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A numerical investigation is conducted on the propulsive performance and acoustic characteristics of two flapping foils in tandem at Re = 200 in uniform flow using the lattice Boltzmann method combined with the immersed boundary method. The effects of phase difference and horizontal spacing on hydrodynamics and acoustics are studied. The results indicate that the propulsive performance and acoustic characteristics of the in-phase foils and anti-phase foils are completely different. As the horizontal spacing increases, the change trend of their propulsive performance is completely opposite. The propulsive performance of the in-phase foils shows a trend of first weakening and then enhancing, while the anti-phase foils show a trend of first enhancing and then weakening. For sound field, the in-phase foils exhibit a dipole-like pattern similar to that of a single foil, while it exhibits a quadrupole-like pattern for anti-phase foils. Compared to the in-phase foils and single foil, the maximum effective sound pressure value of the anti-phase foils is smaller. Meanwhile, the relationship between propulsive performance and acoustic characteristics is analyzed, and it is found that the correlation coefficients follow the same variation pattern. The presented work can offer valuable insights for designing and controlling new bio-inspired autonomous underwater vehicles aimed at suppressing flow noise. [ABSTRACT FROM AUTHOR]

We present a numerical study to investigate the efficacy of impedance boundary conditions in capturing the interfacial dynamics of a particle subjected to an acoustic field and study the concomitant time-averaged acoustic streaming and radiation force fields. While impedance boundary conditions have been utilized to represent fluid–solid interface in acoustofluidics, such models assume the solid material to be locally reactive to the acoustic waves. However, there is a limited understanding of when this assumption holds true, raising concerns about the suitability of impedance boundary conditions. Here, we systematically investigate the applicability of impedance boundary conditions by comparing the predictions of an impedance boundary approach against a fully coupled fluid–solid model. We contrast the oscillation profiles of the fluid–solid interface predicted by the two models. We consider different scatterer materials to identify the extent to which the differences in interfacial dynamics impact the time-averaged fields and highlight the divergence within the predictions of the two models. Our findings indicate that, although impedance boundary conditions can yield qualitatively similar results to the full model in certain situations, the predictions from the two models generally differ both qualitatively and quantitatively. These results underscore the importance of exercising caution when applying these boundary conditions to model general acoustofluidic systems. [ABSTRACT FROM AUTHOR]

To explore the influence of cavitation on the internal fluid excitation characteristics of pumps, numerical simulations and performance testing evaluations were performed on the IS65-50-125 centrifugal pump. The prototype pump's exterior characteristic and cavitation performance curves, as well as its bubble volume distribution, were successfully replicated using numerical computations. The effect of cavitation on the internal pressure pulsation characteristics of the centrifugal pump under various operating situations was comprehensively investigated, indicating a relationship between the degree of cavitation and the root mean square values of pressure pulsation. Special emphasis was placed on the changes in features at intermediate and high frequencies, as well as the processes of rising bubble volume and vortex shedding at the impeller trailing edge on pressure pulsation. To validate the simulation results, a centrifugal pump vibration and noise testing platform was built, and studies on vibration intensity and internal sound field noise were conducted. The experimental results revealed that the vibration intensity and internal sound field sound pressure level of the centrifugal pump rose as cavitation conditions deteriorated, confirming the modeling results. This study's significant innovation is the precise identification of the pump's performance changes under different operating conditions by monitoring pressure pulsation changes at various frequencies, as well as an in-depth discussion of the impact mechanism of cavitation phenomena on the internal fluid excitation behavior of centrifugal pumps. The study demonstrates differences in pressure pulsation characteristics on the suction and pressure sides under various cavitation situations, as well as the process of vortex creation and shedding generated by bubbles in the impeller input channel during severe cavitation. This gives new theoretical basis for pump vibration and noise reduction, as well as significant improvements in centrifugal pump performance and stability. [ABSTRACT FROM AUTHOR]

We consider the effects of passive tracer clustering (e.g., the magnetic field energy in stellar atmospheres) in a random acoustic velocity field. A method for numerical modeling of a two-dimensional random acoustic field is proposed. The field is described by a correlation tensor defined by traveling isotropic waves, taking into account dissipation. Two metrics for measuring the clustering effects are used: concentration and density. Using numerical modeling, we show that the tracer concentration is almost always clustered. The situation with the density is different; as the dissipation tends to zero, the time to reach the clustered states increases significantly. In addition, due to the tracer transport out of the density clustering regions, only a part of the tracer is clustered. For the presented analyses, we considered ensembles of Lagrangian particles and introduced and applied the statistical topography methodology. [ABSTRACT FROM AUTHOR]

Understanding bubble oscillations is critical in science, engineering and medicine. An important aspect of bubble dynamics is the transition from spherical to non-spherical oscillations when the bubble is subjected to external stimuli like time-dependent pressure fields. The mechanisms underpinning this process are not well understood for the complex pressure fields encountered in most applications. Here, we perform high-fidelity numerical simulations of the oscillations of a non-condensable gas bubble induced by the acoustic field generated by the collapse of a nearby cavitation bubble. We compare our simulation results with controlled laboratory experiments and demonstrate that our computational approach predicts spatial and temporal bubble oscillation patterns. This work opens new opportunities to understand the oscillation of bubbles induced by complex acoustic fields. [ABSTRACT FROM AUTHOR]

Expanding pipes with orifice plates are often utilized as silencers for fluid machinery. However, intense tonal sounds can be generated from a flow through such expanding pipes. To clarify the mechanism of tonal sound from a flow through a circular expanding pipe with two orifice plates and the conditions for intense acoustic radiation, the flow and acoustic fields are directly solved based on the compressible Navier–Stokes equations. Phase-averaged flow fields indicate the occurrence of periodic vortex shedding in the free shear layers of the expanding pipe, resulting in acoustic radiation. The effects of the orifice radius and freestream Mach number on the acoustic radiation are focused on. The computational results demonstrate that vortex rings or spiral vortices are generated in the cavities formed by the orifice plates, where the primary vortical shape changes, depending on the freestream Mach number and orifice radius. The collision of the vortex ring and spiral vortex with the orifice plate or downstream edge of the expanding pipe leads to the occurrence of circumferentially in-phase and one-wavelength-mode pressure fluctuations, respectively. The orifice radius also affects the convective velocity of vortices and the position of the acoustic source, varying the frequency of the acoustic radiation. The findings of this research provide the first clarifications of fluid–acoustic interactions in an expanding pipe with orifice plates. [ABSTRACT FROM AUTHOR]

This paper presents a semi-analytical model for calculating the mean deformation of a cylindrical or quasi-spheroidal liquid object placed in standing acoustic waves under the near-field approach. The interaction between the waves and the object is characterized by the angular distribution of the radiation pressure P rad (θ) over the object surface. The key parameters of the model are the Helmholtz number, α, and the object shape aspect ratio, called ϵ for elliptic shapes and ϵg for arbitrary shapes. For elliptic cross-sectional rigid objects, effects are globally dominated by suction for small α or compression for large α, whatever ϵ. When suction predominates, two opposed compression maxima and two opposed suction minima are observed. When compression predominates, an interference pattern with several extrema is observed. For potentially deformable objects, a potential flattening is found whatever α, which is all the more important as ϵ decreases. A general deformation model is developed to quantify the action/reaction loop between the acoustic field and the deformable object of the aspect ratio, ϵg. The acoustic Bond number Boa and a curvature-based parameter, ϵ κ , are introduced. As Boa increases, the object flattens whatever α. For small α, convex deformation is observed until a maximum Bond number is reached when ϵg = ϵ κ = 0. There, an abrupt change occurs from a convex shape to a planar liquid sheet. Otherwise, a local transition from a convex to a concave shape occurs, for which ϵ κ = 0, while ϵ g ≠ 0. Our model successfully predicts numerical and experimental results from the literature. [ABSTRACT FROM AUTHOR]

