Acoustic Excitation Research Articles

Numerical study of static pressure on the sonochemistry characteristics of the gas bubble under acoustic excitation

Abstract The extreme environment formed during bubble collapse can cause a series of chemical reactions inside the bubble, and multiple products (i.e., ˙OH, H˙, O, H2, and HO2˙, etc.) are produced, which is called sonochemistry. In this study, a new model is used to predict the sonochemistry characteristics inside an oxygen bubble oscillating in water. The influences of static pressure, ultrasonic frequency, and the equilibrium radius on the temperature inside the bubble and the yields of chemical products are analyzed. The numerical calculation results are obtained during bubble oscillations under a steady state, which is different from the previous studies that focus on the sonochemical characteristics at the bubble collapse. Numerical studies show that with the change in the equilibrium radius, the maximum bubble temperature fluctuates drastically, and the maximum yields of H2 and ˙OH show a Gaussian curve trend. The cavitation activity corresponding to the equilibrium radius depends on the combination of static pressure and ultrasonic frequency.

Active noise control with resonant flush-mounted piezoelectric cells in the presence of airflow: Wind tunnel experiments

Methods and technologies to control the noise coming from turbulent flows impinging on outlet guide vanes in turbofan engines are tremendously needed nowadays to comply with new legislation on acoustic environment pollution. The following paper relies on preliminary research proposing a new experimental design of active noise control system to be further integrated into a jet engine outer guide vane prototype, consisting of multiple flush-mounted piezoelectric cells. Here, three active cells are placed on the bottom wall of a wind tunnel with upstream acoustic excitation generating grazing incident waves for different airflow velocities. Using a downstream microphone as an error sensor, the active system succeeds in improving the transmission loss of the sample by 2.5 dB at the target frequency (670 Hz) corresponding to the main piezo acoustic mode with airflow velocities up to 20 m.s−1. These early experimental results confirm the ability of the concept to interact with the ambient acoustics without disturbing the airflow as in passive solutions using porous materials, for instance, leading the path to future experiments with realistic turbulence generating acoustic sources on the leading edge of the vane profile.

Numerical study of an acoustic field effect on boundary layer transition mechanism

The laminar-to-turbulent transition mechanism is fundamentally important for the concepts aiming at reducing CO2 emissions and noise pollution. This research is focused on understanding the acoustic excitation effect on the transition mechanism with a laminar separation bubble. The transition over the suction side of the NACA 0012 airfoil at an angle of attack of 2 degrees and Reynolds number of 2 × 105 with zero free-stream turbulence intensity is considered. The investigation is performed based on three flow cases obtained for a smooth and wavy surface modified airfoils using the large-eddy simulation method. The acoustic excitation is controlled by keeping or removing a naturally occurring acoustic feedback loop by introducing surface waviness on the airfoil’s pressure side. The flow solution for the standard NACA 0012 airfoil provides the acoustic excitation case, whereas the airfoil with the wavy surface on the pressure side provides the case without acoustic excitation. The results confirm that a shorter separation bubble and earlier laminar-to-turbulent transition can be triggered in response to the acoustic excitation. Furthermore, the coupling of the acoustic field and hydrodynamic instabilities is investigated. Finally, it is shown by referring to experimental data that the Kelvin-Helmholtz instabilities inside the separation bubble are amplified by the acoustic excitation.

Acoustic and Optical Investigations of the Flame Dynamics of Rich and Lean Kerosene Flames in the Primary Zone of an Air-Staged Combustion Test-Rig

