Acoustic Radiation Field Research Articles

Fluid–acoustic interactions around an expanding pipe with orifice plates

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.

An efficient hybrid collocation scheme for vibro-acoustic analysis of the underwater functionally graded structures in the shallow ocean

In this paper, a novel hybrid collocation scheme based on the generalized finite difference method (GFDM) and the Burton–Miller singular boundary method (BMSBM) is developed to study the vibration and acoustic radiation characteristics of an underwater functionally graded (FG) structure immersed in the shallow ocean. By utilizing the individual strengths of both methods, the GFDM is adopted for the vibration analysis of the FG structures, while the BMSBM is applied for the acoustic wave radiation analysis. Numerical examples verify the accuracy and efficiency of the proposed hybrid collocation scheme for the natural frequency and acoustic radiation behavior of the underwater structures consisting of the functionally graded materials (FGMs). The influences of the material parameters, structural geometries and shallow ocean environment on the natural frequencies of the structural vibration and the underwater acoustic wave radiation field are investigated in detail. Numerical study shows that the variation trend of the natural frequencies of the structural vibration is consistent with that of the resonance peak frequencies of the underwater acoustic radiation field. The present results may provide valuable references for the design and application of underwater marine structures consisting of the FGMs.

Reynolds-Number Effects on the Acoustic Disturbance Environment in a Mach 6 Nozzle

To understand the impact of the Reynolds number on the disturbance field inside a high-speed wind tunnel, we use direct numerical simulations to model turbulent boundary layers along the walls of a quasi-two-dimensional nozzle configuration. These simulations are performed at four different unit Reynolds numbers, ranging from to . The predominantly hydrodynamic fluctuations inside the boundary layer are nearly unaffected by the freestream forcing of the impinging acoustic radiation from the opposite wall. Within the nozzle-core region, the disturbance environment is found to be spatially uniform and solely acoustic in character. Comparisons with tunnel noise measurements are made using the numerical data. For the first time, comparisons of fluctuations in the same physical quantity (the streamwise mass flux) have been published, as opposed to earlier comparisons involving static-pressure fluctuations based on the simulations and pitot-pressure fluctuations recorded in the wind tunnel. The NASA 20 in. Mach 6 wind tunnel observations corroborate the predicted trend of decreased root-mean-square variations in pressure and mass flux as the Reynolds number rises. Additional details of the acoustic radiation field are quantified and should be useful toward a digital synthesis of the tunnel disturbance environment that would enable realistic simulations of the natural transition process.

Numerical Analysis of Aeroacoustic Characteristics around a Cylinder under Constant Amplitude Oscillation

The present study numerically investigated a cylinder under oscillating motions at a low Reynolds number. The effects of two oscillation frequencies and amplitudes on the lift drag coefficient, near-field surface pressure fluctuation, and far-field noise were studied. The models were examined at a Mach number of 0.05, corresponding to a Reynolds number of 1.0 × 105. In this paper, the incompressible Navier–Stokes equations (INSE) and linearized perturbed compressible equations (LPCE) were coupled to form a hybrid noise prediction method, which was used to solve the flow field and acoustic radiation field. Based on the simulation results of the acoustic radiation field, the frequency characteristics of the acoustic waves were analyzed by the dynamic modal decomposition (DMD) method. It was observed that when the oscillation amplitude was the same, the variation amplitude and mean value of the lift-drag coefficient increased with the increase in the oscillation frequency. Under the same small oscillation frequency, the oscillation amplitude had little effect on the lift-drag coefficient. However, for the same large oscillation frequency, the variation amplitude of the lift-drag coefficient increased as the oscillation amplitude increased. In addition, both the amplitude and frequency had a significant effect on the directionality of the noise and the intensity of the sound waves. The main energy of the sound field was mainly concentrated on the first and second narrowband frequencies by using the DMD method to analyze the sound pressure level spectrum.

