effects that accompany the acoustic excitation (e.g., hydronium ion generation, convective relaxation of diffusion mass transfer limitations, and prevention of bubble build‐up and their removal from the electrode), the resultant overpotential reduction of 1.4 V at −100 mA cm−2 and corresponding 14‐fold increase in current density, together with a net‐positive energy saving of 27.3%, showcases the potential of the technology as a scalable platform for effectively enhancing the efficiency of green hydrogen production.

Optical Fiber Serpentine Arrangements for Vibration Analysis Using Distributed Acoustic Sensing

Distributed acoustic sensors (DASs) are able to monitor in real-time acoustic excitation and mechanical vibration of the external environment for many kilometers along the length of the optical fiber. In the oil industry, the technology allows the monitoring of well structural health, identifying flow profile, acquiring seismic data, updating reservoir information, and feeding geological models. The spatial resolution of DAS is a limiting factor on the quality of information gathered. In this article, we propose and analyze a novel method of measuring local dynamic strain of structures using DAS. The method uses serpentine configurations to measure the frequency spectrum of the in-phase strain signals at discrete regions of the structure, enabling DAS to determine, among other vibration parameters, the strain frequency response function (SFRF). An experimental study of the sensor application with different arrangements in a small structure has been performed. The structure in the experiment consists of a free–free aluminum beam of approximately 2.7 m. Experiment results show agreement with numerical results developed by the finite element method (FEM) and using reference sensors, demonstrating a significant improvement of the DAS SFRF accuracy using the serpentine configuration and up to 12.9 dB increase in peak-to-noise ratio (PNR) for low excitation forces. The recovered response enabled the reconstruction of mode shapes.

Radiation-enhanced acoustically driven magnetoelectric antenna with floating potential architecture

A magnetoelectric (ME) antenna driven by a high-overtone bulk acoustic resonator (HBAR) can play a potentially positive role in the bandwidth enhancement benefit from its narrow harmonic frequency interval, in which radiation characteristics remain to be explored. HBAR ME antennas with floating potential architecture (FPA) and grounded are fabricated and demonstrated separately. The measured far-field radiation characteristics show that the FPA can significantly enhance the gain and radiation efficiency of the HBAR ME antenna by more than 10 dB compared to the grounded setup. Meanwhile, the time domain amplitude sweep demonstrates the high-power tolerance (>23.2 dBm) of the HBAR ME antenna. Two-dimensional finite element method analysis reveals that the FPA-induced electric field excites additional longitudinal-wave resonance, resulting in the coupling of dual longitudinal and shear waves, which is the intrinsic mechanism of its radiation enhancement from the perspective of acoustic excitation. Not only limited to HBAR antennas, the mechanism of FPA described here is also a promising candidate for radiation enhancement in acoustically driven antennas.

Complex gravity-acoustic impact on M-flame structure

Ensuring fire safety is one of the most important problems in the creation of habitable spacecraft. It is necessary to thoroughly study the issues of the reliability of the functioning of space systems, including those using chemical fuel. The paper studies experimentally the dynamics of an M-shaped premixed methane-air flame under normal and reversed gravity at various external acoustic excitation. The stable M-shaped flame is studied experimentally by means of high-speed video recording of flame chemoluminescence and PIV-methods. The paper shows that at low frequencies of acoustic excitation, the flame front remains practically undisturbed. At a 160 Hz frequency, the oscillatory motion is energetically more saturated than the main gas flow. With a further frequency increase, the main flow again becomes dominant in terms of energy, and the effect on the flame structure decreases.