Abstract In this study, the thermoacoustic behavior of nonpremixed kerosene flames from rich to lean combustion conditions is investigated. Flame-transfer-functions (FTF) measured purely acoustically are compared with results based on flame chemiluminescence. OH*, CH*, C2*, and CO2* were selected as potential measures for representing steady and fluctuating heat release when burning nonpremixed kerosene. In addition, their ability for the quantification of equivalence ratio fluctuations will be highlighted. The measurements were performed in the primary zone of an atmospheric rich-quench-lean (RQL) combustion test-rig. The new experimental approach allows a characterization of the primary zone independent of the secondary zone. Rich and lean operating points were analyzed by fueling an aero-engine prototype injector with and without acoustic excitation. To improve the quality of the acoustic wavefield reconstruction a thermocouple correction method was implemented. The flame dynamics determined with the multimicrophone method (MMM) exhibit a frequency and equivalence ratio depending effect of rich combustion conditions. The results for the steady behavior of the chosen radicals by altering equivalence ratio and thermal power indicate proportionality of the chemiluminescence to thermal power. Furthermore, the CH*/C2* ratio is found to be a promising indicator for the global equivalence ratio in the combustion chamber. The flame-transfer-functions based on chemiluminescence show a good qualitative agreement with the multimicrophone method. Based on the experimental findings a calibration curve for the different radicals to obtain quantitatively correct flame-transfer-functions from chemiluminescence is presented.

Numerical investigation of sonochemical production in a single bubble under dual-frequency acoustic excitation

A numerical computation study about sonochemical activity of an oxygen bubble oscillations in a dual-frequency acoustic field is presented in this paper. The variation of excitation frequency causes the change of magnitude and duration of pressure acting on the bubble, affecting the instantaneous bubble radius, internal bubble temperature as well as chemical reactions. The numerical results indicate the formation of O2, HO2 •, •OH, H•, O, H2 and H2O2 in an O2 bubble during oscillations. The main oxidant is •OH radical. The influence of several parameters (e.g. couples of frequencies, the total pressure amplitude and the ratio of pressure amplitudes of the two waves) on the molar yields of free radicals is analyzed. There is an optimum bubble collapse temperature (around 5648 K) for the •OH production. Moreover, the corresponding frequency intervals of enhanced effect and weakened effect under different coupled frequencies (i.e. 40, 140, 213, 355 and 515 kHz) are also investigated.

Musculoskeletal Biomaterials: Stimulated and Synergized with Low Intensity Pulsed Ultrasound.

Clinical biophysical stimulating strategies, which have significant effects on improving the function of organs or treating diseases by causing the salutary response of body, have shown many advantages, such as non-invasiveness, few side effects, and controllable treatment process. As a critical technique for stimulation, the low intensity pulsed ultrasound (LIPUS) has been explored in regulating osteogenesis, which has presented great promise in bone repair by delivering a combined effect with biomaterials. This review summarizes the musculoskeletal biomaterials that can be synergized with LIPUS for enhanced biomedical application, including bone regeneration, spinal fusion, osteonecrosis/osteolysis, cartilage repair, and nerve regeneration. Different types of biomaterials are categorized for summary and evaluation. In each subtype, the verified biological mechanisms are listed in a table or graphs to prove how LIPUS was effective in improving musculoskeletal tissue regeneration. Meanwhile, the acoustic excitation parameters of LIPUS that were promising to be effective for further musculoskeletal tissue engineering are discussed, as well as their limitations and some perspectives for future research. Overall, coupled with biomimetic scaffolds and platforms, LIPUS may be a powerful therapeutic approach to accelerate musculoskeletal tissue repair and even in other regenerative medicine applications.

Sub-THz acoustic excitation of protein motion.

The application of terahertz radiation has been shown to affect both protein structure and cellular function. As the key to such structural changes lies in the dynamic response of a protein, we focus on the susceptibility of the protein's internal dynamics to mechanical stress induced by acoustic pressure waves. We use the open-boundary molecular dynamics method, which allows us to simulate the protein exposed to acoustic waves. By analyzing the dynamic fluctuations of the protein subunits, we demonstrate that the protein is highly susceptible to acoustic waves with frequencies corresponding to those of the internal protein vibrations. This is confirmed by changes in the compactness of the protein structure. As the amplitude of the pressure wave increases, even larger deviations from average positions and larger changes in protein compactness are observed. Furthermore, performing the mode-projection analysis, we show that the breathing-like character of collective modes is enhanced at frequencies corresponding to those used to excite the protein.