Far field acoustic radiation and vibration analysis of combined shells submerged at finite depth from free surface

A completely submerged underwater vehicle is modeled as a conical-cylindrical-spherical combined shell, the vibro-acoustic response near free surface of which is rarely analyzed in existing studies. In this paper, a Ritz-Legendre spectral method is proposed to analyze the vibro-acoustic response of combined shells under different submerged depths from the free surface. A theoretical structural model is established based on the Love shell theory, virtual spring technology and coordination relationship between displacements and rotation angle of adjacent subshells. The half-space external acoustic field model with the free surface is formulated by the Legendre spectral method, image method and Kirchhoff–Helmholtz boundary integral equation. The unified displacement admissible function of each subshell is expanded as Legendre orthogonal polynomial along generatrix direction and Fourier series along circumferential direction. The free surface treated as an ideal infinite acoustic soft boundary is substituted into acoustic wave equation and the half-space Green's function is obtained. The key technology to construct final coupled governing equation is how to expand the half-space Green's function with two-dimensional Fourier transform. Computed results are compared with those of the references and finite element method. The convergence, applicability, and correctness of the present method are verified. Influence of the free surface on the vibration and far-field acoustic radiation of combined shells under different submerged depths is analyzed, providing engineering significance for predicting external acoustic radiation of complicated combined shells with free surface.

Structure-Acoustic Coupling Analysis of Vibration and Underwater Acoustic Radiation of a Ring-Stiffened Conical Shell

The underwater acoustic radiation of the submarine power cabin has recently become a hot topic in the industry and also in the academia. In this article, the vibration and underwater acoustic radiation of a ring-stiffened conical shell with bases are investigated numerically by means of the combination of the finite element method and boundary element method. The acoustic radiation field is obtained by the traditional acoustic field model and ISO acoustic field model, respectively. A series of numerical examples are given, and the results are compared. Besides, the sound pressure at different positions with frequency is further studied. It is shown that the sound radiated by the structure mainly propagates to the side directions of the shell and propagates relatively less to the front side and the rear side.

Analysis of characteristic of acoustic radiation from axisymmetric pressure-resistant egg-shaped shells in the ocean environment

Due to the need for an ocean voyage, the shape and material of pressure-resistant structures of submersibles need to meet higher requirements. At present, the static mechanical properties of pressure-resistant systems have been extensively studied. However, it is still necessary to further explore its acoustic radiation noise to provide a reference for the overall acoustic design. A combination of the finite element method and wave superposition method (FEM-WSM) is used to calculate the acoustic radiation field of axisymmetric pressure-resistant structures in the ocean environment. Taking spherical, egg-shaped, and multi-body egg-shaped shells as examples, explores the distribution of acoustic radiation field of axisymmetric pressure-resistant shells under different shapes, materials, and submerging depths.

On the effect of the correction of modelled airfoil tonal noise for a spanwise extension

Airfoil self-noise at high Reynolds numbers is caused by turbulent boundary layers. In modelling such a case with Large Eddy Simulation (LES) or Direct Numerical Simulation (DNS), the acoustic source and thus surface pressure fluctuations are incoherent along the modelled spanwise length, assuming the simulated span is wide enough to capture turbulent flow structures. In literature, the spanwise coherence of surface pressure is normally inspected at 0.95% of the chord. A sound pressure level (SPL) correction for an increase in spanwise length can be applied, for example, for comparison of the modelled SPL spectra with experimental measurements. For a larger spanwise surface pressure coherence length, the SPL correction increases. In contrast, airfoils at low Reynolds numbers emit self-noise with a tonal signature. An acoustic feedback mechanism propagates instabilities upstream and is responsible for the tonal amplification. Consequently, the source of noise is not highly localized and may not be fully incoherent at all frequencies. Therefore, the chordwise position at which the surface pressure coherence length is to be calculated for a SPL correction for spanwise length, is not straightforward. In this paper, the sources of tonal acoustic noise and the effect of the position of spanwise coherence length calculation for SPL correction is investigated. A NACA 0012 airfoil is modelled using LES, at a chord-based Reynolds number of Rec = 1.1×105 and an angle of attack of α = 3°. The far field acoustic pressure is calculated with the Ffowcs-Williams Hawkings acoustic analogy Formulation 1C. The airfoil source regions are separated by their boundary layer flow characteristics and their relative contributions to the far field SPL are analysed to find a chordwise point on the airfoil surface which has a spanwise coherence length that is representative of the nature of the far field acoustic radiation. The spanwise coherence length at this position can be used to calculate the SPL correction for an increase in spanwise length. Two chordwise points are identified for which the resulting corrected SPL spectra agree well overall with experimental data.