Radial extracorporeal shockwave therapy (rESWT) is a noninvasive medical technique that treats a range of musculoskeletal conditions. To understand its biological effects and develop personalized treatment plans, it is crucial to fully characterize the acoustic field that rESWT generates. This study presents a quantitative assessment of rESWT's acoustic field, achieved through experiments and simulations. The study measures the acoustic fields using a needle-type hydrophone under different machine settings and establishes and calibrates a computational model based on the experimental measurements. The study also determines the spatial distributions of peak pressure and energy flux density for different driving pressures. High-speed photography is used to visualize cavitation bubbles, which correspond to the negative pressure distribution. The study finds that the axial pressure distribution is similar to the acoustic radiation from an oscillating circular piston, whereas the radial pressure distribution cannot be described by acoustic radiation. Furthermore, the study develops a machine learning model that predicts positive pressure distributions for continuous driving pressure. Overall, this study expands our understanding of the acoustic fields generated by rESWT and provides quantitative information to explore underlying biological mechanisms and determine personalized treatment approaches. [ABSTRACT FROM AUTHOR]

Aero-engines and rocket engines regularly experience unstable combustion phenomena. In order to reveal the complex mechanism, it is necessary to measure the jet gas–liquid interface disturbances. However, most techniques require optical access and the measurement fails when the optical access is limited. Ultrasonic method can be considered as an alternative under this condition. The present work proposes an acoustic approach for measuring the jet gas–liquid interface based on ultrasonic scattering. A thorough investigation of the scattered acoustic field by the jet is conducted by experimental methods. The high-speed imaging technology is used to verify the ultrasonic measurement results. The direct measurement results demonstrate a relationship between the perturbation of the jet gas–liquid interface and that of ultrasonic scattered acoustic pressure in certain measuring direction. The scattered acoustic field is analyzed theoretically and practically by using customized metal scatters of certain size. Based on these findings, a database is created to enable the ultrasonic measurements to be calibrated. The corrected result shows that the correlation of ultrasonic measurements and results from cameras has been greatly improved, and the maximum relative error of the ultrasonic measurements is 30.9%, the average relative measurement error is 2.1%. It is proved that the method of determining the gas–liquid interface of jet by ultrasonic scattering wave is feasible. The method may also be used for the measurement of the overall jet fluctuations and breakup. [ABSTRACT FROM AUTHOR]

Acoustic tweezers have shown great potential in revolutionizing fields such as noninvasive surgery and drug delivery. However, the current theoretical prediction and quantitative analysis of acoustic radiation force and torque are limited by the assumption of a homogeneous medium and spherical micro-objects. In this study, a theoretical model to address this limitation is proposed by considering the propagation of acoustic waves in layered heterogeneous media with irregular interfaces. The approach begins by constructing a propagation model based on plane wave decomposition and scalar diffraction theory to describe the behavior of acoustic waves passing through irregular interfaces. The spatial distribution of the acoustic field after passing through these interfaces is then determined using the angular spectrum method. To account for the geometric characteristics of axisymmetric objects, a conformal transformation approach is incorporated, allowing us to efficiently establish a theoretical model for the acoustic radiation force and torque exerted on these objects suspended in the layered medium. To validate the model, the computed acoustic pressure fields and radiation forces are compared with results obtained from full three-dimensional numerical simulations. The agreement between the two sets of results demonstrates the effectiveness of the proposed method. This theoretical model is expected to contribute significantly to the development of acoustic tweezers technology, enabling the manipulation of irregular micro-objects in multi-layer medium and providing important theoretical support for the application of acoustic tweezers in complex medium. [ABSTRACT FROM AUTHOR]

Tilted tongue is one of the very effective solutions to reduce the noise of centrifugal fans. In this study, the accuracy of the numerical simulations is verified by experimentally measured aerodynamic performance and acoustic spectra. Two localized tilting designs of the tongue are proposed with the aim of reducing the aerodynamic noise of the centrifugal fan. Numerical results show that the different approaches significantly affect the noise reduction of the fan, with the best design exhibiting up to 2.33 dB of noise reduction. The internal flow and acoustic fields are analyzed in conjunction with the dynamic modal decomposition method to reveal the effect of the local tilt of the tongue on the flow and acoustic characteristics of the fan. The structural change induces the separation vortices at the trailing edge of the blades and the generation of secondary flow at the outlet wall of the volute shell, which is the main reason for the slight degradation of the aerodynamic characteristics of the fan. The causes of the blade passage frequency (BPF) noise reduction are not identical for different orders. BPF noise reduction stems from diminishing source intensity and dispersing source phase. Noise reduction in 2BPF is mostly due to weakening sound source intensity, with less conspicuous phase dispersion. This study provides good theoretical support for the design of backward curved centrifugal fans in the context of noise reduction. [ABSTRACT FROM AUTHOR]

The Ffowcs-Williams & Hawkings (FWH) acoustic analogy is adopted to reconstruct the acoustic far field of a system consisting of an upstream hydrofoil and a downstream propeller, considering the former at incidence angles of 0 ° , 10 ° , and 20 °. Also comparisons against the same propeller working in isolated conditions are reported. Fluid dynamic data from earlier high-fidelity, Large-Eddy Simulations (LES) on a grid consisting of 1.7 × 109 points are utilized. The analysis demonstrates that, with some exceptions at the smallest frequencies, the acoustic far field is dominated by the loading sound coming from the propeller, achieving its highest values of acoustic pressure in the upstream and downstream directions. In contrast, the lowest values occur on the propeller plane, whose minima are aligned with the spanwise direction of the hydrofoil. A strong dependence on the incidence angle of the hydrofoil is found, although decreasing toward higher frequencies. Interestingly, while at the shaft and at the blade frequencies the acoustic pressure coming from the hydrofoil-propeller system is always higher than that from the open-water propeller working alone, as expected, at higher harmonics of the blade frequency this is not the case. This may be due to phenomena of destructive interactions across the acoustic sources on the surface of the propeller or the result of a shift of the acoustic signature toward even higher frequencies, beyond the range covered by the database available to the present study. [ABSTRACT FROM AUTHOR]

In this work, we experimentally investigate the suspension behavior of droplets subjected to standing acoustic waves. We focus on the droplet sizes beyond the Rayleigh limit, i.e., when the droplet size is comparable to the wavelength of the acoustic wave. We show that an acoustic field can disrupt the uniform motion of aqueous droplets in oil and cause them to either suspend or settle, depending on the interplay between acoustic and gravity forces. Remarkably, in contrast to droplets within the Rayleigh limit, the critical acoustic power or minimum pressure amplitude required to suspend droplets beyond the Rayleigh limit is dependent on the droplet size. As the droplet size increases, the critical acoustic power increases significantly. Building upon this understanding, a novel sorting method is proposed based on critical acoustic power. [ABSTRACT FROM AUTHOR]