Investigation of the orifice flow of over-the-rotor liner and its interaction with the rotor flow field

The emergence of ultra-high bypass ratio engines calls for advanced noise control technology. A promising technology is installing the acoustic liner at the casing over the rotor (OTR), forming the so-called OTR liner. Experiments have shown that this treatment could compensate for the reduced noise absorption due to the shortened intake length. Compared with the conventional liner receiving a pure acoustic pressure excitation, the OTR liner is exposed to the combined aerodynamic pressure perturbance and high acoustic pressure excitation, leading to complex flow mechanisms in the liner orifices. In this work, the large-eddy simulation method is used to investigate the flow phenomenon of OTR liner where each liner orifice is explicitly modeled to capture the flow behaviors at the blade tip proximity. Interesting findings are as follows: first, OTR liner orifices have an asynchronous dynamic response with a phase lag corresponding to the blade traversing speed; second, oscillating flow, resembling that in a classical Helmholtz resonator, is formed in the upstream liner orifices, while a bias flow appears in the downstream liner orifices. The dramatically different flow behaviors lead to non-uniform energy dissipation among orifices, implying the uniform impedance assumption in conventional acoustic liner definition fails when installing the liner near the rotor blade. Results also show that the orifice flow interacts with the blade tip leakage flow, affecting the development of the tip leakage vortex that may impact the rotor–stator interaction noise.

Internal sound pressure and power radiation from a submerged cylindrical enclosure with interior acoustic excitation

This paper presents the modelling and analysis of the effect of an interior acoustic excitation on a submerged cylindrical enclosure. The enclosure has ring stiffeners and acoustic absorption material on the internal wall. The acoustic excitation generates an incident and scattered sound field inside the enclosure. The incident and scattered sound forms the internal pressure which induces a structural response and, thus, the radiated sound. The previous work (James 1985) with an internal noise source located on the vertical axis is expanded to allow forces and interior acoustic sources at arbitrary locations. The formulation of the internal pressure is developed by solving the unknown coupling term between the internal fluid and the hull. The coupling term was neglected by James (1985). However, this term is significant in magnitude for the internal pressure and must, therefore, be included. The analytical results are compared with numerical finite element/boundary element models with good agreement.

Acoustics of a pressurized air-filled pipe open at one-end at varying depths

A rigid pipe with a loudspeaker at one end and open at the other end, was submerged in a water tank at various depths. Microphones were placed along the length of the pipe to measure the acoustic response of the duct due to the acoustic excitation from an internal loudspeaker. Compressed air was injected into the pipe to equalise the internal pressure of the pipe caused by the water pressure acting on the open end of the pipe. The results were that the acoustic resonance frequencies of the duct did not alter with depth.

Control of acoustic scattering of trailing edge flow by distributed compliance

In this paper, an approach for the reduction of trailing edge noise due to flow scattering from a semi-infinite splitter plate is proposed. It utilizes the fluid–structure interactions of well-designed multiple compliant elastic panels to suppress the flow instabilities within the boundary layers over the splitter plate to reduce overall trailing edge noise scattering. The approach is studied numerically using high-fidelity direct aeroacoustic simulation at low Reynolds numbers based on a panel length of 5×104. The noise reduction efficacy of the approach is analyzed by studying two different cases, and their underlying physical mechanisms are explored. First, the boundary layer over one side of the plate is subjected to a weak monochromatic acoustic excitation to produce laminar instabilities. Second, the boundary layer is subjected to a weak broadband excitation within the boundary layer. For each case, the panel system is uniquely designed with thorough consideration of the flow characteristics of the boundary layer instabilities of the problem. Comprehensive aeroacoustic analyses reveal that a significant sound power level reduction of 4.2 and 7.4 dB can be achieved by designed configurations for both kinds of excitation without any drag penalty. Nonlinear fluid–structure interactions of carefully designed elastic panels result in a weak correlation between the near-field flow instabilities and far-field noise. The flow-induced panel structural resonance is proven to effectively absorb the energy of boundary layer instabilities and their scattering at the trailing edge. Key characteristics for the design of compliance systems under different flow conditions are discerned and discussed.

