Lighthill's Acoustic Analogy Research Articles

Effects of wall heating on wall pressure fluctuations and flow noise in a low-Reynolds-number turbulent channel flow with temperature-dependent viscosity

Wall pressure fluctuations and flow noise substantially degrade sonar detection performance and the acoustic stealth performance of underwater vehicles. This paper numerically investigates the effects of wall heating on wall pressure fluctuations in turbulent channel flow of water with temperature-dependent viscosity, exploring a novel method for controlling wall pressure fluctuations and flow noise in underwater vehicles. Large-eddy simulation (LES) is employed for the numerical calculation of the flow field, while a hybrid method combining LES with Lighthills acoustic analogy is employed to predict flow noise. The numerical results show that when the temperature difference between the wall and the incoming flow is 30 K and 50 K, the peak root-mean-square pressure fluctuations decrease by 6.76% and 8.91%, respectively. Wall heating stabilizes the pressure field near the wall, with the spectral levels of wall pressure fluctuations showing average decreases of approximately 1 dB and 2 dB. Wall heating weakens the energy-containing structures of wall pressure fluctuations and increases the overall convection velocity by 1.22% and 3.81%, respectively. Flow structure analysis reveals that the weakening of energy-containing structures results from the suppression of the vortex structures in the near-wall region. In the wall heating cases, peak turbulent kinetic energy decreases by 12.6% and 15.8%, respectively. Moreover, the sound pressure level of flow noise decreases with increasing wall temperature, with the maximum noise reduction exceeding 3 dB. Previous studies have not yet explored the effects of viscosity reduction caused by wall heating on wall pressure fluctuations and flow noise. This study demonstrates that wall heating is a promising method for reducing wall pressure fluctuations and flow noise.

Simulation and experimental research on the propagation mechanisms of acoustic energy inside the pipeline to detect the leakage

Leakage from pipelines has negative impacts on property and environment. The diameters of internal acoustic detectors in pipelines are small and not easily blocked, making it suitable for pipelines under various working conditions. Studying the characteristics of pipeline leakage sound signal and the propagation mechanisms within pipelines is crucial for timely detection of small leaks. In this study, a 3D simulation model of pipeline leakage was established, and a CFD/CAA hybrid calculation method was used to obtain effective leakage flow field information via LES. The equivalent sound sources in the flow field were extracted based on the Lighthill acoustic analogy theory. Determine the characteristic frequency range of the leakage sound signal was 1–2 kHz. This study founded that an increase in the leakage pressure and aperture size both lead to an enhancement in the leakage sound signal, and the propagation of leakage sound signal in pipelines is affected by background flow. An experimental pipeline leakage system was designed, and the accuracy of the simulation results was verified experimentally.

Impact of helical grooves on drag force and flow-induced noise of a cylinder under subcritical Reynolds numbers

The drag force and flow-induced noise of underwater vehicles significantly affect their hydrodynamic and stealth performance. This paper investigates the impact of helical grooves on the drag force and flow-induced noise of underwater vehicles through numerical simulations of the flow around cylinders with two types of helical grooves under various subcritical Reynolds numbers. The simulation scheme employs the large-eddy simulation framework combined with the Lighthill acoustic analogy method. The results show that the helical-groove structure can achieve reductions of up to 30% in drag and 5 dB in noise. These helical grooves have a significant effect in terms of suppressing the formation of a Karman vortex street downstream of the cylinder. Under subcritical Reynolds numbers, the drag-reduction effect of the helically grooved cylinder decreases as the number of helical grooves increases, while the noise-reduction effect increases with increasing number of helical grooves.