The study explores twin bi-component drop evaporation at various acoustic frequencies and amplitudes. The drops have a fixed central spacing. The acoustic streaming around the drops varies with frequency and amplitude. The streaming rises with the amplitude at a given frequency. The acoustics push the outer flow away from the drop. Furthermore, the flow transforms from horizontal to vertical at higher amplitudes, indicating a rise in the outer flow. Correspondingly, the drops' internal circulation and evaporation rise. The maximum evaporation is observed at higher amplitudes and lower frequencies due to intense streaming. The intense streaming accelerates the circulation, forcing the oscillatory circulation to be steadier. In contrast, the circulation is unsteady for lower amplitudes and higher frequencies. The steady circulation escalates the convective current inside the drops and boosts the evaporation rate. An empirical expression is developed to predict the outer flow velocity and evaporation rate at various amplitudes for any twin bi-component drops under diverse acoustic conditions. The study's novelty lies in demonstrating how the amplitude plays a crucial role in modifying the outer flow and internal circulation to elevate the evaporation rate despite frequency. [ABSTRACT FROM AUTHOR]

The role of micro-vortex generators (MVGs) in supersonic jet noise reduction is investigated. Studies are performed to understand the noise reduction mechanisms of these devices. MVGs are implemented on a scale model representative of GE-F404 nozzles. Configurations consisting of single and dual arrays of MVGs were tested for pressure ratios relevant to takeoff operating conditions. A combination of laboratory measurements and large-eddy simulations are used. Particle image velocimetry reveals that MVGs placed inside the nozzle form oblique shocks that interact with the jet plume's shock cell structure and greatly modify the shock-cell structure issued from the nozzle throat. This modification can weaken the jet plume shock-cell structure and even reduce the jet core velocities. Convective velocities estimated from the high-speed Schlieren imaging showed a significant reduction in magnitude caused by MVGs. When compared to the baseline design, the MVG nozzles showed noise reduction up to 10 dB in the upstream direction and near 5 dB in the peak downstream radiation angle at cold jet conditions. Near-field acoustic measurements showed a significant reduction of low-frequency turbulent mixing noise. Scaling analysis showed that MVGs are capable of delivering the same noise reduction performance across different model scales. [ABSTRACT FROM AUTHOR]

The low-pressure turbine (LPT) has become a potential noise source for future ultra-high by-pass ratio engines. In this paper, the feasibility and mechanism of wavy leading edge (WLE) noise control in the LPT cascade model are analyzed. The flow field and acoustic data are obtained with the large eddy simulation and Ffowcs Williams–Hawkings methods, which are validated using experimental data. The acoustic results are compared for different models; the maximum noise reduction can achieve 8.6 and 3.7 dBA in the frequency bands of FR#2 (315–4000 Hz) and FR#4 (6300–16 000 Hz), respectively; the noise reduction does not vary proportionally to the WLE parameter. The noise source is identified in the baseline model, and then the effect of WLE amplitude and wavelength on the noise source and its control on pressure fluctuations are evaluated. The pressure statistics demonstrate that WLE with a smaller wavelength and a larger amplitude can reduce the impingement of stator wakes on the leading edge of the rotor and stabilize the pressure fluctuation. To analyze the mechanism of WLEs on noise control, the pressure spectrum in terms of amplitude and coherence coefficient is utilized to explain the excellent noise performance of the WLE model in FR#2. The proposed similarity coefficient of coherence can quantify the destructive interference level and thus the coherence characteristics of the sound source. Generally, the noise reduction level can be predicted by the combination of the similarity coefficient and the amplitude spectrum of the pressure fluctuations for the WLE models. [ABSTRACT FROM AUTHOR]

The effect of nozzle exit geometry on the triadic interactions between convective Kelvin–Helmholtz instabilities and the quasi-steady shock structure of supersonic jets is discussed in this study. A circular and rectangular nozzle with an aspect ratio (AR) of 2 were evaluated using results from near field acoustics and high-speed Schlieren images. The study investigated different types of screech instabilities in both nozzle configurations, including the multimode instability in rectangular jets. The near field acoustic measurements showed comparable trends in screech staging in both nozzle configurations. The Schlieren results revealed that the standing waves in the jet near field were influenced by the oscillation mode of the jets. The study also explored a new technique of noise visualization from Schlieren images based on near field acoustics and temporal Fourier decomposition. Spatial Fourier decomposition demonstrated that the resonance closure mechanism across both nozzle configurations was dependent on the relative influence of the first two shock cells. Comparisons between the AR 2 and AR 9 jet showed that jet aspect ratio can significantly affect the energy distribution from the triadic interactions. [ABSTRACT FROM AUTHOR]

As an effective tool for contactless manipulation of submicrometer scale objects, the controllability of acoustic streaming velocity and flow field morphology determines the accuracy of object migration and the completeness of three dimensional (3D) imaging. This paper proposes an equivalent acoustic streaming driving force model that is applicable to both two dimensional (2D) and 3D calculations and constructs a numerical method for submicrometer microsphere migration and rotation velocity in acoustic streaming. The results show that the relationship between the peripheral vortex size Lp/wc and the relative acoustic streaming velocity vas/vf satisfies Lp/wc = 0.125vas/vf0.36 under certain geometrical conditions. Reducing the spatial confinement and increasing the inter-vortex distance will increase the energy release efficiency, reduce the pressure gradient distribution and convective dissipation rates, increase the vortex intensity and radiation range, and consequently, increase the vortex characteristic size. In complex 3D vortex flow fields, suspended objects are affected by velocity distributions and exhibit motions such as cross-flow lines and rotation. For larger vortex structure sizes, full 3D imaging is more favorable due to the increased rotation speed and period of motion along the orbit of the submicrometer microspheres. This study helps us to reveal the modulation mechanism of acoustic streaming field flow characteristics, enrich the basic theory of alternating orbital motion and forces on objects in vortex structures, and provide guidance for acoustic flow-based contactless object manipulation. [ABSTRACT FROM AUTHOR]

Data from Large-Eddy Simulations on a grid consisting of 3.8 billion points are utilized to reconstruct the acoustic far field of a propeller–hydrofoil system, using the Ffowcs-Williams and Hawkings acoustic analogy. Results demonstrate that the lowest frequencies, around the shaft frequency, are dominated by the sound coming from the surface of the downstream hydrofoil, especially in its normal direction. In contrast, at the blade frequency and higher frequencies, the acoustic signature of the upstream propeller gains the lead, especially in the upstream and downstream directions. Minima of the sound pressure levels correlate in all cases with the spanwise direction of the hydrofoil. Despite the dramatic change of the flow conditions for growing incidence angles of the hydrofoil, characterized by a substantial rise of the turbulent fluctuations on its suction side, no similar increase in the acoustic pressure in the far field occurs at large incidence angles, even at the onset of separation phenomena, which are able to affect the highest frequencies only. [ABSTRACT FROM AUTHOR]