Impact of Transverse Acoustic Excitation and Air Swirl on a Confined Hollow Cone Sheet

Abstract This study investigates the effect of imposing symmetric transverse acoustic excitation and counter-rotating air swirl flow on a confined hollow cone spray sheet. We study the impact of these forcing conditions on two unstable modes—sheet flapping and sinuous wave growth modes—and their phasic relationship. We used the shadowgraphy technique, and only the sheet edges were analyzed using the method of snapshot proper orthogonal decomposition (POD). In this study, we used postprocessing techniques to estimate the nondimensional breakup length, phase averaged cone angle, and continuous wavelet transform (CWT) frequency. Results show that imposing acoustic excitation alone causes the sheet edges to flap, axisymmetrically, at the pressure antinode, while at the pressure node and base flow condition, the asymmetric sinuous wave mode is dominant. At the pressure intermediary, we find the possible dominance of both modes. While the dominant effect of air swirl alone was difficult to ascertain, results from CWT analysis show that POD mode 1 and mode 3 have very similar spectral content, with mode 3 showing a flapping mode. Finally, air swirl flow has a more dominant effect than acoustics in causing sheet breakup.

Dynamics of acoustically coupled single- and multi-port jet diffusion flames

The present experimental study investigated the dynamics of single- and multi-port gaseous jet diffusion flames exposed to acoustic excitation via a standing wave situated in a closed waveguide at atmospheric pressure. High-speed imaging of the oscillatory flame was analysed via proper orthogonal decomposition (POD), revealing distinct signatures in both mode shapes and phase portraits for transitions in the acoustically coupled combustion process. For Reynolds numbers between 20 and 100, and for low to moderate forcing amplitudes, the flame exhibited sustained oscillatory combustion (SOC) that was highly coupled to the acoustic forcing. Frequency analysis of the temporal POD modes accurately recovered the forcing frequency and its higher harmonics. At higher forcing amplitudes, a multi-frequency response was observed, resulting from a combination of the forcing frequency and much lower frequency oscillations due to periodic lift-off and reattachment (PLOR) of the flame, preceding a transition to flame blow-off (BO). For both single- and triple-jet flames, transitions from SOC to PLOR to BO were characterized by significant alterations in primary modal energetic content, deflection and eventual smearing in phase portraits, and the development of additional frequencies in modal spectra, although transitional behaviour for the triple jet flames involved additional complexity in the dynamics due to its structure. These features provide the potential for the development of reduced-order models that can characterize and predict acoustically coupled combustion behaviour.

Acoustic kinetic energy and soot suppression efficiency of acetylene diffusion flame under acoustic excitation at varying resonant frequencies in Rijke tube

The acoustic kinetic energy and soot suppression efficiency of acetylene diffusion flame under acoustic excitation at varying resonant frequencies in Rijke tube is investigated experimentally. The acoustic kinetic energy is introduced for the first time to explain the mechanism of soot suppression by acoustic oscillation. The soot suppression efficiency and acoustic kinetic energy is investigated by changing the acoustic parameters and the length of the Rijke tube. The results show that the efficiency of the soot suppression and the acoustic pressure increase significantly at the resonant fundamental frequency (167 Hz and 185 Hz) and resonant frequency (346 Hz and 373 Hz). The standing wave at the resonant fundamental frequency is a half wave, while the standing wave at the resonant frequency is a full wave. Soot suppression efficiency and acoustic kinetic energy reach the maximum at the wave nodes. When the efficiency of soot suppression is the same, the acoustic kinetic energy at the resonant frequency is twice that at the resonant fundamental frequency at the wave nodes. The flame brush, flame fluctuation velocity and the flame temperature are studied in detail by changing the acoustic parameters. The results show that the efficiency of soot suppression increase with the increase of flame fluctuation velocity and flame temperature. This is considered that the acoustic kinetic energy accelerates the flame fluctuation velocity which enhances the heat and mass transfer in the combustion field, resulting in an increase in the flame temperature, thereby soot is re-oxidized. This study can provide some reference for the practical application of acoustic oscillate combustion in soot suppression.

