Acoustic Flow Field Research Articles

Vibroacoustic characteristics analysis of a planetary gear reducer considering the exterior housing structure

Abstract. Previous studies have attempted to identify weak links in the dynamic characteristics of the planetary gear reducer (PGR) exterior body structure (EBS). Through numerical simulation, these studies analyzed the mode and natural frequencies and the vibration types of each order of the EBS. However, these scholars have never focused on the main factors affecting the dynamic characteristics of the EBS of this subject. This study in the topic has analyzed the vibroacoustic characteristics of an EBS and optimized its design using numerical simulation. Herein, the contribution of the vibration transfer path from the excitation points on the exterior body to the machine foot is emphatically revealed, and the influences of the main structural parameters on the transfer characteristics are discussed. An optimal EBS for a PGR with lower acoustic vibrations is designed in detail, and a composite EBS with damping vibration attenuation and acoustic absorption is proposed. The radiation acoustic characteristics without acoustic protection and damping materials are analyzed. These research results are implemented in order to realize the dynamic characteristics, transmission, and radiation acoustic characteristics as objects of optimization, and the structural design parameters of the PGR exterior body are deeply optimized using dynamic modification and sensitivity analysis. This topic focuses on the vibroacoustic coupling of EBS in stationary fluid and average flow field. Based on the analysis method of theoretical modeling and numerical calculation, the EBS dynamic response and vibroacoustic characteristics under the action of frontal excitation external acoustic flow field are studied, which will be beneficial to explore the comprehensive optimization design of PGR dynamic and vibroacoustic properties.

Numerical simulation analysis of a PGR exterior body structure considering vibroacoustic characteristics

Previous studies have attempted to identify weak links in the dynamic characteristics of the planetary gear reducer (PGR) exterior body structure (EBS). Through numerical simulation, these studies analyzed the modals and natural frequencies and the vibration types of each order of the EBS. However, these scholars have never focused on the main factors affecting the dynamic characteristics of the EBS of this subject. This study in the topic has analyzed the vibroacoustic characteristics of an EBS and optimized its design using numerical simulation. Herein, the contribution of the vibration transfer path from the excitation points on the exterior body to the machine foot is emphatically revealed, and the influences of the main structural parameters on the transfer characteristics are discussed. An optimal EBS for a PGR with lower acoustic vibrations is designed in detail, and a composite EBS with damping vibration attenuation and acoustic absorption is proposed. The radiation acoustic characteristics without acoustic protection and damping materials are analyzed. These research results are implemented in order to realize the dynamic characteristics, transmission, and radiation acoustic characteristics as objects of optimization, and the structural design parameters of the PGR exterior body are deeply optimized using dynamic modification and sensitivity analysis. This topic focuses on the vibroacoustic coupling of EBS in stationary fluid and average flow field, based on the analysis method of theoretical modeling and numerical calculation, the EBS dynamic response and vibroacoustic characteristics under the action of frontal excitation external acoustic flow field are studied, which will be beneficial to explore the comprehensive optimization design of GPR dynamic and vibroacoustic properties.

Acoustic streaming in second-order fluids

In this article, inner acoustic streaming for second-order fluids has been studied analytically by employing asymptotic expansions for a thin Stokes layer and low acoustic Mach number. In addition, a multiple-timescale approach has been adopted to separate the primary oscillatory flow and the steady acoustic streaming. The study considers two sample cases: (i) motionless boundary and (ii) vibrating boundary and compares the characteristics associated with their streaming. It is observed that both the primary oscillatory flow and acoustic streaming flow fields are suppressed in second-order fluids due to the extra stress components present in the fluids. This study considers both compressible and incompressible Stokes layers to bring out the acoustic streaming characteristics associated with fluid compressibility. For the compressible Stokes layer, stronger acoustic streaming flow results for the motionless boundary, leveraging the deeper interaction between the primary oscillatory pressure field and the steady streaming. In the case of a vibrating boundary, the primary oscillatory pressure field is independent of the Stokes layer compressibility, and hence, the acoustic streaming flow remains unaltered. The extra stresses in second-order fluids reduce the acoustic body force density, and the maximum reduction has been observed for the vibrating boundary. In order to understand Lagrangian streaming, Stokes drift has also been calculated and compared for all the scenarios. The theoretical analysis and fundamental insights derived from this study have potential for applications in diverse fields such as particle manipulation, biosensing, cell sorting, and removal of loosely bound material such as non-specifically bound proteins.