Unsteady flow behaviors and flow-induced noise characteristics in a closed branch T-junction

In the present study, dynamic delayed detached eddy simulation is utilized to explore turbulent flow in T-junctions at a Reynolds number of ReD = 2.0 × 104. Three systems with varying corner cavity depth-to-diameter ratios (Ld/D = 1, 2, and 4) are examined to elucidate the interplay between unsteady flow and flow-induced noise. The analysis employs Lighthill's acoustic analogy to scrutinize surface dipole acoustic sources and their noise propagation characteristics. Coherent flow structures, characterized as wavepackets, are identified through spectral proper orthogonal decomposition, demonstrating consistent dominance in modes and dipole distributions across the systems. In the system with Ld/D = 1, wavepackets originating from the downstream region of the junction exhibit a pronounced flapping behavior attributed to the Kelvin–Helmholtz instability. Most dissipate with the mainstream flow, whereas a portion interacts with the wall, forming dipole acoustic sources. For systems with Ld/D = 2 and 4, the dominant mode transitions to the junction adjacent to the corner cavity, expanding continuously after separation until obliquely colliding with the wall, resulting in expanded dipole distributions. Mechanisms underlying flow-induced noise generation are unveiled by extracting transient vorticity fields within oscillation cycles. For shallow corner cavity depths (Ld/D = 1), periodic oscillatory vorticity shedding from the junction's sidewall significantly contributes to far-field sound pressure. As the cavity is deep enough to support one or more full recirculations of the fluid (Ld/D = 2 and 4), periodic vorticity shedding from the trailing edge directly impacts the wall above the junction, simultaneously suppressing flapping behavior at the leading edge and weakening overall dipole acoustic source intensity.

Aeroacoustic simulation of human phonation based on the flow-induced vocal fold vibrations including their contact

This paper deals with the numerical simulation of the flow induced sound as generated in the human phonation process including vocal folds (VF) contact. The hybrid approach is used allowing to separate the acoustic and the biomechanical description of the considered problem. The two-dimensional fluid–structure interaction (FSI) problem is modelled by a simplified linear elastic model coupled to the incompressible Navier–Stokes equations in the arbitrary Lagrangian–Eulerian form and special attention is paid to the treatment of the contact problem in the FSI context. Following approach of [Sváček and Horáček, 2021] the incompressible flow model is modified by an addition of fictitious porosity term and the penalization inlet boundary condition is used. Further, the Lighthill acoustic analogy is applied for calculation of sound sources and their propagation through a vocal tract model. All partial differential equations, describing the elastic body vibration, the fluid flow and the acoustics, are approximated by finite element method. Numerical results of one-sided vocal fold paralysis are presented.

Study on Mechanism and Suppression Method of Flow-Induced Noise in High-Speed Gear Pump

The flow-induced noise mechanism of a 5000 rpm high-speed gear pump is explored. On the basis of the CFD technology and the Lighthill acoustic analogy theory, a numerical model of the flow-induced noise of a high-speed gear pump is constructed, and the effect of oil suction pressure (0.1–0.2 MPa) on the internal flow field and flow-induced noise characteristics of the high-speed gear pump is investigated. To evaluate the accuracy of the numerical simulation, a noise testing platform for high-speed gear pumps was developed. Adding an oil replenishment groove to the high-speed gear pump suppresses its flow-induced noise. The results indicate that the discrete noise at the fundamental frequency and its harmonic frequency is the primary component of the flow-induced noise of the pump and that the oil-trapped area is the principal source of vibration. The overall sound pressure level of flow-induced noise in the inlet and outlet areas decreases with distance from the oil-trapped area, and the sound pressure level in the outlet area is greater than that in the inlet area. The oil replenishment groove may considerably minimize cavitation noise, enhance the oil absorption capacity, and reduce the outer field’s overall sound pressure level by 4–5 dB.

A hybrid finite volume - spectral element method for aeroacoustic problems

We propose a hybrid Finite Volume (FV) - Spectral Element Method (SEM) for modeling aeroacoustic phenomena based on the Lighthill's acoustic analogy. First the fluid solution is computed employing a FV method. Then, the sound source term is projected onto the acoustic grid and the inhomogeneous Lighthill's wave equation is solved employing the SEM. The novel projection method computes offline the intersections between the acoustic and the fluid grids in order to preserve the accuracy. The proposed intersection algorithm is shown to be robust, scalable and able to efficiently compute the geometric intersection of arbitrary polyhedral elements. We then analyze the properties of the projection error, showing that if the fluid grid is fine enough we are able to exploit the accuracy of the acoustic solver and we numerically assess the obtained theoretical estimates. Finally, we address two relevant aeroacoustic benchmarks, namely the corotating vortex pair and the noise induced by a laminar flow around a squared cylinder, to demonstrate in practice the effectiveness of the projection method when dealing with high order solvers. The flow computations are performed with OpenFOAM [50], an open-source finite volume library, while the inhomogeneous Lighthill's wave equation is solved with SPEED [34], an open-source spectral element library.