A mathematical model for the acoustic pressure field induced by an enclosed cavity upon a solid sphere impacting into water is established. The relation between sound emission, cavity ripples, and the falling sphere is elaborated analytically. To describe the water domain above the sphere, physical and co-moving cylindrical coordinate systems are introduced. The water domain is analyzed by the perturbation method and potential flow theory. The air inside the cavity is assumed to be uniform and adiabatic. In the physical coordinates, the zeroth-order cavity surface is governed by the Rayleigh–Plesset Equation and is cylinder-like. In the co-moving coordinates, the sphere is stationary and subjected to an upward free stream. The slender cavity geometry permits linear surface waves with a vertical normal-mode solution, which corresponds to the first-order cavity ripples. The formula of ripples suggests that the cavity nose is a falling, pulsating, flat, cylindrical acoustic source in water. The induced acoustic wave is formulated in another spherical coordinate system. The sound frequency equals the vibration frequency of the cavity nose and also equals the formation/propagation frequency of the ripples. Moreover, the natural frequency of the cavity nose and the natural normal mode of the ripples are formulated. A water entry experiment is implemented and validates the theory. The normal mode of the cavity ripples, the criteria of the sound emission, and the initiation of the ripples and sound emission are discussed. [ABSTRACT FROM AUTHOR]

Microfluidic manipulation has been widely applied in bio-chemical research and applications, including bacteria/cell/protein mixing, separation, focusing, concentration, and trapping. One of the current severe challenges of this technique is to manipulate particles smaller than micrometer scale. In addition to multi-physical assists like acoustic and electrical fields, optimization of a structural design is a promising way to improve the functional capability of a microchannel. In our recent work, we built a robust and versatile numerical simulation model, validated with experiments, to reveal the mechanism of inertial microfluidic particle focusing within the double layered microchannel. In this study, a comprehensive investigation on the (geometrical and dimensional) optimization was further conducted with various numerical case studies. Based on the results, the fundamentals of the double layered microchannel with grooves were deeply revealed. In detail, the effects of microchannel geometric characteristics were discussed, including aspect ratio, groove curve radius, and groove spacing. In addition, an optimization strategy of geometrical and dimensional design was proposed to deeply exploit the manipulating potential of the microchannel. Based on the simulating calculation, the proposed optimized design of microchannel can remarkably improve the manipulating performance breaking through the manipulating limitation of manipulatable microparticle size, from microscale (4–10 μm) into nanoscale (500–800 nm), compared with the conventional microchannel. [ABSTRACT FROM AUTHOR]

The study of differential schemes with high accuracy and low dispersion is of great significance for the numerical simulation of aeroacoustics over complex geometries. The midpoint-and-node-to-node explicit finite difference is employed to solve the flux derivatives of the linear Euler equations for improving the robustness. To obtain the numerical flux at the center of the grid element, we adopted a hybrid interpolation method of center and upwind interpolations, combined with a symmetrical conservative metric method, to achieve high-resolution discretization of the acoustic field variables and geometric variables of the structural grid. To suppress spurious oscillations and improve the resolution of discontinuous regions, a family of mapping functions is developed to establish different smoothness indicators and applied to the sixth-order weighted compact nonlinear hybrid scheme (WCNHS), forming a new WCNHS scheme based on piecewise exponential mapping functions (WCNHS-Pe). The approximate dispersion relation shows that the dispersion error and numerical dissipation of WCNHS-Pe are smaller than those of WCNHS with a simple mapping function to the original weights in Jiang and Shu and other mapping function-weighted WCNHS schemes. We have applied various WCNHS schemes to the numerical simulation of the Shu–Osher problem, propagation of Gaussian impulses on two-dimensional wavy grids, sound transmission at discontinuous interfaces, propagation of Gaussian impulses on three-dimensional wavy grids, etc. Numerical results indicate that the WCNHS-Pe scheme has better discontinuity capture capability, higher resolution, and lower dispersion under the same differential stencil, and is suitable for computational aeroacoustics of complex geometries. [ABSTRACT FROM AUTHOR]

We investigate experimentally the attraction between two closely spaced, oscillating microbubbles. Above a certain value of the applied acoustic field, the bubbles jump to a new equilibrium location, where they are separated by a thin fluid layer whose the thickness is much smaller than the bubble radii. We demonstrate that this new equilibrium is caused by the sign reversal of the radiation interaction force acting between the two bubbles, attributed to the multiple rescattering effects of the waves emitted by the bubbles. Theoretical investigation reveals that a new stable equilibrium appears at short distances, resulting in a quasi-contacting bubble pair. [ABSTRACT FROM AUTHOR]

The study investigates how an acoustic field influences evaporation and internal circulation of twin drops when their in-between horizontal spacing varies. The acoustic source is a simple sine wave (i) with and (ii) without white noise at various frequencies. The circulation and outer flow are visualized. Maximum evaporation rate and circulation are found for the lowest frequency and highest spacing. The rate rises with the spacing for a given frequency up to a critical distance. The evaporation becomes almost identical beyond the critical spacing. A correlation among the spacing, evaporation rate, and outer flow velocity is demonstrated. The rate becomes lowest for a given frequency at the least spacing since the vapors accumulated in the surrounding are not swept out by the acoustic-induced flow. The visualization shows a horizontal outer flow, which becomes vertical with the rise in spacing because the acoustic wave can sweep the vapor out. The horizontal flow for the least spacing transforms itself to vertical when the wave amplitude is raised. The evaporation thus rises because the wave now sweeps the vapors out. We show that the perception that any acoustic wave enhances the evaporation of multiple nearby drops is incorrect. The evaporation and circulation decline faster with the rise in frequency since the surrounding flow becomes weak. Thus, we show how the spacing influences the evaporation when acoustic is incident and how the evaporation can be raised by sweeping the accumulated vapor out using higher amplitude acoustics for the closer drops. [ABSTRACT FROM AUTHOR]

This paper aims to analyze the sound and pseudo-sound generation mechanism and its corresponding control method in a centrifugal pump. The delayed detached eddy simulation and vortex sound theory are applied to calculate the flow and acoustic fields. The prediction results agree well with the experimental results. Based on the vorticity binary decomposition, the influence of vorticity and its components of rigid vorticity and shear on the sound and pseudo-sound fluctuation is discussed. The results show that the sound source is dominated by the shearing part rather than rigid vorticity. Sound and pseudo-sound pressure levels could be amplified when the fluctuation of vorticity and its components gets intense, but the pseudo-sound is less sensitive. The amplitude of sound pressure is threefold over the pseudo-sound in the spiral casing, while the pseudo-sound becomes higher than the sound at the outlet as the former decay slowly along the outlet tube. Increasing the volute tongue angle could reduce the fluctuation of vorticity and its components in the outlet tube, leading to a decrease in as much as 44% in sound pressure amplitude and 10% in pseudo-sound. [ABSTRACT FROM AUTHOR]