Averaging of thermoacoustic azimuthal instabilities

We consider the acoustic flow field of rotationally symmetric systems, like an annular combustor and the flow in a round duct, in absence of a mean azimuthal flow field. We focus on azimuthal instabilities, which manifest as either spinning (rotating) waves or standing waves, or a linear combination of the two. These instabilities are often excited by some level of background noise that makes the system randomly change between spinning and standing states, undergo amplitude variations and changes to the azimuthal orientation of the solution. To account for this random change, we make use of a novel ansatz to track as a function of time the amplitude, orientation, nature (standing/spinning) and temporal phase of these instabilities. To capture the effect of the background noise, we apply stochastic averaging on the governing equations and obtain a novel differential equation. The equation allows to study the effects of acoustic sources and sinks on the statistics of the solution, and of explicit and spontaneous symmetry breaking and noise intensity. We focus on this last effect and show how the noise intensity affects the system preference for spinning and/or standing states. We find for example that, when present, background noise pushes the system away from spinning states and towards standing states, consistently with experiments and numerical simulations.

A step towards the live identification of pipe obstructions with the use of passive acoustic emission and supervised machine learning

A single passive acoustic emission sensor was used to collect signals coming from an obstructed pipe in a water recirculation system. Four geometrically different obstructions were investigated. The flow field of water around each obstruction was visualised with the use of 2D particle image velocimetry (PIV) to identify the different flow features. In parallel, the acoustic emission signals were acquired by locating a piezoelectric sensor on the outer wall of the pipe at the tip of the obstruction. The acoustic emission signals were then pre-processed and the frequency domain was extracted for 100 recordings in each case. Signals were processed further by using principle component analysis and a matrix is created for supervised machine learning algorithms. This methodology was applied over a range of four flow rates, all in fully developed turbulent flow. Results showed that different obstructions generated different acoustic signals and flow fields, which reflected the different flow fields observed with PIV. The average velocity and amplitude of the acoustic signals increased in magnitude with increasing flow rate. The machine-learning algorithm with highest prediction values was quadratic support-vector machine with predictions in the area of 95% accuracy or above. This makes the combination of machine learning and a single passive acoustic sensor a viable option to predict pipe obstructions and the type of obstruction. This may lead to a useful application for urban water supply or sewage systems as well as agricultural practice for field irrigation or the detection of nozzle blockages.

Quantitative experimental determination of evaporation influencing factors in single droplet levitation

Acoustic levitation has been used successfully as a model system for spray polymerization processes and the determination of particle morphology. The experimental extension for the investigation of heat and mass transfer in levitated liquid droplets requires much more effort. In order to enable the levitation method as a universal tool for the tracking of heat and mass transfer, influencing experimental parameters and their effect on relevant characteristics like droplet deformation, temperature and surrounding flow field were analyzed. These parameters were elucidated experimentally to reveal their mutual dependencies. It was shown that the evaporation rate is affected by the aspect ratio of a droplet, which is directly correlated with its wet-bulb temperature. This was primarily explained by varying acoustic flow field patterns based on classical fluid mechanics, gas velocities and acoustic boundary layer thicknesses, which were visualized and quantified by particle image velocimetry. In order to study the wet-bulb temperature, thermocouple measurements on the levitated droplet were performed. It turned out, that the wet-bulb temperature is dependent on the power input and the resulting sound pressure level as it changes the acoustic field around the droplet. Moreover, the temperature measurement revealed the necessity to establish a temperature correction to account for heat conduction effects along the thermocouple shaft into the droplet. The illustrated experimental setup and measurement procedure is supposed to provide guidelines for the experimental determination of heat and mass transfer in evaporating droplets.

Experimental Study of Acoustically Enhanced Heat-Transfer from Copper Sphere in a Steady Flow Field

The combined effects of acoustic field and flow field on the forced convective heat transfer rate during cooling of a stationary sphere were investigated experimentally. The acoustic frequency is in the range of 100 to 2500 Hz, with the velocity ratio of acoustic velocity and flow velocity is in the range of 0.59 to 4.76. The relationship among sound pressure levels, frequency, flow velocity and the diameter of the sphere with the temperature gradient of the sphere were analyzed. The results show that higher velocity ratio cause higher heat-transfer rate. However, when the sphere size is 5 mm, the heat transfer rate is always lower than that without an acoustic field during our experiment, and when the sphere size is 10 mm and the velocity ratio is higher than 3.38, the heat transfer rate is higher than that without an acoustic field, but when the size of the copper sphere is 15 mm, the velocity ratio is decreased to 1.89. There is a special frequency, at which Nu reaches a maximum. This special frequency increases with increasing SPLs. The conclusions highlight a good application prospect in combustion for the boiler.