A theoretical study of self-soise generation in turbulent jets using one-dimensional turbulence and Lighthill's acoustic analogy

Modeling and simulation of turbulent jet noise is an ongoing numerical challenge relevant to noise pollution control. In the present study, the concept of a novel dimensionally reduced modeling approach based on the so-called one-dimensional turbulence model (ODT) is discussed. ODT aims to resolve source terms due to velocity and kinematic pressure variations in the acoustic near-field at all relevant scales, but only for a single physical coordinate. In the standalone formulation of the model, the physical coordinate taken as a notional line of sight that is pointing in radial cross-stream direction and advected downstream with the axial velocity of the jet while the microscales evolve. Thereby, turbulence is represented by a stochastically sampled sequence of discrete mapping events that punctuate the deterministic molecular diffusive advancement. It is discussed how an ensemble of small-scale resolving independent flow realizations can be used to estimate turbulent noise sources from a range of scales. The main objective is to present an approach for the prediction of broadband self-noise generated by a turbulent jet using ODT and Lighthill's equation. Dedicated experimental measurements are conducted for verification and validation of the numerical modeling approach.

Toward the use of a reduced‐order and stochastic turbulence model for assessment of far‐field sound radiation: Low Mach number jet flows

AbstractThis work presents a novel framework for the evaluation of far‐field sound radiation, specializing to a low Mach number jet flow. The framework comprises an analytical and a numerical part. In the analytical part, a low Mach number asymptotic analysis is presented to obtain the spectral sources of sound radiation starting from a pressure wave equation that is directly obtained from the Navier–Stokes equations. The derivation procedure is based on the fundamental premise of Lighthill's acoustic analogy. In the numerical part, a reduced‐order model for turbulent flow, the one‐dimensional turbulence model, is used to simulate the velocity field of a low Mach number turbulent round jet, assumed fully developed and statistically steady, hence homogeneous in the azimuthal direction and for a cross‐sectional slab assumed locally homogeneous in axial direction, too. The generated velocity field is used for the calculation of the spectral pressure sources, and consequently, to estimate the far‐field sound pressure level (SPL) of the jet. The results of the analysis are compared against available experimental SPL measurements from a subsonic jet. A reasonable agreement is obtained for the SPL. Furthermore, the analysis sheds light into the different contributions of the subsonic flow to sound radiation. The framework is readily extendable for any kind of low Mach number flow, including variable density flows and jet flames.

Simulation study on the flow-induced vibration and noise in marine centrifugal pumps

Due to the three-dimensional non-axisymmetric shape of the volute, its interaction with the impeller increases the internal flow field complexity and instability of the marine centrifugal pump, representing the main reason for pump body vibration and hydraulic noise. First, the basic principle of fluid machinery noise calculation was explained based on the Lighthill acoustic analogy method and Curle’s theory. Then, the fluid pressure pulsation and flow characteristics of centrifugal pumps were analyzed using the Fluent software, after which the Actran software was used to examine the flow-induced vibration noise properties. The results showed that the clearance size significantly impacted the pump head, while the peak value of the flow-induced vibration response was mainly twice the blade frequency. Although a larger ring clearance increased the structural vibration, it had little impact on the sound pressure level changes in the external sound field. The results provided a new method for the structural optimization and low-noise design of marine centrifugal pumps.

