Computational Aeroacoustics Method Research Articles

A mathematical‐boundary‐recognition domain‐decomposition Lattice Boltzmann method combined with large eddy simulation applied to airfoil aeroacoustics simulation

AbstractBeing a direct computational aeroacoustics method, Lattice Boltzmann method (LBM) has great potential and broad application perspective in the field of numerical simulation of aerodynamic noise due to its low dispersion and low dissipation. A series of numerical algorithms and the related improvements based on the standard LBM method are proposed and developed in this paper to adapt to the airfoil noise calculation with complex grid at middle‐high Reynolds number. First, a new mathematical‐boundary‐recognition algorithm based on Green's formula is proposed to deal with complex curved geometric models, which is validated by three‐element airfoil 30P30N benchmark. Then, in order to reduce grid redundancy and improve computing efficiency, the grid refinement technique of domain decomposition model (DDM) is adopted and also improved, which is verified by calculating the flow and sound fields around 2D and 3D cylinders at Reynolds number equal to 90,000. Finally, three different LES turbulence models are combined with the standard MRT‐LBM method, where different finite difference schemes are used to solve Reynolds stress tensor which is different from the traditional one. Through the direct acoustic numerical simulation of NACA0012 airfoil at Reynolds number equal to 200,000, the effects of Smagorinsky models and Wall‐adapting local eddy‐viscosity (WALE) model on aerodynamic noise prediction are compared and analyzed. Overall, the proposed methodology is shown to be appropriate for predicting the aerodynamic noise at low Mach number and can successfully simulate the generation and propagation of far field acoustics.

Numerical Investigation of the Flow Structure and Acoustical Noise in a Suppressor

The noise created when a firearm is fired has a lot of adverse effects on humans and the environment, so analyzing and attenuating this noise is essential. This study aimed to examine propellant flow and the sound generated by this flow using a hybrid computational fluid dynamics and computational aeroacoustics method. The compatibility of this numerical study was also validated by comparing it with the experimental result. The impact of critical parameters, such as the shape and number of baffles of the suppressor, was studied. Finally, the overpressure and acoustics results of different suppressors were compared with each other and with the unsuppressed condition. According to the result, the suppressor with curved baffles shows a better performance. When the exit pressure at the tip of suppressor compared to the initial inlet pressure to the suppressor the percentage drop in overpressure was 76.01%, 78.79% and 81.3% for the suppressor with one, three and five curved baffles, respectively. For condition without suppressor, 169.49[Formula: see text]dB of SPL was recorded. When using a suppressor without a baffle, this value was reduced to 162.13[Formula: see text]dB. For the suppressor with one, three and five curved baffles, the SPL value was 160.234[Formula: see text]dB, 159.43[Formula: see text]dB and 158.11[Formula: see text]dB, respectively. Generally, this study shows that the suppressor’s efficiency is directly proportional to its shape and number of baffles.

Numerical and experimental investigation of aerodynamic noise from a cooling fan in a turbulent flow

The investigation into aerodynamic noise arising from rotating surfaces bears significant practical implications. Among these, engine cooling fans have persistently been identified as primary contributors to vehicle noise. This study aims to provide a deeper comprehension of the complex unsteady flow dynamics surrounding an automotive cooling fan and its consequential impact on the generation of aerodynamic noise. Prediction of sound generated from fluid flow, despite its difficulty, can be simulated using modern numerical techniques and computational fluid dynamics (CFD). This research undertakes both numerical and experimental investigations. The experiment involves evaluating fan pressure against mass flow rate at 1500 rpm, while aerodynamic noise assessment transpires at two rotational speeds of 1500 rpm and 2500 rpm. Empirical sound pressure level data of the fan is meticulously captured using the requisite measurement apparatus. In addition, numerical investigations of the aerodynamic noise of an automotive cooling fan based on computational fluid dynamics and computational aeroacoustic (CAA) method have been performed. Initially, a steady-state aerodynamic simulation is carried out, followed by the simulation of the flow field through the application of the improved delayed detached eddy simulation (IDDES) formulation. The pressure rise of the fan versus the mass flow rate is obtained. The satisfactory concurrence observed between the simulation outcomes derived from the numerical method and the experimental dataset stands as notable validation. For the aeroacoustic computation, the Ffowcs Williams-Hawkings model is invoked to deduce sound pressure levels at specific flow points. The broadband noise sources model is also used to obtain the sound power level on the blade surface. Conclusively, it emerges that maximal noise manifestation occurs at a 90° degree angle, corresponding to the hub’s frontal orientation, with discernible dominance of tonal noise within lower frequency ranges (sub-1000 Hz).