Characterization of the aircraft bay/landing gear coupling noise at low subsonic speeds and its suppression using leading-edge chevron spoiler

When the aircraft opens the bay door to let the landing gear either drop or retract, the incoming flow will result in a significant amount of coupling noise from the bay and the landing gear. Here, an experimental study was reported to characterise the acoustic performance and flow field at low subsonic speeds. Also, we examined a passive control method leading-edge chevron spoiler to suppress the noise. The experiment was performed in a low-speed aeroacoustic wind, the bay was simplified as a rectangular cavity and the spoiler was mounted to the leading edge. Both acoustic and aerodynamic measurements were performed through two microphone arrays, pressure transducers and particle image velocimetry. It was found that installation of the landing gear model can attenuate cavity oscillation noise to some extent by disturbing the shear layer of the cavity leading edge. Moreover, acoustic measurement confirmed the noise control when the spoiler was used. In addition, a parametric study on the effects of chevron topology was performed, and an optimised value was found for each parameter. From the aerodynamic measurement, the noise reduction was explained from the perspective of fluid dynamics. It was observed that installation of the chevron can raise the leading-edge shear layer and break up the large-scale vortices, thereby controlling the Rossiter mode noise and the landing gear model noise at certain frequencies.

Analysis on acoustic performance and flow field in the split-stream rushing muffler unit

Airflow velocity is a dominant factor of influencing exhaust resistance and turbulence noise in exhaust muffler of internal combustion engine. In this paper, a new principle of reducing the airflow velocity, based on split-stream rushing in muffler, was proposed. The main idea of this principle is to split the exhaust airflow into two streams first, and then lead them rushing when meeting from two opposite-located holes so that achieve the result of reducing the exhaust airflow velocity through the muffler. The basic structure of the split-stream rushing muffler unit was presented and its acoustic performance was analyzed by using the acoustic wave theory. Finally, the numerical simulation and experimental way were used to verify the internal sound field and velocity changes in the new muffler unit. The results show that the proposed muffler unit can effectively reduce the exhaust airflow velocity through the silencer and exhaust backpressure.

Trapping and exclusion zones in complex streaming patterns around a large assembly of microfluidic bubbles under ultrasound

We experimentally observe streaming patterns around pulsating bubbles. Under flow, closed recirculation zones appear, that we can predict with a dipolar-like description of the acoustic streaming flow field. This can be extended to model the complex patterns around a large group of bubbles.

Inertial effects on acoustic Rayleigh streaming flow: Transient and established regimes

The effect of inertia on Rayleigh streaming generated inside a cylindrical resonator where a mono-frequency standing wave is imposed, is investigated numerically and experimentally. To this effect, time evolutions of streaming cells in the near wall region and in the resonator core are analyzed. An analogy with the lid-driven cavity in a cylindrical geometry is presented in order to analyze the physical meanings of the characteristic times.Inertial effects on the established streaming flow pattern are then investigated numerically using a code solving the time averaged Navier–Stokes compressible equations, where a mono-frequency acoustic flow field is used to compute the source terms.It is shown that inertia of streaming cannot be considered as the leading phenomenon to explain the mutation of streaming at high acoustic levels.

Acoustic Flow-Based Control of a Mobile Platform Using a 3D Sonar Sensor

Moving a sensor through its environment creates signature time variations of the sensor’s readings often referred to as flow cues. We analyze the acoustic flow field generated by a sonar sensor, capable of imaging the full frontal hemisphere, mounted on a mobile platform. We show how the cues derived from this acoustic flow field can be used directly in a layered control strategy, which supports a robotic platform to perform a set of motion primitives, such as obstacle avoidance, corridor following, and negotiating corners and T-junctions. The programmable nature of the spatial sampling pattern of the sonar allows efficient support of the varying information requirements of the different motion primitives. The proposed control strategy is first validated in a simulated environment and subsequently transferred to a real mobile robot. We present simulated and experimental results on the controller’s performance while executing the different motion primitives. The results further show that the proposed control strategy can easily integrate minimal steering commands given by a user (electric wheelchair application) or by a high-level navigation module (autonomous simultaneous localization and mapping (SLAM) applications).