A coupled approach for predicting transmission loss of silencers with consideration of three-dimensional wave effects in inlet and outlet ducts

A coupled approach combining finite element method and duct mode expansion is proposed to determine the transmission loss of large silencers with consideration of three-dimensional wave effects in inlet and outlet ducts in the presence of flow. The finite element formula is established for the linearized potential equation (LPE), and the acoustic variables in the boundary integrals on the inlet and outlet surfaces are expressed analytically in terms of mode expansion. Since the introduced modal amplitude coefficients add unknown variables, the intrinsic functions are used as weighted functions to construct integral equations on the inlet and outlet surfaces to form a closed set of equations. Expressions of incident and transmitted sound powers are derived, and the transmission loss of silencer may be determined for any-giving mode incidence. For the symmetric silencers, specific combinations of acoustic modes are presented to improve the stability and efficiency of the approach. The dissipative and reactive silencers with rectangular and circular cross-sectional inlet and outlet ducts are examined, good agreements between predictions and benchmark data verified the present approach. Effects of Mach numbers and geometric parameters, flow fields and acoustic excitations on the acoustic attenuation behavior of the silencers as well as transmission loss predictions from different strategies are discussed.

Roughness induced transition delay in a swept-wing boundary layer in presence of freestream disturbances, Part 2: Acoustic stabilization

Results of experimental investigations on control of the laminar–turbulent transition location in a 45-degree swept-wing boundary layer by the distributed micro-sized roughness (MSR) elements in presence of freestream perturbations are discussed in this two-part work. Part 1 of this study Borodulin et al. (2023) is devoted to the freestream turbulence effects, while Part 2 (the present paper) concentrates on consideration of the acoustic waves’ effects. This paper contains: (i) a complementary description of the experimental setup and measurement techniques related to the acoustic excitation and (ii) the results of studies of the freestream acoustic-field effects on the MSR transition control. The robustness of the MSR transition-control mechanism has been studied by altering the wind-tunnel flow quality: by introducing a significant background sound and/or by simultaneous variation of both the freestream turbulence level and the level of the acoustic wave excitation. At enhanced freestream turbulence, the laminar–turbulent transition is found to move upstream regardless of the sound level and frequency, while the MSR-elements lost their effectiveness in transition control in all cases. At low freestream turbulence levels and in absence of the MSR, the acoustic waves did not affect basically the transition location. However, if the laminar–turbulent transition was delayed by the MSR, the sound of a certain frequency was found to be able to cause a significant additional transition delay.

Sound transmission loss behavior of honeycomb sandwich plate structure under thermal loading

This paper presents an accurate and analytical method for investigating the sound transmission loss (STL) characteristics of honeycomb sandwich plate structure subjected to combined thermal and acoustic excitation. In the analysis, the governing equations of sandwich plate structure in thermal environments are established by first order shear deformation theory. Two types of thermal conditions with and without heat flux are considered. The fluid-structure coupling interaction between the plate structure and acoustic excitation is described analytically by applying velocity continuity condition at fluid-structure interfaces. By using the modal expansion approach, the sound transmission loss is described analytically. The experimental measurements of honeycomb sandwich plate are carried out to validate the theoretical model, and good agreement is achieved. Finally, the influences of honeycomb core dimensions, material parameter and temperature distribution on sound transmission loss behavior of honeycomb sandwich structure have been investigated. It has found that the resonance dips of STL curves move to higher frequencies with the increase of ratio of elastic modulus and honeycomb core wall thickness, but the increase of the honeycomb core side length causes the resonance dips to move to low frequencies. The magnitudes of the STL curves are found to be decreasing with increase of temperature values.