An Efficient Hybrid Computational Process for Interior Noise Prediction in Aeroacoustic Vehicle Development

Numerical methodologies for aeroacoustic analyses are increasingly crucial for car manufacturers to optimize the effectiveness of vehicle development. In the present work, a hybrid numerical tool based on the combination of a delayed detached-eddy simulation and a finite element model, which relies on the Lighthill's acoustic analogy and the acoustic perturbation equations, is presented. The computational aeroacoustics is performed by the software OpenFOAM and Actran, concerning respectively the CFD and the FEM. The aeroacoustic behavior of the SUV Lamborghini Urus at a cruising speed of 140 km/h has been investigated. The main aerodynamic noise phenomena occurring in the side mirror region in a frequency range up to 5 kHz are discussed. The numerical simulations have been verified against the measurements performed in the aeroacoustic wind tunnel of the University of Stuttgart, operated by FKFS. The predicted exterior noise propagation into the far field has been validated by comparing the sound pressure level with the experimental data measured by exterior microphones, which were located outside the turbulent region beside the wake of the side mirror. Furthermore, the noise transmission into the cabin through the side window has been modeled. Simulation results have been validated by means of interior microphones installed on the driver seat. Both the exterior and the interior noise predictions show very good correlations with experiments. Lastly, a comprehensive investigation of the most critical aeroacoustic sources has been carried out. The numerical tool has been proven to be in good accordance with the microphone array with respect to the distribution of the sound pressure level in the proximity of the side mirror. Besides, the main vortex structures involved in the generation mechanisms of wind noise have been investigated by a CFD analysis. The entire CAA process has been proven to be accurate and suitable for combined analysis between the generation mechanisms of wind noise and the resulting transfer into the interior cabin to the driver's ear as well.

Analysis of Elliptical-Jet Acoustic Directivity and Efficiency Using a Vortex-Sheet-Based Wave-Packet Model

The hydrodynamic characteristics and associated far-field acoustics of installed elliptical jets were studied using linear models based on the Navier–Stokes equations. A compressible elliptical vortex-sheet model predicted that the growth rates of the varicose and wagging modes decreased with the aspect ratio, whereas the growth rate of the flapping mode increased, which was consistent with previous studies using finite-thickness stability models. Using a propagation model adapted from the Lighthill acoustic analogy, it was found that the acoustic efficiency of the “varicose” mode is significantly higher than the “flapping” and “wagging” modes for both free- and installed jets. Increasing the aspect ratio of the elliptical jet produces a slight reduction in sound for the mode, a significant decrease in the mode, and a significant increase for the mode. A similar trend was observed for the installed case. An exponential relationship between the jet–plate distance and scattered sound was found for elliptical jets, as observed for circular jets. Other geometric parameters such as the orientation angle of the ellipse were also analyzed.

Near-field acoustic holography for underwater moving surfaces

Near-field acoustic holography has been the standard technique to accurately image static structure vibration from nearby acoustic pressure measurements. The classical application bases the method on the stable solution of an integral surface representation of the acoustic pressure. In this work, we will be concerned with the extension of NAH to image a vibrating structure moving underwater. In an underwater medium, even at slow speeds, the action between the flow and structure produces many radiation sources that could severely limit the accuracy of the integral surface representation used for static NAH. Using the Lighthill acoustic analogy, we use an integral surface representation for the structure’s radiated sound and a quadrupole source that models the corresponding flow-induced sound. Finally, we will justify the validity of the proposed method for COMSOL numerically generated pressure data that results from the internal excitation of a cylindrical structure moving over an underwater medium. This work has been supported by the Office of Naval Research.

Numerical Analysis of Flow Field and Aerodynamic Noise in Intake Structure

The CFD numerical simulation was used to simulate the flow field and sound field of the intake system, combined with the acoustic finite element method. Based on the Lighthill acoustic analogy and the Möhring acoustic analogy method, the internal sound source distribution of the intake system, the intensity of the nozzle noise and the acoustic directivity were analyzed. Improvement measures were proposed from the perspective of noise transmission, and the influence of different wall thicknesses on the wall stimulated radiation noise, as well as the effects of attaching sound absorbing materials and spraying damping materials on wall radiation noise were studied.

