Acoustic Solver Research Articles

Theoretical model of scattering from flow ducts with semi-infinite axial liner splices

In this paper we present a theoretical model to study sound scattering from flow ducts with a semi-infinite lining surface covered by some equally spaced rigid splices, which is of practical importance in the development of silent aeroengines. The key contribution of our work is the analytical and rigorous description of axial liner splices by incorporating Fourier series expansion and the Wiener–Hopf method. In particular, we describe periodic variations of the semi-infinite lining surface by using Fourier series that accurately represent the layout of rigid splices in the circumferential direction. The associated matrix kernel involves a constant matrix and a diagonal matrix. The latter consists of a series of typical scalar kernels. A closed-form solution is then obtained by using standard routines of Wiener–Hopf factorisation for scalar kernels. A couple of appropriate approximations, such as numerical truncations of infinite Fourier series, have to be adopted in the implementation of this theoretical model, which is validated by comparing favorably with numerical solutions from a commercial acoustic solver. Finally, several numerical test cases are performed to demonstrate this theoretical model. It can be seen that the proposed theoretical model helps to illuminate the essential acoustic effect jointly imposed by axial and circumferential hard–soft interfaces.

Unsteady Fast Random Particle Mesh method for efficient prediction of tonal and broadband noises of a centrifugal fan unit

In this study, efficient numerical method is proposed for predicting tonal and broadband noises of a centrifugal fan unit. The proposed method is based on Hybrid Computational Aero-Acoustic (H-CAA) techniques combined with Unsteady Fast Random Particle Mesh (U-FRPM) method. The U-FRPM method is developed by extending the FRPM method proposed by Ewert et al. and is utilized to synthesize turbulence flow field from unsteady RANS solutions. The H-CAA technique combined with U-FRPM method is applied to predict broadband as well as tonal noises of a centrifugal fan unit in a household refrigerator. Firstly, unsteady flow field driven by a rotating fan is computed by solving the RANS equations with Computational Fluid Dynamic (CFD) techniques. Main source regions around the rotating fan are identified by examining the computed flow fields. Then, turbulence flow fields in the main source regions are synthesized by applying the U-FRPM method. The acoustic analogy is applied to model acoustic sources in the main source regions. Finally, the centrifugal fan noise is predicted by feeding the modeled acoustic sources into an acoustic solver based on the Boundary Element Method (BEM). The sound spectral levels predicted using the current numerical method show good agreements with the measured spectra at the Blade Pass Frequencies (BPFs) as well as in the high frequency range. On the more, the present method enables quantitative assessment of relative contributions of identified source regions to the sound field by comparing predicted sound pressure spectrum due to modeled sources.

Farassat's Formulations in Marine Propeller Hydroacoustics

The aim of this work is to show the potential of the Acoustic Analogy in the analysis of the underwater noise generated by a marine propeller. In particular, some formulations proposed by Feri Farassat are used to conduct an investigation on an interesting and rather unknown aspect of the problem: The identification of the most significant noise generating mechanisms taking place underwater and related to the propeller. Sample numerical results are presented, that were obtained by coupling an incompressible hydrodynamic code to an acoustic solver based on the Ffowcs Williams-Hawkings (FWH) equation, suitably designed to manage the huge set of data from the hydrodynamic simulation. A surprising outcome is that, contrary to popular belief and regardless of the low blade rotational speed, a reliable hydroacoustic analysis of a marine propeller seems to require the computation of the FWH equation's nonlinear quadrupole sources and cannot leave apart an accurate estimation of the three-dimensional turbulence and vorticity fields.

Acoustic simulation of a patient's obstructed airway

This research focuses on the numerical simulation of stridor; a high pitched, abnormal noise, resulting from turbulent airflow and vibrating tissue through a partially obstructed airway. Characteristics of stridor noise are used by medical doctors as indication for location and size of the obstruction. The relation between type of stridor and the various diseases associated with airway obstruction is unclear; therefore, simply listening to stridor is an unreliable diagnostic tool. The overall aim of the study is to better understand the relationship between characteristics of stridor noise and localization and size of the obstruction. Acoustic analysis of stridor may then in future simplify the diagnostic process, and reduce the need for more invasive procedures such as laryngoscopy under general anesthesia. In this paper, the feasibility of a coupled flow, acoustic and structural model is investigated to predict the noise generated by the obstruction as well as the propagation of the noise through the airways, taking into account a one-way coupled fluid, structure, and acoustic interaction components. The flow and acoustic solver are validated on a diaphragm and a simplified airway model. A realistic airway model of a patient suffering from a subglottic stenosis, derived from a real computed tomography scan, is further analyzed. Near the mouth, the broadband noise levels at higher frequencies increased with approximately 15–20 dB comparing the stridorous model with the healthy model, indicating stridorous sound.