Investigation of Harmonic Response in Non-Premixed Swirling Combustion to Low-Frequency Acoustic Excitations

The propagation mechanism of flow disturbance under acoustic excitations plays a crucial role in thermoacoustic instability, especially when considering the effect of non-premixed combustion on heat release due to reactant mixing and diffusion. This relationship leads to a complex coupling between the spatial distribution of the equivalence ratio and the propagation mechanism of flow disturbance. In the present study, the response of a methane-air non-premixed swirling flame to low-frequency acoustic excitations was investigated experimentally. By applying Proper Orthogonal Decomposition (POD) analysis to CH* chemiluminescence images, the harmonic flame response was revealed. Large Eddy Simulation (LES) was utilized to analyze the correlation between the vortex motion within the shear layers and the harmonic response under non-reacting conditions at excitation frequencies of 20 Hz, 50 Hz, and 150 Hz. The results showed that the harmonic flame response was mainly due to the harmonic velocity pulsations within the shear layers. The acoustically induced vortices within the shear layer exhibited motion patterns susceptible to harmonic interference, with spatial distribution characteristics closely related to the oscillation modes of the non-premixed combustion.

Creating Geometric Imperfections in Thin-Walled Structures Using Acoustic Excitation

Abstract Thermomechanical buckling of slender and thin-walled structural components happens without warning and can lead to catastrophic failure. Similar phenomena are observed during plasmolysis (contraction of a plant cell’s protoplast) and rupture of viral capsids. Analytical formulas derived from stability analyses of elastic plates and shells that do not account for the effects of random geometric imperfections introduced during the manufacturing process or biological growth may vastly over-estimate buckling capacity. To ensure structural safety, the formulas must therefore be combined with empirical data to define “knockdown factors” which are in turn used to establish safety factors. Towards improved understanding of the role of imperfections on mechanical response, ingenious methods have been used to fabricate and test near-perfectly hemispherical shells and those containing dimple-like defects. However, a method of inducing imperfections in the form of randomly shaped surfaces remains elusive. We introduce a protocol for realizing such imperfect shells and measuring the pressure required to buckle them. Silicone is poured onto an elastomeric mold under an acoustic excitation, which can be either random sound, or if desired the same as the modal frequency of the mold. Illustrative micro-computed-tomography images and buckling pressure experiments of a nearly perfect shell and an imperfect one show that the method is effective in introducing randomly shaped imperfections of significant magnitudes. This proof-of-concept study demonstrates that the experimental results when combined with computational simulations can lead to improved understanding of stochastic buckling phenomena.

Characteristics of the flow-induced noise from rectangular rods with different aspect ratios and edge geometry

The dynamic lift force, wake characteristics and the susceptibility to self-excited acoustic resonance is experimentally investigated for rectangular rods with aspect ratios of l/h=0.5, 1, and 2, where l is the rod length in the flow direction and h is the rod length perpendicular to the flow. The effect of rounding different edges on the flow-sound interaction mechanism is also considered. Rectangular rods with an aspect ratio of l/h=0.5 showed a significant escalation of the acoustic pressure and the dynamic lift force during acoustic resonance excitation. However, when the aspect ratio of the rod increased to l/h=2, an early excitation of the third acoustic mode is observed accompanied with a significant reduction in the dynamic lift force compared to the values observed before the onset of resonance excitation. It is revealed that the wake structure around the rectangular rod with an aspect ratio of l/h=2 has significant dependence on the edge geometry and this results in different shedding modes. The first shedding mode is the typical vortex shedding with a Strouhal value of Stl≈0.16−0.18 and it occurs for rods with aspect ratios of l/h=2 when either all the edges are sharp or the two upstream edges are sharp. Although the vortex shedding is dominant for this case, a masked shear layer instability along the lateral faces of the rectangular rod is what triggers the early excitation of the third acoustic mode. When either all the edges are rounded or the two upstream edges are rounded, the shear layer instability mode becomes dominant with a Strouhal value of Stl≈0.5−0.52 and the dynamic lift force decreases substantially. This mode classically excites the first and the third acoustic modes at their frequency coincidence and no typical single-street vortex shedding is detected downstream of the rod. The work presented in this paper reveals that rounding the upstream edges of a rectangular rod with an aspect ratio of l/h=2 can switch the shedding pattern making the shear layer instability mode dominant and hence a reduction of the dynamic loading and an attenuation of the acoustic energy is obtained.