Aerodynamic Noise Simulation of a High-Speed Maglev Train Operating inside a Fully Enclosed Sound Barrier

The aerodynamic noise from the new high-speed Maglev train (NHMT) is significant. One way to reduce it is to use a fully enclosed sound barrier (FESB). This paper studies the aerodynamic noise characteristics of the NHMT operating inside a FESB at 600 km/h, taking into account the compressibility of air and the quadrupole sound sources. The transient flow field around the NHMT is simulated by adopting an improved delayed detached-eddy simulation. According to Lighthill acoustic analogy theory, the aerodynamic noise inside and outside the FESB can be simulated using the acoustic finite element method. The sound insertion loss (IL) of the FESB is analyzed by comparing the noise outside the FESB with the noise generated by the NHMT operating on open tracks. The results indicate that the noise is distributed in the streamlined shoulder area of the head and tail car and the wake. The far-field noise belongs to broadband noise, and the noise outside the FESB is similar to an incoherent line source. The IL of the FESB is 25.2 dB(A) at 25 m from the track centerline and 3.5 m above the track surface. Therefore, the noise reduction effect of a FESB is much better than a traditional upright sound barrier.

Unsteady flow behaviors and noise generation mechanisms of tandem orifices in a circular duct

Turbulent flow through tandem orifices in a circular duct is numerically modeled through dynamic delayed detached-eddy simulations to clarify the unsteady flow behaviors and noise generation mechanism. The characteristics of four configurations with different separation distances L/D = 0 (single orifice), 1, 2, and 4 are compared at a Reynolds number of 10 000. The acoustic sources and their noise-propagation behaviors are analyzed using Lighthill's acoustic analogy. The coherent flow structures (wavepackets) are determined through spectral proper orthogonal decomposition to clarify the resulting flow noise mechanism. The dominant noise sources are acoustic dipoles that are alternately energetic on the orifice's leading and trailing faces subjected to intermittent interaction with the unsteady flow. The total sound pressure level (SPL) for a single orifice is alternately dominated by the shedding and flapping behaviors of the large-scale vortical structures in the low-frequency range and Kelvin–Helmholtz (K–H) type wavepackets at high frequencies. For the tandem orifice configuration with L/D = 1, the total SPL is dominated by the contribution of the trailing face, attributable to the interactions between the K–H-type double-wavepacket structures. Both the upstream and downstream wavepackets start to split and generate four-wavepacket structures in the high-frequency range. In the cases of L/D = 2 and 4, the total SPL is dominated by the dipole sources at the downstream leading face that subjected to the intermittent interaction with the upstream separated shear layers. The dominant flow structures are independent wavepackets through each orifice at low frequencies, while at high frequencies the four-wavepacket structures break into two independent double-wavepacket structures close to each orifice.

Analysis of noise attenuation for offshore platform silencer in duct with flow

Recently, the NORSOK standards, which regulate noise and vibration on offshore platforms, were strengthened by prohibiting the use of glass wool for environmental and safety reasons. In addition, the use of sound-absorbing materials in panels and silencers was prohibited. Therefore, developing a silencer that does not use sound-absorbing materials is necessary. In this study, a bandgap expressed by acoustic characteristics of a periodic structure was applied to develop a silencer without sound-absorbing materials. The bandgap is formed in a frequency range in which propagation of acoustic waves is suppressed by setting a specific distance between single units based on periodic structural theory. The designed silencer evaluates the noise attenuation effect of the periodic structure based on a fluid-structure coupling finite-element equation for a plane wave in a stationary medium. In addition, the silencer model was selected by analyzing the regenerated noise originating from the periodic structure in the flow duct. This flow noise is used to analyze the propagation of sound waves based on the FW-H equation. This is achieved by employing the Lighthill acoustic analogy to predict the flow-induced sound pressure. Furthermore, compressed air was injected at the end of the duct in the flow noise experiment to perform measurements. The results from the acoustic and flow noise experiments were compared to confirm the effect of the flow in the duct on noise generation. Moreover, the flow noise analysis based on the flow velocity elucidated the effect of the periodic structure on the flow. In addition, the analysis results were used to investigate its performance of the silencer with the periodic rod arrangement considering the flow noise in the duct and predict its acoustic characteristics in the bandgap frequency range.