Pipe jet noise reduction using co-axial swirl pipe

ABSTRACTThe present experimental work highlights the acoustic far field and flow field characteristics of confined co-axial swirling pipe jets. Co-axial confinements with six vanes at angles of 0°, 20° and 40° are considered here. Two pipe lengths of L/D=0.5 and 2 are studied. The Mach numbers studied range from 0.85 to 1.83. An increase in the pipe length causes suppression of the transonic tones in non-swirl pipe jets. Swirl reduces the low frequency noise components and increases the high-frequency components compared to non-swirl jet. The broadband shock associated noise is mitigated by the swirl pipe jets. However, the screech tone is completely eliminated by the swirl pipe jets. Further, swirl pipe jets radiate low levels of noise at all the emission angles compared to non-swirl pipe jets, for both the pipe length cases at supersonic Mach numbers. Increase in the pipe length enhances the shock associated noise and OASPL for the non-swirl pipe jet. Centreline pitot survey and schlieren visualisation show a reduction in core length, reduction in the number of shock cells, weakening/destruction of the shock cells by the swirl pipe jets compared to the non-swirl pipe jets.

Acoustic field and convection pattern within liquid material during ultrasonic processing

When ultrasound propagates in a liquid alloy, nonlinear effect takes place such as cavitation effect and acoustic streaming, which accelerates the solute and thermal transportation during alloy solidification, and consequently, improves the solidification microstructures and mechanical properties of the metallic alloy. Therefore, it is significant to investigate the ultrasound propagation characteristics in liquid. Here, by choosing water as a model transparent material, the acoustic fields and flow fields induced by 20 and 490 kHz ultrasounds are investigated by numerical simulation, and the effects of frequency and ultrasonic horn radius are studied. Firstly, the simulation results demonstrate that the sound pressure under 20 kHz ultrasound decreases obviously along the ultrasonic propagation direction, and the maximum of sound pressure value is equal to the initial pressure. In this case, the cavitation effect only occurs in the region close to the ultrasonic horn. By contrast, when the ultrasonic frequency increases to 490 kHz, the sound pressure is higher than that of 20 kHz ultrasound, and displays periodical vibration characteristic along the wave propagation direction. The cavitation volume correspondingly expands to a large extent with a regular striped distribution. It can also be found that increasing the ultrasonic horn radius under 20 and 490 kHz ultrasounds can effectively promote the sound pressure level in water, and hence leads to the remarkable enlargement of cavitation volume. Secondly, the calculated results of flow field indicate that the streamlines in water are similar under the two ultrasounds with different frequencies. A jet produced by the center of horn spreads down and divergences to both sides after reaching the bottom. For both frequencies as the horn radius increases, the radius of jet increases and the average velocity in water first increases and then decreases, whose maximum value appears when the horn radius is 40 mm. Meanwhile, the average velocity under 20 kHz ultrasound is larger than that under 490 kHz ultrasound for each horn radius. Finally, particle image velocimetry method is employed to measure the velocity field in water. Both the positions of eddy and the velocity distribution are the same as the simulation results, which verifies the reliability of the present theoretical calculation model. The scenario in this work is analogous to the acoustic field and the flow field in liquid alloy, which is beneficial for the design of parameter optimization during ultrasonic processing in alloy solidification.

Comparison of various algorithms for improving acoustic attenuation performance and flow characteristic of reactive mufflers

The parametric optimization of the reactive mufflers is researched by numerical analysis, regarding the performance of the acoustic and flow fields synthetically. The finite element method, based on the Helmholtz equation and the Navier–Stokes equation respectively, is utilized in the analysis of the acoustic and flow fields. And the initial and boundary conditions are set up in the physical fields respectively. The weighting multi-objective function about acoustic and flow fields is formulated. In addition, the optimization results of multidisciplinary, obtained by the Nelder Mead algorithm (NMA) based on the sensitivity analysis, the Monte Carlo algorithm (MCA) and Genetic Algorithm (GA) based on the random sampling, are analyzed comparatively. The optimization results indicate that the NMA can maximize the transmission loss (TL) and minimize the pressure drop with the given weight factor. Finally, numerical optimization examples confirm the validity and reliability of the proposed optimization method in the acoustic-flow field.

Acoustic streaming, fluid mixing, and particle transport by a Gaussian ultrasound beam in a cylindrical container

A computational study is reported of the acoustic streaming flow field generated by a Gaussian ultrasound beam propagating normally toward the end wall of a cylindrical container. Particular focus is given to examining the effectiveness of the acoustic streaming flow for fluid mixing within the container, for deposition of particles in suspension onto the bottom surface, and for particle suspension from the bottom surface back into the flow field. The flow field is assumed to be axisymmetric with the ultrasound transducer oriented parallel to the cylinder axis and normal to the bottom surface of the container, which we refer to as the impingement surface. Reflection of the sound from the impingement surface and sound absorption within the material at the container bottom are both accounted for in the computation. The computation also accounts for thermal buoyancy force due to ultrasonic heating of the impingement surface, but over the time period considered in the current simulations, the flow is found to be dominated by the acoustic streaming force, with only moderate effect of buoyancy force.