Hybrid FEM–SBM solver for structural vibration induced underwater acoustic radiation in shallow marine environment

This study focuses on the efficient computational model for predicting the structural vibration induced underwater acoustic radiation in shallow marine environment. In the proposed model, the finite element method (FEM) is used to calculate the vibration response of the shell structures and the singular boundary method (SBM) with near-field and far-field Green’s functions is used to simulate the underwater acoustic radiation excited by shell structural vibration in shallow water marine environment, namely, the hybrid FEM–SBM solver. For axisymmetric shell structures, the axisymmetric numerical formulation of the FEM–SBM solver is derived to save the computational cost. For general shell structures, the fast direct solver based on the recursive skeletonization factorization is introduced into the FEM–SBM solver to accelerate the computation of the underwater acoustic field. Numerical results demonstrate that the proposed FEM–SBM solver provides more efficient and reliable solutions in comparison with the coupled FEM/FEM solver (COMSOL software). The effect of the sound velocity profiles on the underwater acoustic radiation field in shallow water marine environment is numerically investigated.

A mixed analytical-numerical method for the acoustic radiation of a spherical double shell in the ocean-acoustic environment

This study presents a mixed analytical-numerical method based on the analytical method and the wave superposition method to predict the acoustic radiation of a spherical double shell immersed in the ocean-acoustic environment. Using the Green's function, the fluid structure coupled vibration as well as the near and far field acoustic radiations can be simultaneously analyzed. Different forms of Green's functions are utilized to deal with the near and far field distribution of sound pressure. To reduce the complexity of the computation, the image-source method is used in the near field while the normal mode method is adopted in the far field. The acoustic radiation fields of the spherical double shell in finite depth ocean-acoustic environment with positive and negative gradient sound speed profiles are calculated using the proposed method. Numerical results compared with results using the finite element method obtained by COMSOL verify that the proposed method improves the computational efficiency without losing computational accuracy. Further, taking the spherical double shell as an example, the effects of seabed, sea surface, and sound speed profile of seawater on the fluid structure coupled vibration, acoustic radiation, and sound propagation of underwater structures are analyzed. Some figures are also included to show these effects clearly. The main idea proposed in this paper can be applied to the integrated calculation of fluid structure coupled vibration and the near and far field acoustic radiations of arbitrary three-dimensional elastic floating bodies in the ocean-acoustic environment.

Vibroacoustic radiation and propagation properties of slender cylindrical shell in uniform shallow sea

Shallow sea, especially the reflection and absorption of seabed, have significant influence on the vibroacoustic radiation and propagation properties of underwater structures. An FEM/BEM algorithm based on modal summation is adopted to calculate the modal source strengths and modal coordinate responses of a slender cylindrical shell, in which the Green function of sound field is obtained via virtual source chain model. Using the modal source strengths and modal coordinate responses obtained, the near/far field radiated sound pressure level (SPL) of the slender cylindrical shell in uniform shallow sea is calculated adopting the 3-parameter seabed model. By comparing the radiated power of all modal shapes, the modes with greater contribution can be acquired. Results show that seabed parameters have influence on the far field acoustic radiation of the slender cylindrical shell, mainly on the modal shapes with large grazing angle waves due to larger bottom loss; while the small grazing angle waves are less susceptible to the seabed and propagate much farther.

Study of integrated calculation method of fluid-structure coupling vibrations, acoustic radiation, and propagation for axisymmetric structures in ocean acoustic environment

Based on a combination of the finite element method and wave superposition method (FEM-WSM), an integrated calculation method of fluid-structure coupling vibrations, acoustic radiation, and propagation for axisymmetric structures in ocean acoustic environments is proposed. The FEM was applied to study the dry mode of axisymmetric structures in a vacuum, and WSM was used to consider the fluid-structure interaction effects and calculate the acoustic radiation field in ocean acoustic environments. The method proposed combines the advantages of FEM and WSM. This method is suited for complex or large axisymmetric structures. Furthermore, it is more efficient than FEM, and the calculation range of the water is not limited. To simplify the calculation, the Green's functions corresponding to the propagation of specific ocean environments were calculated differently in the near and far fields. Using spherical and elliptical axisymmetric shells as examples, the results from the method proposed and COMSOL were compared to verify the correctness of the proposed model. In the calculation of the acoustic radiation field, the numerical errors from matrices, including partial derivatives of the Green's function with huge condition numbers, required the use Tikhonov regularization, and the efficiency was not influenced.