Numerical study on the aerodynamic and acoustic scale effects for high-speed train body and pantograph

Based on computational fluid dynamics (CFD) and Lighthill acoustic analogy theory, the aerodynamics and acoustic scale effects of high-speed train bodies and pantographs are numerically studied, by changing the model scale. The model scales are 1/1, 1/2, 1/4 and 1/8 respectively. The aerodynamic force, flow field behavior, surface pressure and far-field noise of the train body and pantograph models, at different model scales are compared and analyzed. The numerical simulation is in good agreement with the experiment. The reliability of the numerical calculation method has been proven. The results show that as the scale of the model decreases, the root mean square (RMS) value and the degree of fluctuation of the aerodynamic drag coefficient of the pantograph increase. In addition, the aerodynamic drag coefficient of the train body increases, but the degree of fluctuation decreases. The similarity of the unsteady flow field of the pantograph at different model scales is strong, but the similarity of the train body is weak. The main frequency and power spectral density distribution of the pulsating pressure on the surface of the pantograph have obvious corresponding relationships with the model scale, but the train body has no obvious regularity. The longitudinal and lateral propagation characteristics of aerodynamic noise from the train body and pantograph are not affected by the model scale, but the sound pressure level changes. Compared with the train body, the far-field aerodynamic noise of the pantograph is less affected by the model scale. The energy distribution of noise is also affected by the scale of the model. As the model scale decreases, the energy concentration range of radiated noise will change from low frequency to medium high frequency.

Study on prediction in far-field aerodynamic noise of long-marshalling high-speed train.

It is still difficult to conduct numerical calculation of the aerodynamic noise of full-scale, long-marshalling, high-speed trains. Based on the Lighthill acoustic analogy theory, the aerodynamic sound source of the high-speed train is equivalent to countless micro-vibrating sound sources. An acoustic radiation model of the dipole sound source of high-speed trains is established, and a method to predict the aerodynamic noise in the far field of long-marshalling high-speed trains is proposed. By this method, combined with numerical simulation technology, the flow field, noise source, and far-field noise characteristics of high-speed trains with different marshalling numbers are studied. The improved delayed detached eddy simulation method is used for flow field calculation, to obtain aerodynamic noise source information regarding the surface of high-speed trains. The numerical calculation method is verified by wind tunnel testing. The results show that the flow field and noise source characteristics of high-speed trains with different marshalling numbers are similar. The greater the length of the train body, the longer the trailing distance of the train wake, and the stronger of a surface noise source the tail car becomes. The spatial distribution characteristics of aerodynamic noise in the far field of high-speed trains do not change significantly with the length of the train body, but the magnitude of the sound pressure level will increase with the increase in length of the train body. The middle car body parts of high-speed trains with different marshalling numbers have similar noise distributions and sound pressure levels. Based on the noise calculation results of the 3-marshalling high-speed train, the far-field noise of the 5-marshalling and 8-marshalling train models is predicted and found to be in good agreement with the far-field noise of the actual train model. The differences in average sound pressure level are 1.01 dBA and 1.74 dBA, respectively.

Lighthill's theory applied to tornadoes as an infrasonic source

It has been shown that tornadoes radiate infrasound, which can be detected at great distances. The physical mechanism underlying the radiation is not well understood. In this work, we use Lighthill's acoustic analogy to predict the pressure signals emitted from numerically simulated tornadoes. Several simulations will be considered including simulations produced using an LES model and provided by researchers from the Penn State Department of Meteorology and Atmospheric Science. The acoustical source produced from the LES tornado simulations shows interesting patterns in a narrow low-frequency band. By comparing different numerical models of tornadoes, we hope to determine what in a tornado is causing the infrasound radiation.