With the increasing demand for underwater detection, interest in the acoustic field of range-dependent ocean waveguides is also growing. For weakly range-dependent ocean waveguides, adiabatic modes represent a compromise between accuracy and computational cost and occupy an important place in the simulation of numerical sound fields. However, either existing adiabatic-mode programs consider too few layers of media or the root-finder tends to miss roots. In addition, none of the programs can solve the acoustic field excited by a line sound source located anywhere in the plane. In this paper, we first derive an expression for the acoustic field excited by a line source by adiabatic modes and then introduce a high-precision spectral method to solve the local eigenmodes. For the lower boundary condition of the acoustic half-space, we use the eigenvalue transformation technique to transform the transcendental algebra system formed by spectral discretization into a generalized eigenvalue problem. Several representative numerical experiments are designed to verify the accuracy of the algorithm. After analyzing the parallelism, the multiprocess and multithread hybrid strategy is adopted to further accelerate the algorithm in parallel, and parallel numerical simulation is carried out on the Tianhe–2 multicore supercomputer; favorable acceleration is achieved. [ABSTRACT FROM AUTHOR]

Large-scale coherent structures (CS) are present on a turbulent circular jet and are known to constitute an important source of jet noise. These structures may be treated as wavepackets of instability modes supported by the time-averaged mean flow. The nonlinear evolution and acoustic radiation of such CS in the fully developed region of a jet have been described in our recent work [Zhang and Wu, "Nonlinear evolution and low-frequency acoustic radiation of ring-mode coherent structures on subsonic turbulent circular jets," J. Fluid Mech. 940(A39), 1–56 (2022)], where the mechanism of envelope radiation, or low-frequency radiation, was identified. The present paper focuses on the distinguished very-near-nozzle region, where the nozzle diameter is comparable with the length scale of the envelope of the CS. The CS resides in the thin shear layer developing from the lip line, and its nonlinear dynamics resembles that on a planar shear layer, only slightly affected by circular geometry, or circularity. However, circularity causes a leading-order effect on the acoustic radiation. One of significant new features is the presence of an inner acoustic field in the potential core. The outer and inner acoustic fields interact with each other, and the equivalent sources of sound have to be determined along with the sound fields. The waves in the inner acoustic field propagate mainly upstream. It is shown for the first time that nonlinear interactions of CS in the thin shear layer excite via receptivity the so-called discrete "trapped acoustic modes" as well as low-frequency Kelvin–Helmholtz modes, which may evolve into preferred modes in the developed region. The envelope radiation provides a key mechanism coupling the dynamics in the thin shear layer with the dynamics in the jet column regions. As the jet develops downstream, the inner acoustic field narrows gradually and disappears eventually. It is demonstrated that the theory developed here for nonlinear dynamics and acoustic radiation is valid from the very-near-nozzle to the developed regions, and thus stands as a unified theory. [ABSTRACT FROM AUTHOR]

Numerical simulations are employed to investigate the sound generated by flow over two traveling wavy foils in a side-by-side arrangement by an immersed-boundary-method-based hybrid approach. The effects of Strouhal number (St), phase difference, and lateral spacing (S) between the foils on the flow performance and the sound pressure field are examined. The results imply that the sound produced by a single foil is dominated by the lift dipole, and that the low-amplitude–high-frequency foil can achieve higher thrust and higher sound pressure compared to the high-amplitude–low-frequency foil. For the two side-by-side foils (i.e., an in-phase and anti-phase foil system), the sound pressure fields exhibit distinct features. Specifically, a dipole-like pattern appears during in-phase motion, whereas a monopole-like pattern exists during anti-phase motion. Moreover, the magnitude of the sound pressure increases slightly with increasing S in the in-phase case. However, the sound pressure decreases rapidly when S < 0.7L (foil length) and then remains nearly unchanged when S > 0.7L in the anti-phase case. Furthermore, the anti-phase foil system could improve thrust by increasing power consumption and could generate lower sound pressure compared to the in-phase one due to the distinct differences in wake patterns. The present work is expected to improve the understanding of sound-generation mechanisms of fish-like locomotion and collective motion for relevant biomimetic underwater vehicles. [ABSTRACT FROM AUTHOR]

An analytical solution utilizing a double finite sine integral transform method is presented in this study for the sound radiation prediction of a ribbed rectangular plate structure under various combinations of clamped and simply supported boundary conditions. An advantage of the modeling method is that the plate–beam structural coupling and the structure–fluid coupling are automatically defined in the integral transformation without the need to manually select the mode shape function. The model is then utilized to investigate the effect of ribs on the radiated sound power and directivity of the plate structure under resonant and non-resonant conditions, respectively. The result shows that the ribbing effect on the omnidirectional radiated sound field of the rectangular plate is consistent with the change of the radiated sound power. However, the radiated sound pressure may increase at certain directions even if the radiated sound power of the rectangular plate is suppressed by the inclusion of ribs. The effect of periodic ribs on the sound radiation of the rectangular plate is also explored. The insight gained from this study can inspire the noise design for structures, such as marine platforms and high-speed rail carriages. [ABSTRACT FROM AUTHOR]

In the fields of organ printing and drug preparation, high-precision and stable dispersion of high-viscosity biomaterials enable precise control of organ morphology and drug release rate. This paper proposes the use of an acoustic surface wave to overcome the problem of unstable interface breakup and weak size controllability when the traditional passive droplet microfluidics is applied to high-viscosity (higher than 0.4 Pa·s) dispersed phases. This paper studies the internal flow behavior of high-viscosity fluid under the influence of an acoustic field and realizes the accurate prediction of formation regime and droplet size. Experimental results show that with the increase in acoustic power, three unique droplet generation regimes (e.g., long jetting, transition, and dripping) exist. The transition regime is most suitable for high-throughput preparation of high-viscosity droplets, and its corresponding flow and acoustic conditions can be predicted by equation μd/μc = 4.8 × 10−8 (μc × vc/A P 0 2 × w)−3.32. Affected by the regime transition, the droplet size increases with the increase in acoustic power. The droplet size prediction can be realized based on the capillary number Caf, which represents the intensity of the acoustic field. [ABSTRACT FROM AUTHOR]

This work investigates the effects of actuation frequency and fluid properties on the relocation of miscible inhomogeneous fluids inside a microchannel under acoustic standing waves. Remarkably, we demonstrate the cases in which relocation is achieved relatively faster in the case of fluids with smaller impedance difference between them (1.5%), than in the case of fluids with larger impedance difference (10%) when actuated at an optimum frequency. Subsequently, we show that, if the impedance difference between two fluids is less, actuation of the microchannel at a single frequency is sufficient for the fast and complete relocation, whereas, in the case of larger impedance difference between the fluids, sweeping at multiple frequencies results in fast and more complete relocation compared to actuation of the system by a single frequency. Furthermore, the role of gravity in the process is also analyzed in detail. [ABSTRACT FROM AUTHOR]