Finite Volume Time-Domain Solver to Estimate Combustion Instabilities

Modeling tools used to estimate thermoacoustic combustion instabilities include classic network models, three-dimensional frequency-domain acoustic solvers, and computational fluid dynamics. Motivated by the large gap in both computational cost and predictive capability between the first two tools and computational fluid dynamics, the present work discusses and tests an approach that bridges this gap: a three-dimensional finite volume, acoustic solver in the time domain. Distinguishing features of the newly developed solver include the ability to capture both linear and nonlinear acoustics, the use of a solution algorithm based on an approximate Riemann solver, and the ability to handle complex geometries with unstructured meshes. This new solver produces results that agree well with analytical solutions for a two-dimensional isothermal cavity and a one-dimensional Rijke tube, as well as with the experimental data of a reheat buzz. For this last problem, a limit cycle is produced with a physical model that bounds heat-release fluctuations. In addition, results from the new acoustic solver for an annular combustor compare well with those of a three-dimensional frequency-domain acoustic solver, demonstrating the capabilities of the new solver to capture multidimensional acoustics.

Airfoil Trailing Edge Noise Generation and Its Surface Pressure Fluctuation

In the present work, Large Eddy Simulation (LES) of turbulent flows over a NACA 0015 airfoil is performed. The purpose of such numerical study is to relate the aerodynamic surface pressure with the noise generation. The results from LES are validated against detailed surface pressure measurements where the time history pressure data are recorded by the surface pressure microphones. After the flow-field is stabilized, the generated noise from the airfoil Trailing Edge (TE) is predicted using the acoustic analogy solver, where the results from LES are the input. It is found that there is a strong relation between TE noise and the aerodynamic pressure. The results of power spectrum density show that the fluctuation of aerodynamic pressure is responsible for noise generation.

Interaction between compressible fluid and sound in a flue instrument

In order to study the generation of (aerodynamic) sound in flue instruments, we numerically apply Howeʼs energy corollary for a two-dimensional model of a flue instrument. Howeʼs energy corollary enables us to estimate the energy transfer between the fluid flow and acoustic field. To implement it, separating the acoustic field from the fluid flow is needed. However the complete method to numerically achieve it has not been established yet. In this work, we develop an approximate method, which has been recently proposed in their experimental studies by Yoshikawa et al (2012 J. Sound Vib. 331 2558–77) and others, and we apply it to the simulation of the model instrument. We first calculate fluid flow and acoustic oscillation simultaneously by a compressible fluid solver. Next referring to the information on the acoustic oscillation obtained we set up a pressure source on an acoustic solver and reproduce almost the same acoustic oscillation with it. Combining those results, we are able to calculate Howeʼs energy corollary. The numerical result shows that the aerodynamic sound is generated from the oscillating jet rather than the vortices shed by the collision of it with the edge of the mouth opening, namely vortex shedding.

A high-order Cartesian-grid finite-volume method for aeroacoustics simulations

A moving-least-square based finite-volume method is developed to simulate acoustic wave propagation and scattering from complicated solid geometries. This hybrid method solves the linearized perturbed compressible equations as the governing equations of the acoustic field. The solid boundaries are embedded in a uniform Cartesian grid and represented using level set fields. Thus, the current approach avoids unstructured grid generation for the irregular geometries. The desired boundary conditions are imposed sharply on the immersed boundaries using a ghost fluid method. The scope of the implementation of the moving moving-least-square approach in the current solver is threefold: reconstruction of the field variables on cell faces for high-order flux construction, population of the ghost cells based on the desired boundary condition, and filtering the high wave number modes near the immersed boundaries. The computational stencils away from the boundaries are identical; hence, only one moving-least-square shape-function is computed and stored with its underlying grid pattern for all the interior cells. This feature significantly reduces the memory requirement of the acoustic solver compared to similar finite-volume method on irregular unstructured mesh. The acoustic solver is validated against several benchmark problems.