Study on the integrated calculation method of fluid–structure interaction vibration, acoustic radiation, and propagation from an elastic spherical shell in ocean acoustic environments

A modeling method based on the wave superposition method is studied in this paper. Different underwater acoustic propagation models are used in the near and far field. Taking the acoustic radiation field calculation of an elastic spherical shell as an example, the near and far field can be analyzed as a unified system, using this method. Many studies use FEM/BEM (finite element method/boundary element method) to calculate acoustic radiation fields of elastic structures, but only a few of these methods can be applied when considering finite ocean depth, seafloor reflection loss, and sound velocity profile. In this study, the Green function is calculated differently in the near and far field to significantly simplify calculations. The virtual source method is used to calculate the acoustic radiation field in the near field and source strength, while the normal mode method is used in the far field. The results are compared with those obtained using the COMSOL finite element software, which showed that this method was both computationally efficient and accurate. Based on numerical examples, the influence of sea surface, seafloor, and sound velocity on the acoustic radiation field of elastic structures in an ocean acoustic environment is quantitatively analyzed.

Roller-Integrated Acoustic Wave Detection Technique for Rockfill Materials

This paper proposes a roller-integrated acoustic wave detection technique for rockfill materials. This technique can be divided into two parts: theoretical analysis and technical implementation. Based on Lamb’s problem and an infinite baffle piston radiation acoustic field model, a relationship model between the sound compaction value (SCV) and the dry density of the natural gravel materials (NGM) was established, namely, A-model. During the modeling process, an innovative differential pulse excitation method (DPEM) was used to find the numerical solution of the vertical displacement of the soil surface under harmonic loads. In this research, a continuous compaction control acoustic wave detection system (CAWDS) was developed and utilized along with real-time kinematic global positioning systems (RTK-GPS). The SCV was adopted as a characterization index for the compaction quality of rockfill materials. A case study on a reservoir project in Luoyang, China indicated that the SCV is highly linearly correlated with the number of compaction times, dry density, and compactness of the NGM. This new technique demonstrated several advantages, such as higher accuracy, discreetness, convenience, and suitability for detecting the compactness of the NGM. This technique is an effective tool for compaction quality control of rockfill materials and has a great potential for further applications.

Acoustic radiation from shear deformable ring-stiffened laminated composite cylindrical shell submerged in flowing fluid

Acoustic radiation from a point driven, infinite, periodically ring-stiffened, laminated composite cylindrical shell submerged in flowing fluid is investigated theoretically. Both the effects of in-plane and out-of-plane vibrations of the ring-stiffeners and the effects of fluid convection on far field acoustic radiation behaviors are concerned. The equations of motion of the laminated composite cylindrical shell is presented on the basis of the first order shear deformation theory. Fourier transform and Poisson summation formula are used to transform the equations into a set of infinite algebraic equations expressed in the wavenumber domain. After truncation, the response of the laminated composite cylindrical shell is solved, and the stationary phase approximate is employed to find the expression for the far field sound pressure. Convergence analysis of the numerical solutions is conducted. The theoretical model and numerical method proposed in this paper are validated by comparison with those presented in available literature. Finally, numerical results are presented to demonstrate the effects of various parameters such as the size and spacing of the ring-stiffener, the thickness and the radius of the cylindrical shell, the lamination angle and the lamination scheme of the composite materials as well as the Mach number on the far field sound pressure.