Characteristics of the generation and propagation of aerodynamic noise are unique to conventional ground vehicles or aircraft due to the wing-in-ground (WIG) phenomenon. This study numerically examines the aeroacoustic characteristics of a multi-directional wing under the WIG effect with different values of clearance. The flow field was simulated by using a large eddy simulation for six groups of wings with different clearances and one group of wings in free space, at a freestream velocity of 0.3 Ma and an angle of attack of 5°. Acoustic simulations were carried out for each condition by using Möhring's analogy, and the acoustic analysis was based on details of the flow field. A multi-vortex system was generated at the trailing edge of the wing when the clearance was lower than 0.2 C. It consisted of an oscillating attached vortex that expanded and shrank with continuous changes in energy, and two periodically shedding vortices were formed that generated a strong wall-bound vortex street downstream of the wing after shedding. An analytical approach was used to simultaneously analyze different dominant frequencies in each region of the sound field. We conclude that different intensities of the WIG effect led to remarkable differences in the characteristic frequency of aerodynamic noise induced by the multi-directional wing. The characteristic frequencies of aerodynamic noise of the multi-directional wing with a low clearance (clearance ≤ 0.2 C) were mainly caused by the wall-bound vortex street while those at a high clearance (clearance ≥ 0.4 C) were mainly caused by vortices on the wingtip. [ABSTRACT FROM AUTHOR]

This work examines the instability of a plane liquid sheet under the action of a transverse acoustic field. The mechanical definition of the acoustic field is introduced first, and the Floquet theory is applied to derive the dispersion equation and dispersion curve. The dominant instability mechanism of each unstable region on the dispersion curve is distinguished by calculating the oscillation frequencies of the disturbance waves. Next, the parameters within the dispersion equation are set as variables to analyze the development of the instability mechanisms of the unstable regions on the dispersion curve and the oscillation modes on the two surfaces of the liquid sheet. The results prove that the distribution of unstable regions can be affected by the amplitude and frequency of the acoustic field, the viscosity and surface tension of the liquid sheet, and the density ratio of the two gas–liquid phases. Variation in the thickness of the liquid sheet causes development and competition within the oscillation modes, which were found to be related to the development of the instability mechanism. Such evolutionary competition between the sinuous and the varicose oscillation modes was also reflected in the experimental study, where it was observed that the disturbance wave has the characteristics of Faraday waves. [ABSTRACT FROM AUTHOR]

Microbubbles excited by acoustic fields inside water oscillate and generate acoustic radiation forces and drag-induced acoustic streaming. These forces can be harnessed in various biomedical applications, such as targeted drug delivery and on-chip biomanipulation. The conventional approach for using microbubbles as actuators is to trap them inside microfabricated cavities. Anisotropic forces are applied by constraining the interfaces where the air interacts with water. The existing analytical models derived for spherical bubbles are incapable of predicting the dynamics of bubbles in such configurations. Here, a new model for bubbles entrapped inside arbitrary shaped cavities with multiple circular openings is developed. The semi-analytical model captures a more realistic geometry through a solution to an optimization problem. We challenge the assumption that bubbles should be excited at their first resonance frequency to optimize their performance. The natural frequencies and the correlated normal vibration modes are calculated, which are subsequently used to compute the acoustic streaming patterns and the associated thrust by a finite element simulation. An experimental platform was built to measure the deflection of beams loaded by microfabricated bubble actuators and visualize the generated streaming patterns. The results highlight the contribution of the computational model as a design tool for engineering applications. [ABSTRACT FROM AUTHOR]

The influences of serrated trailing edge on the aerodynamic and aeroacoustic performance of a flapping wing during hovering flight are investigated using a hybrid framework of an immersed boundary Navier–Stokes solver for the flow field and the Ffowcs Williams–Hawkings (FW–H) analogy for the sound field. A rigid rectangular wing with an aspect ratio of 2 undergoes pitching and stroke motions at a Reynolds number (Re) of 310 and a Mach number (M) of 0.012. Simulations are conducted by varying the dimensionless wavenumber k * from 2 π to 10 π and wave amplitude 2 h * from 0.25 to 1.0. We find that at k * = 8 π and 2 h * = 1.5 (D4), the average sound power level is reduced by up to 6.8 dB within the Strouhal number (St) between 2.0 and 4.0 compared to that of a plain trailing edge while the lift coefficient is maintained. The directivity at St = 0.2, St = 0.4, St = 2.2, and St = 2.4 is discussed. It is found that the serrations of D4 do not affect the directivity for the first two frequencies and significantly reduce the magnitude of the directivity for the last two frequencies. The serrations of D4 considerably alter the flow field near the wing surface and reduced the surface pressure fluctuations near the wing tip, leading to the noise reduction. The lift coefficient of D4 is not significantly changed, because the reduction in the pressure-contributed lift is compensated by an increase in the shear stress-contributed lift. The serrations with higher 2 h * and k * have larger shear stress-contributed lift. [ABSTRACT FROM AUTHOR]

The dynamics of a cavitation bubble cluster under the influence of an acoustic field is a complex multiphase system that can be observed in acoustic cavitation. In the present study, a three-dimensional computational technique based on the multiphase lattice Boltzmann method (LBM) with multiple relaxation time (MRT) is adopted to investigate the acoustically driven cavitation bubble cluster dynamics near a solid wall at different wetting conditions. Herein, the Peng–Robinson–Stryjek–Vera equation of state with an acentric factor is incorporated in the LBM to accurately impose the physical properties of actual fluids. The validity and capability of the adopted MRT-LBM are confirmed by the excellent agreement of the present results compared to the computed data based on the Rayleigh–Plesset equation for a heterogeneous cavitation phenomenon. The obtained results for the acoustically driven cavitation bubble cluster dynamics demonstrate that the shielding effect of top bubbles prevents the pressure pulse from reaching the lower bubbles. Therefore, the cluster core and the bubbles near the solid surface are more affected by the destruction of the upper layer bubbles than the acoustic field. Also, it is found that the wettability of the solid wall significantly affects the irradiated impulsive pressure waves around the cluster. To justify this result from the physical point of view, the magnitude of the primary and secondary Bjerknes forces is measured and, accordingly, the growth and collapse of bubbles in the cluster under the influence of the acoustic field are discussed in detail. [ABSTRACT FROM AUTHOR]

Surface acoustic wave-based particle/bioparticle manipulation has emerged as a promising tool for disease diagnosis. The effects of the titled angle of the acoustic field θ and the microchannel aspect ratios β on the particle migration mode, the force of particle, and the three-dimensional focusing behavior are studied by using simulation and high-speed microscopic visualizations experiments. The acoustic field tilt range is from 0° to 15°, and the wavelength is 160 μm. Particle migration trajectory is observed from high-speed photographic images. Compared with most parallel acoustic fields, the particle migration efficiency of the tilted acoustic field is higher because the acoustic radiation force (Fr) continues to act on the particles in the lateral direction. The tilted angle of the acoustic field is not a fixed value (usually 15°), and there is an optimal angle to match the maximum lateral migration of the target particles. A model is put forward to predict the optimal acoustic field tilt-angle for acoustofluidic devices, which can achieve 96% separation of 15 μm target particles. The change in the direction of the Fr drives the particles to create two typical migration states during the lateral migration process, named continuous migration and intermittent migration. The phenomenon of multi-layer particle focus in the vertical Z-direction of the microchannel is experimentally observed for the first time, which mainly depends on whether the microchannel has enough height to make multiple acoustic pressure nodes in the vertical direction. Two or even three layers of particle focus lines can be observed in the vertical direction at the microchannel aspect ratios β > 0.5. The research results provide new insight into the high-throughput development of microfluidic devices. [ABSTRACT FROM AUTHOR]