An Update and Comparative Study of Acoustic Modeling and Solver Technologies in View of Pass-By Noise Simulation

An Update and Comparative Study of Acoustic Modeling and Solver Technologies in View of Pass-By Noise Simulation

Prediction and Analysis of Combustion Instabilities in a Model Rocket Engine

This paper is a compilation of the research efforts of five different groups (Bertin Technologies with Centre National d’Etudes Spatiales, Institut de Mécanique des Fluides de Toulouse, ONERA, Purdue University, and Technische Universität München) on the prediction of combustion instabilities in a model rocket engine. This research was initiated by the Rocket Engine Stability Initiative group for the 2nd Rocket Engine Stability Initiative Workshop on Combustion Instability Modeling, which took place in October 2010 at Astrium, GmbH, in Ottobrunn, Germany. The target experiment consists of a single shear coaxial injector using methane and decomposed hydrogen peroxide as reactants. Both the inlet of the injector and the outlet of the chamber are choked, resulting in well-defined acoustic boundary conditions. The length of the oxidizer tube could be varied continuously, and its influence on the stability is studied. Many numerical strategies are tested, addressing different physical phenomena at play during unstable combustion. Acoustic solvers, both with and without mean-flow effects are used to draw stability maps. The weak spot of these solvers is that they require the flame response to acoustic perturbations as an input. Large-eddy simulations, requiring no such a priori knowledge, are performed with the intent to elucidate flame stabilization and flame response mechanisms.

Analysis and Control of Internal Flow Induced Noise of Rotor Oil Pump

The fluid noise of rotor oil pump is studied in this paper, and the turbulent and flow sound field model of rotor oil pump are built. Based on CFD software, three dimensional unsteady internal flow field numerical simulation of some type rotor oil pump is carried out. And, the velocity and pressure simulation results at different speeds are obtained. Based on acoustic finite element and infinite element method, CFD software and acoustic solver software are used to simulate the flow induced noise of rotor oil pump. And, the sound pressure level values of monitor points in the pump are obtained. This paper puts forward some methods to reduce noise, and the experiment shows these methods can reduce the flow noise effectively.

Numerical Investigations of Tonal Noise of Counter-Rotating Open Rotors

In the work are presented the results of a three dimensional computational analysis of tonal noise of two counter-rotating open rotors. The analysis was performed using 3DAS (3 Dimensional Acoustics Solver) CIAM in-house solver, designed for three dimensional calculation of turbomachinery tone noise generation, propagation and radiation in the near and the far acoustic fields. The results of the computations were compared with the results of experiments performed in the wind tunnel of Central Aerohydrodynamic Institute (TsAGI), Russia, within the project DREAM (Validation of radical engine architecture systems) of European Seventh Framework Programme (FP7). The comparison showed satisfactory correspondence between directivity diagrams for the tone noise on combinational harmonics.

Turbofan duct geometry optimization for low noise using remote continuous adjoint method

In this paper, a remote continuous adjoint-based acoustic propagation (RABAP) method is proposed for low noise turbofan duct design. The goal is to develop a set of adjoint equations and their corresponding boundary conditions in order to quantify the influence of geometry modifications on the amplitude of sound pressure at a near-field location. The governing equations for the 2.5D acoustic perturbation equation solver (APE) formulation for duct acoustic propagation is first introduced. This is followed by the formulation and discretization of the remote continuous adjoint equations based on 2.5D APE. The special treatment of the adjoint boundary condition to obtain sensitivities derivatives is also discussed. The theory is applied to acoustic design of an axisymmetric fan bypass duct for two different tone noise radiations. The 2.5D APE is further validated using comparisons to an experiment data of the TURNEX nozzle geometry. The implementation of the remote continuous adjoint method is validated by comparing the sensitivity derivative with that obtained using finite difference method. The result obtained confirms the effectiveness and efficiency of the proposed RABAP framework.

Numerical Research on Internal Flow Induced Noise of Rotor Oil Pump

The fluid noise of rotor oil pump is studied in this paper, and the turbulent and flow sound field model of rotor oil pump are built. Based on CFD software, three dimensional unsteady internal flow field numerical simulation of some type rotor oil pump is carried out. And, the velocity and pressure simulation results at different speeds are obtained. Based on acoustic finite element and infinite element method, CFD software and acoustic solver software are used to simulate the flow induced noise of rotor oil pump. And, the sound pressure level values of monitor points in the pump are obtained. This paper puts forward some methods to reduce noise which has been verified by the experiment result effectively.

Numerical analysis of flow induced noise propagation in supercavitating vehicles at subsonic speeds.

Flow supercavitation begins when fluid is accelerated over a sharp edge, usually at the nose of an underwater vehicle, where phase change occurs and causes low density gaseous cavity to gradually envelop the whole object (supercavity) and thereby enabling higher speeds of underwater vehicles. The process of supercavity inception/development by means of "natural cavitation" and its sustainment through ventilated cavitation result in turbulence and fluctuations at the water-vapor interface that manifest themselves as major sources of hydrodynamic noise. Therefore in the present context, three main sources are investigated, namely, (1) flow generated noise due to turbulent pressure fluctuations around the supercavity, (2) small scale pressure fluctuations at the vapor-water interface, and (3) pressure fluctuations due to direct impingement of ventilated gas-jets on the supercavity wall. An understanding of their relative contributions toward self-noise is very crucial for the efficient operation of high frequency acoustic sensors that facilitate the vehicle's guidance system. Qualitative comparisons of acoustic pressure distribution resulting from aforementioned sound sources are presented by employing a recently developed boundary integral method. By using flow data from a specially developed unsteady computational fluid dynamics solver for simulating supercavitating flows, the boundary-element method based acoustic solver was developed for computing flow generated sound.