Numerical investigation on flow-induced structural vibration and noise in centrifugal pump

A full scale structural vibration and noise induced by flow was simulated by a hybrid numerical method. An interior flow field was solved by large eddy simulation firstly. The sliding mesh technique was applied to take into account the impeller-volute interaction. A sensitivity analysis on effects of near-wall grid size and sampling time on amplitude of pressure pulsations was performed to impose appropriate vibration exciting source. Computed modal of pump components was validated by experimental results, before the volute vibration and sound field were simulated using a coupled vibro-acoustic model. The numerical results indicated that the amplitude of pressure fluctuation, especially on those points located at near the volute tongue, strongly depended on near-wall grid size. The dominated frequency of the vibration velocity of volute was also blade-passing frequency (BPF), which was in according with frequency spectral characteristics of unsteady pressure fluctuation. Directivity distribution of radiation acoustic field excited by volute vibration was typical dipoles. This study shows that it is feasible to use the hybrid numerical method to evaluate the flow-induced vibration and noise generated in centrifugal pump.

Analysis of Transient Acoustic Radiation Field from Pulse-driven Finite Aperture Piezoelectric Transducer

A method is presented for the precise analysis of the discontinuous transient radiation field from a circular piezoelectric transducer in the liquid medium. The transducer is excited by the step pulse voltage signal, on the contrary to the conventional burst sine pulse signals. Specifically, time domain Rayleigh integral formula is extended to include the medium attenuation to meet the actual phenomena in the far field region. As a results, some peculiar characteristics intrinsic to the transient pulse radiation fields are elucidated. Simulation results are also compared with experiments to verify the validity of the proposed methods.

Vibration Analysis of a Shell Structure by Finite Element Method

Plate-shell structures, especially cylindrical shells and spherical shells, are widely used in engineering fields, such as aircraft and tanks, missiles, submarines, ships, hydraulic pumps, infusion pipelines and gas pipelines, and so on. These structures are usually in a fluid medium, which are related to the structure fluid-solid coupling and acoustic radiation field. As many experiments show that enclosed air in a thin walled structure, just like the violin, affects some modes of vibration significantly, air coupling between vibrating sides of the structure cannot be neglected. In order to explore the sound pressure distribution of vibrational frequencies, this paper, considering the material anisotropy, analyzes a typical complex shell structure of the violin by finite element method, including acoustic-structure coupling analysis and post-processing, especially sound pressure vibration frequency extraction. Finally, we get the conclusion that the distribution of sound pressure vibration frequency is similar to the normal distribution.

Study on the radial vibration and acoustic field of an isotropic circular ring radiator

Based on the exact analytical theory, the radial vibration of an isotropic circular ring is studied and its electro-mechanical equivalent circuit is obtained. By means of the equivalent circuit model, the resonance frequency equation is derived; the relationship between the radial resonance frequency, the radial displacement amplitude magnification and the geometrical dimensions, the material property is analyzed. For comparison, numerical method is used to simulate the radial vibration of isotropic circular rings. The resonance frequency and the radial vibrational displacement distribution are obtained, and the radial radiation acoustic field of the circular ring in radial vibration is simulated. It is illustrated that the radial resonance frequencies from the analytical method and the numerical method are in good agreement when the height is much less than the radius. When the height becomes large relative to the radius, the frequency deviation from the two methods becomes large. The reason is that the exact analytical theory is limited to thin circular ring whose height must be much less than its radius.

A new type of high power composite ultrasonic transducer

A new type of high power composite ultrasonic transducer was proposed and studied. The composite transducer consists of a sandwich longitudinal piezoelectric transducer, an isotropic metal hollow cylinder with large radial dimension, and the front and back metal radiation mass. By means of its special structure and Poisson’s effect, the composite transducer can produce vibrations both in its longitudinal and radial directions, and therefore, it can radiate sound waves in three-dimensional space. The electro-mechanical equivalent circuit of the composite transducer was derived and the resonance frequency equation was obtained analytically. Numerical methods were used to simulate the vibration of the composite transducer, and the vibrational displacement distribution, the resonance frequency and the radiation sound field are given. Some composite transducers are designed and manufactured; their resonance frequencies and the radiation acoustic field are measured and compared with the analytical and numerical results. It can be seen that the measured frequencies and acoustic field contour are in good agreement with the analytical and numerical results. It is expected that this kind of composite ultrasonic transducer can be used in more and more power ultrasonic applications, such as ultrasonic cleaning, ultrasonic extraction, ultrasonic sonochemistry and other ultrasonic liquid processing, where high ultrasonic power and large ultrasonic processing space are needed increasingly.