The effect of rotor blade installation angle on the structure-borne noise of adjustable-blade axial-flow fans is analyzed based on the fluid–solid coupling method. The co-simulation environment ANSYS Workbench is adopted to perform one-way fluid–solid coupling analysis. Following this, the properties of the flow field and noise field with different installation angles are simulated. The flow field simulation results reported significant vorticity near the rotor and stator, and a larger installation angle may cause higher pressure fluctuation. The sound field results showed that the frequency spectrum characteristics for the sound pressure level and the sound power level are almost the same while the installation angle changes from −8° to 8°, and the peaks of frequency spectrum occur at the blade passing frequency and its harmonics. The total sound pressure level (TSPL) and the total sound power level (TPWL) all show increasing trends ranging from −8° to +8°. The maxima of TSPL and TPWL reach 134.1 and 176 dB, while their minima reach 123.1 and 163 dB, respectively. Thus, reduction of the installation angle can reduce the structure-borne noise. Besides, the structure-borne noise generated by adjustable-blade axial-flow fans is low-frequency noise, which lies in the range of 0–500 Hz. [ABSTRACT FROM AUTHOR]

Flow over rectangular cavities can become unstable and excite the acoustic modes of the surrounding duct, resulting in severe noise and vibration. In this work, acoustic resonance excitation by two opposite and aerodynamically isolated rectangular cavities is experimentally and numerically investigated to identify the effect of the flow-acoustic coupling on the synchronization of shear layer instabilities. Compressible unsteady Reynolds-averaged Navier–Stokes simulation is used to model the self-excitation of resonance and characterize the fully coupled flow and acoustic fields. Moreover, the location and the strength of the acoustic sources and sinks are evaluated using Howe's integral formulation of the aerodynamic sound. It is revealed that double symmetric cavities generate a higher rate of acoustic energy transfer due to the synchronization of the shear layer instabilities over the two cavities in an antisymmetric pattern, leading to a stronger acoustic resonance than all other cases. On the other hand, the two shear layers over two opposite cavities with different aspect ratios were mismatched in phase and vortex convection velocity. As a result, the net energy transfer in an asymmetric cavity configuration occurred at a similar rate to a single rectangular cavity, driving a weaker acoustic resonance excitation. [ABSTRACT FROM AUTHOR]

We propose a new direct coupling scheme based on the overset technique to tackle moving boundary problems within the lattice Boltzmann framework. The scheme is based on the interpolation of distribution functions rather than moments, that is, macroscopic variables, and includes an additional hypothesis ensuring mass and momentum conservation at the interface nodes between fixed and moving grids. The method is assessed considering four test cases and considering both the vortical and the acoustic fields. It is shown that the direct coupling method results are in very good agreement with reference results on a configuration without any moving subdomain. Moreover, it is demonstrated that the direct coupling method provides an improvement of the accuracy of the lattice Boltzmann overset algorithm for aeroacoustics. In particular, a convected vortex test case is studied and reveals that the direct coupling approach leads to a better ability to conserve the vortex structure over time, as well as a reduction in spurious acoustic distorsions at the fixed/moving interface. [ABSTRACT FROM AUTHOR]

In this paper, an annular array consisting of 64 piezoelectric ceramics was established to make acoustic vortices. The trapped particles and streaming lines in the fluid chamber were theoretically designed and manipulated in experiment. The acoustic field was simulated by Bessel functions and the principle of Huygens by a hydrophone with the annular array excitation. An fast Fourier transform comparison method was proposed in experiments to get the acoustic vortices and phase patterns. The results showed that the patterns of the acoustic field were much different from each other with the variation of excitation phases and the phase patterns implied the vortices in fluid of the chamber, which were affected by the annular arrays of piezoelectric slice excitation. The research concluded that the trapped area and streaming lines can be manipulated by adjusting the phase of piezoelectric slices and the experimental data were helpful to guide the design of acoustic tweezers. [ABSTRACT FROM AUTHOR]

Directivity of the acoustic field of leapfrogging vortices is investigated in the present paper toward the goal to gain insights into sound propagation of free vortical flows and uncover the fundamental physics of aeroacoustics. To this end, frequency-domain Lighthill's acoustic analogy is derived and validated quantitatively. It is sufficient to account for the Doppler effect and investigate the acoustic directivity. Our study finds that the mean flow causes the convection of the propagation of the sound waves and variation of the magnitude of the acoustic pressure. The magnitude of the rms (root-mean-square) pressure fluctuation is equal to that of the acoustic pressure without the mean flow multiplied by the negative square of Doppler's factor. Furthermore, based on the spatial decomposition approach of acoustic sources, the noise radiated from leapfrogging vortices results from the linear superposition of the acoustic fields generated by the upper and lower vortex pairs alone. The superdirectivity of sound originates from the interference of the acoustic sources. The present findings and approaches are promising to provide a framework to study more complex flows. [ABSTRACT FROM AUTHOR]

To help realize stable droplet manipulation, we present a study on the physical mechanism of interfacial instability and droplet atomization dynamics in acoustic levitation with ultrasonic phased arrays. Acoustic levitation is highly applicable in the fields of analytical chemistry and biology because container-less processing can prevent undesirable wall effects, such as nucleation and contamination resulting from the container walls. Although many studies demonstrated the atomization behavior in single-axis acoustic levitation, the breakup characteristics of levitated droplets in ultrasonic phased array levitation have been less studied. Here, we visualized the atomization behavior of ethanol, ethanol solution, and silicone oil droplets using a high-speed camera. The time evolution of the interfacial velocity of the droplets indicates a threshold for the droplet atomization. To elucidate atomization dynamics, the critical interfacial velocities of the levitated droplet immediately before its atomization are discussed and compared to the theoretical prediction based on the Kelvin–Helmholtz instability. Our experimental findings provide deeper physical insights into the levitation stability of droplets in ultrasonic phased array levitation for futuristic lab-in-a-drop applications. [ABSTRACT FROM AUTHOR]

The construction of particle manipulation in the near-field sound field has been extensively studied. In this article, a scheme for far-field particle control through a non-diffracted wave based on the X wave is proposed, which has theoretical completeness and algorithmic simplicity for the construction of far-field acoustic tweezers. The analytical expression of the acoustic radiation force (ARF) acting upon spherical particles of any order X wave is deduced. The spectral pattern of ARF exerted by the X wave in the fluid medium is given. The change law of ARF exerted by the X wave with specific parameters is obtained through calculation. Through the drawn image, the possibility of obtaining a wide range of negative ARFs through X wave is verified. In addition, the far-field invariance of the ARF exerted by the X wave as non-diffraction wave is verified, which provides the possibility of the subsequent construction of far-field acoustic tweezers as well as the basis algorithms for designing acoustic schemes for specific particle manipulation. [ABSTRACT FROM AUTHOR]