Vibro-Acoustic Coupling Analysis of High Velocity Aircraft Based on Hybrid Acoustic Simulation Method

High-speed aircraft is in service condition of high temperature, high pressure and strong noise during flight. Strong noise will damage structures and equipments of an aircraft, and the existence of high temperature and high pressure will greatly increase the damage degree. Therefore, research on aircraft structure characteristic and surface acoustic characteristic is very significant. The hybrid method of CFD and acoustic solver combination method can accurately obtain aerodynamic noise load on the aircraft structure surface in a high velocity environment. The hybrid method can make acoustic and vibration analysis for aircraft structure, and get influence of noise load on it. The simulated results show that the presence of high temperature and high pressure can make the natural frequency of typical components and whole machine model reduction, and the acoustic load on the aircraft structure model surface is larger at low frequency, smaller at high frequency.

Accuracy improvement of axisymmetric bubble dynamics using low Mach number scaling

Bubble collapse and associated shock wave emission are characterized by the compressibility of both gas and liquid. The bubble motion may, however, be in the low Mach number flow regime when the bubble is near the maximum size at the early stage and at its full rebound. Although it is quite well known that a compressible flow solver encounters difficulties in the low Mach number regime, the influence of the low Mach number on the simulation of bubble collapse and rebound is not clear. In the present work, an axisymmetric compressible solver based on the acoustic Riemann solver is used to simulate the dynamics of bubble motion. The artificial viscosity terms in the Riemann solver are rescaled for the low Mach regime by following the concept of numerical sound speed, which was originally developed in the AUSM family scheme. The numerical results are compared with the solutions of the Rayleigh model for bubble collapse and the Keller–Miksis model for bubble collapse and rebound. It is found that the low Mach number scaling improves the accuracy of the size of a collapsing bubble considerably.

Computational Aeroacoustic Simulation of Landing Gear

RANS / NLAS numerical simulation method is adopted in this paper to carry out study on the aerodynamic noise analysis of basic landing gear configuration. Reynolds average N-S equation is solved with nonlinear turbulence model to establish the landing gear initial flow field, based on which, the NLAS (nonlinear acoustic solver) processed the turbulence fluctuation reconstruction to obtain the near-field acoustic characteristics of landing gear. Combined with the flow characteristics and the associated noise spectrum analysis, aerodynamic noise characteristics of landing gear are achieved. The work in this paper can provide useful research foundation on the following noise reduction design of landing gear.

Bayesian Image Reconstruction in Quantitative Photoacoustic Tomography

Quantitative photoacoustic tomography is an emerging imaging technique aimed at estimating chromophore concentrations inside tissues from photoacoustic images, which are formed by combining optical information and ultrasonic propagation. This is a hybrid imaging problem in which the solution of one inverse problem acts as the data for another ill-posed inverse problem. In the optical reconstruction of quantitative photoacoustic tomography, the data is obtained as a solution of an acoustic inverse initial value problem. Thus, both the data and the noise are affected by the method applied to solve the acoustic inverse problem. In this paper, the noise of optical data is modelled as Gaussian distributed with mean and covariance approximated by solving several acoustic inverse initial value problems using acoustic noise samples as data. Furthermore, Bayesian approximation error modelling is applied to compensate for the modelling errors in the optical data caused by the acoustic solver. The results show that modelling of the noise statistics and the approximation errors can improve the optical reconstructions.

An implicit cell-centered Lagrange-Remap scheme for all speed flows

A second-order time-accurate implicit scheme is constructed for the Lagrange-Remap (LR) strategy. The numerical flux is given by the simple acoustic Riemann solver. The Riemann solver is modified by introducing a scaling coefficient so that the scheme can deal with very subsonic (low Mach) flows as well as supersonic flows. The Lagrange step solves implicitly the hyperbolic equations of pressure and velocities under the isentropic assumption by the trapezoidal time integration method. The new LR scheme maintains exactly the conservation of mass, momentum and energy, and it is general for materials with any equation of state. Numerical tests show that the LR scheme using the simple but general Riemann solver can resolve shock waves sharply for supersonic flows, and resolve well the acoustic waves in low Mach number flows as low as M=0.001.