Analysis and Improvement on Acoustic Performance of Muffler with the Consideration of Temperature and Flow Velocity

Aiming for the acoustic performance of expanded-muffler, the acoustic fields, flow field, temperature field and flow regenerated noise inside the expanded-muffler were studied. Furthermore, with the consideration of temperature and flow velocity, the influence of temperature and flow velocity on acoustic performance was obtained. Based on the research, the acoustic performance of an exhaust muffler was studied. By adjusting the structural parameters, without affecting the engine output power, the insertion loss of muffler has average increased by 5.1 dB (A).

Acoustic streaming in gas-fluidized beds of small particles

An increasing number of applications rely on the use of gas-fluidized beds of small particles to enhance processes based on the mass/heat transfer between a gas and a solid. On the other hand, experimental studies have shown that application of high-intensity acoustic fields serves to further intensify fluidized bed processes. It is well known that the fluid streaming developed around the surface of a solid body subjected to a sound wave leads to a notable increase of the mass/heat transfer. However, several additional phenomena are observed in sonoprocessed fluidized beds requiring a close examination. In a sonoprocessed fluidized bed, acoustic streaming is superposed on the fluidizing gas flow, which exhibits large fluctuations within the bed at a local level due to long-range hydrodynamical interactions. Attenuation due to viscous friction and thermal conduction on the surface of the small particles is expected to play a relevant role. Fine particles may be entrained by the acoustic flow field whereas acoustic streaming on the surface of not entrained particles gives rise to an increased drag. Entrainment of fine particles in the sound wave may serve to improve fluidization uniformity in dense beds or to induce particle agglomeration in diluted beds. Acoustic streaming leads also to a large-scale fluid recirculation characterized by velocities sufficiently high as to affect the fluidized bed dynamics. A substantial modification of the dynamics of fluidized beds may thus occur depending on intervening physical parameters such as particle size and density, sound wave intensity and frequency, fluid properties, fluidization regime and system size. In this manuscript, the main features of acoustic streaming and the associated phenomena due to the application of high-intensity acoustic fields on fluidized beds are analyzed from a review on the fundamentals of acoustic streaming and observations on sonoprocessed fluidized beds. Results inferred from this analysis are expected to help assess the optimum conditions under which sound assisted process intensification would be maximized in each particular application.

Calculation on cylinder pressure fluctuation by using the wave equation in KIVA program

Cylinder pressure fluctuation during combustion process of internal combustion engine is closely related to combustion noise and knock. The current studies are based on cylinder pressure test to obtain information on combustion noise and knock, but there is little for simulation of combustion pressure fluctuation. Based on effects of combustion process in the combustion chamber on cylinder pressure by using wave equation, the mechanism of pressure fluctuation during combustion is researched with three-dimensional acoustic wave equation and flow field model of KIVA program. The cylinder pressure fluctuation curve, temperature field and acoustic field are obtained from the coupled calculation of the wave equation and KIVA program. The frequency spectrum analysis is taken with the cylinder pressure oscillation of cylinder pressure measured and calculated. The calculation result is consistent with the experimental result. This indicates that the cylinder pressure fluctuation can be correctly calculated with the wave equation. Analysis of calculation results of temperature field and acoustic field shows that sound field changes faster than flame propagates, and distribution of sound field is more complicated. Combustion pressure oscillation in the diesel engine is under highly unstable condition. This indicates that the combination of cylinder pressure fluctuation model and combustion model is an effective method to study the pressure oscillations and a new method to study the combustion noise and knock.

Flow and geometry induced scattering of high frequency acoustic duct modes

Cut-on cut-off transition of acoustic modes in hard-walled ducts with irrotational mean flow is well understood for Helmholtz numbers of order unity. Previous finite-element simulations of this phenomenon, however, appear to indicate the possibility of energy scattering into neighbouring modes at moderately large Helmholtz numbers. In this paper, such scattering phenomena are explained and predicted in slowly varying aeroengine ducts using a multiple-scales approach. It is found that, for sufficiently high frequencies, two mechanisms exist whereby energy can be scattered into neighbouring modes by an incident propagating mode. One mechanism occurs only when there is a mean flow inside the duct and induces scattering at significantly lower frequencies than the other mechanism which remains present without mean flow. A coupled system of ordinary differential equations is derived and then solved numerically for a number of example cases to obtain the corresponding transmitted and reflected amplitudes of the scattered modes as well as the overall acoustic pressure field. The theory appears to demonstrate that some exchange of energy between the acoustic and mean flow fields occurs during scattering.