Artificial microswimmers have gained significant attention recently for use in biomedical applications, such as drug delivery, and microfluidic applications, such as mixing. The small scale of the swimmers requires external actuation mechanisms, such as magnetic fields. Acoustic actuation, due to its biocompatible nature, can be used alongside magnetic fields to improve swimming performance. Several experimental studies have already shown the propulsion of both non-helical and helical swimmers with acoustic fields. Here, we present a simple model to evaluate the acoustic radiation force on helices. The methodology can be applied to solve for the force on other complicated structures as well, reducing the high computational cost required with other methods, such as the finite-element method. The approach is coupled with a resistive force theory-based model of slender magnetized helices to evaluate the complete three-dimensional trajectories of the swimmers under acoustic and magnetic fields. Traveling waves are shown to reduce swimmer wobbling significantly while also generating a significant push. Standing waves, on the other hand, are observed to place the swimmer in a unique position and orientation, which can be exploited for accurate positioning or micro-mixing. [ABSTRACT FROM AUTHOR]

The flow–sound interaction mechanism and its effect on the vortex dynamics in the wake of circular finned cylinders are experimentally investigated using phase-locked particle image velocimetry at Reynolds numbers between 7 × 104 and 9.5 × 104. In addition, a hybrid experimental–numerical technique using the theory of vortex sound is employed to quantify the acoustic sources and sinks in the vicinity of finned cylinders with different fin-to-root diameter ratios, Df/Dr = 1.5, 2.0, and 2.5. The results show that changing the diameter ratio of the fins induces fundamental changes in the wake structure and the vortex shedding process downstream of the cylinder. Finned cylinders induce stronger vortex cores with a shorter formation length compared to their equivalent bare cylinders. Moreover, the flow topology over the spanwise direction shows that acoustic resonance results in uniform cylindrical vortex cores with less three-dimensional distortion, which demonstrates that the flow field becomes highly two-dimensional during resonance excitation. Quantification of the energy transfer between the flow and the sound fields reveals an enhancement in the acoustic energy production closer to the cylinder with a significant dependence on its fin-to-root diameter ratio. [ABSTRACT FROM AUTHOR]

This paper explores the flow-induced noise propagation mechanism of centrifugal pump from the view of flow and sound field synergy concept. First, the unsteady synergetic relationship between flow and sound fields is deduced, and the synergy angle is defined to describe the synergy degree. It is shown that the domain-averaged synergy angle (θave) changes little with flow time, which implies that the synergy degree is basically unchanged with flow time. With increasing rotational speed or flow rate, the time-averaged θave (θtave) in the impeller and the volute moves far away from 90° gradually, i.e., the synergy degree increases. Meanwhile, the noise outside the pump increases, and the variation of both the noise outside the pump and θtave tends to be gradual. The results manifested that the flow-induced noise propagation mechanism of the centrifugal pump can be well described by the change in synergy degree and the increase in synergy degree can cause the noise tending to propagate outside. In addition, the impact of the blade outlet angle on the noise propagation characteristics is investigated. Considering the synergy degree in the impeller and the volute comprehensively, the deviation of θtave from 90° decreases from 6.48° to 4.74° as the angle increases from 15° to 35°, i.e., θtave tends to approach 90°, and the synergy degree decreases gradually, indicating that increasing the blade outlet angle can weaken the tendency of noise propagating outside by decreasing the synergy degree. These conclusions can guide noise control research and engineering design. [ABSTRACT FROM AUTHOR]

Acoustic levitation can be employed to hold liquid drops in midair, enabling novel applications in X-ray scattering of proteins, amorphous crystallization of solutions, or contactless mixing. Multiple studies have characterized the physical behavior of a levitated drop inside an acoustic field. Here, we present a numerical and experimental study on the acoustic levitation of water drops in a single-axis acoustic levitator consisting of an ultrasonic transducer and an opposing reflector. Instead of modeling an abstract incident acoustic field, our model considers the shape of the drop as well as the real geometry of the levitator. We also use a high-speed camera to observe the disintegration and the undesired oscillations of the drops. Our results show that the insertion of a drop in the levitator provokes a shift in its resonant frequency that depends on the shape of the drop. Second, the levitation behavior depends on whether the levitator operates slightly below or above the resonance. Third, if the levitator is driven above the resonant frequency, it is possible to levitate with more strength and avoid disintegration of the drop. This research provides an insight on how to achieve more stable experiments that avoid the bursting and undesired oscillations of the levitated sample. We hope that it will facilitate numerous experiments involving acoustically levitated liquid drops. [ABSTRACT FROM AUTHOR]

Acoustic levitation has been the cornerstone of many interesting studies across multiple application domains ranging from biomedical engineering to spray drying. In the sphere of colloidal or nanofluid droplets, acoustic levitation allows researchers to probe deep into the physical mechanisms concerning stability, heat and mass transfer processes, and subsequent particle self-assembly. It also offers a plethora of opportunities to custom engineer the transport mechanisms, thereby enabling unique morphological features of the dried precipitate. The high degree of spatial control in a levitator and ease of experimental diagnostics ensure one to study any such transport process in great detail. In this review, we have systematically elucidated three important paradigms in acoustic levitation of nanofluid droplets. First, we have provided a detailed understanding of the fluid mechanics of the process by delving into the pressure and velocity fields the droplet encounters. We have provided descriptions about the key nondimensional number responsible for successful levitation of the droplet. Second, we have studied the transport processes in nanofluid droplets and investigated the important transport mechanisms that are affected by flow and the acoustic field of the levitator. In particular, we look into the heat and mass transfer limitation for particle laden droplets. Third, we have analyzed the particle self-assembly and formation of nanoporous viscoelastic shell. Subsequently, we provided detailed insights into the morphological transitions of the shell through buckling and cavity ingression. We also showcase how the morphology of the shell can be controlled using differential heating and doping. Finally, we conclude by showcasing some unique application context-like photonic crystal behavior that can emerge from unique particle assembly in acoustic levitation. [ABSTRACT FROM AUTHOR]

The turbulent flow over a porous trailing edge of a NACA 0018 airfoil is experimentally investigated to study the link between the hydrodynamic flow field and the acoustic scattering. Four porous trailing edges, obtained from open-cell metal foams, are tested to analyze the effects on far-field noise of the permeability of the material and of the hydrodynamic communication between the two sides of the airfoil. The latter is assessed by filling the symmetry plane of two of the porous trailing edges with a thin layer of adhesive that acts as a solid membrane. Experiments are performed at a zero degree angle of attack. Far-field noise measurements show that the most permeable metal foam reduces noise (up to 10 dB) with respect to the solid trailing edge for Strouhal numbers based on the chord below 16. At higher nondimensional frequencies, a noise increase is measured. The porous inserts with an adhesive layer show no noise abatement in the low frequency range, but only a noise increase at higher frequency. The latter is, therefore, attributed to surface-roughness noise. Flow field measurements, carried out with time-resolved planar particle image velocimetry, reveal correlation of near-wall velocity fluctuations between the two sides of the permeable trailing edges only within the frequency range where noise abatement is reported. This flow communication suggests that permeable treatments abate noise by distributing the impedance jump across the foam in the streamwise direction, promoting noise scattering from different chordwise locations along the inserts. This is further confirmed by noise source maps obtained from acoustic beamforming. For the frequency range where noise reduction is measured, the streamwise position of the main noise emission depends on the permeability of the insert. At higher frequencies, noise is scattered from upstream the trailing edge independently of the test case, in agreement with the roughness-generated noise assumption. [ABSTRACT FROM AUTHOR]