Investigation of Effect of Mean Flow on Active Noise Control System in Duct

The increasing demand for a quieter living environment has driven the development of noise reduction and control technologies. Previous research has demonstrated the significant impact of mean flow on the performance of noise reduction devices. While efforts have been made to enhance passive noise control systems in the presence of mean flow, research on the influence of mean flow on active noise control (ANC) systems is limited, particularly in terms of computational modeling. This study addresses this gap by investigating the effect of mean flow on ANC system performance using a validated virtual controller within a computational framework, complemented by experimental investigations. The acoustic propagation inside the computational domain is computed using the Computational Aeroacoustics method, and the ANC system is modeled using FxLMS algorithm with the virtual-controller method as proposed in our earlier work. Through experiments, it reveals that the presence of mean flow in duct reduces the performance of the ANC system. The numerical investigation exhibited a consistent trend in line with the experimental findings, showing a strong agreement between the 3D numerical simulations and experimental data. These numerical results offer compelling evidence that the computational framework used in this study accurately models the impact of mean flow on ANC systems.

Research on the directional characteristics of wind noise emitted by bionic rods

In this paper, the directional characteristics of wind noise emitted by different bionic rods were studied based on a hybrid computational aeroacoustics method. The noise reduction mechanism of surface grooves, pits, and bumps was analyzed, respectively. The basic principle of noise reduction is to reduce the influence of the vortex shedding on the rod by changing the shape of the rod or passive control technology to reduce the dipole sound source. The unsmooth transverse surface will increase the loss of flow on the leading edge of the rod and reduce the vertical effect of vortex shedding on the rod. The convex leading edge of the rod can help to transfer the vertical noise from low frequency to high frequency and reduce the vertical effect of wake vortex shedding to reduce the peak sound pressure level. The cost of those was the increase in the aerodynamic drag and the increase in noise in the flow direction (the increase in the amplitude of drag fluctuation). In particular, the longitudinal v-groove structure on the surface of the elliptical rod can reduce the circumferential aerodynamic noise while keeping the aerodynamic drag coefficient unchanged.

Investigating the aerodynamic drag and noise characteristics of a standard squareback vehicle with inclined side-view mirror configurations using a hybrid computational aeroacoustics (CAA) approach

This study investigates the aerodynamic noise generated and radiated from a standard squareback body with various inclined side-view mirrors using a hybrid computational aeroacoustics method based on a stress-blended eddy simulation coupled with the Ffowcs-Williams and Hawkings acoustic analogy. The results indicate that in the absence of the side-view mirror, the idealized A-pillar is identified as the subsequent major contributor to the overall noise radiated from the vehicle body, and the coefficient of drag decreases by approximately 13.3% despite a minimal change in the projected frontal area. However, the behavior of the drag coefficient becomes nonlinear and highly dependent on the complex flow features, including the vortex shedding patterns and the interaction between the flow and side surface of the body, with increasing mirror inclination angle. In contrast, the radiated noise exhibits a constant decrease as the mirror inclination angle (θ) increases to 32°. Additionally, when the side-view mirror is considered as the sole source, the noise radiated is minimal for an inclination angle of 16°, and a further increase in inclination angle has no significant reduction on the noise radiated but alters the overall drag coefficient of the vehicle. These findings have practical implications for the design of side-view mirrors to reduce aerodynamic noise in automotive applications and highlight the complex tradeoffs between noise reduction and changes in the drag coefficient that must be considered in such designs.

Assessment of The Acoustic Scattering Matrix of a Heat Exchanger Using ssCFD-LNSE Simulation

The risk of thermoacoustic instability is present in any combustion appliance. The instability results from a closed loop feedback between unsteady combustion, heat-transfer and acoustic modes of the system. To predict the system acoustics all constituting elements of the appliance need to be modelled. A heat-exchanger is the element where the gas faces complex fluid dynamics and heat transfer processes. Therefore, modelling of (thermo)-acoustic properties of a heat exchanger is challenging. In this paper, a computational approach is proposed to characterize the acoustic properties of a generic heat exchanger in both laminar and turbulent flow regimes. A hybrid Computational Fluid Dynamics - Computational Aero-Acoustics (CFD-CAA) method is used based on full linearized Navier-Stokes equation, called ssCFD-LNSE. The fundamental idea in this approach is to efficiently model acoustic wave propagation with inclusion of mean flow and temperature fields. ssCFD-LNSE is performed by splitting the quantities of the total field into a mean part (obtained from CFD) and a (acoustic) perturbation part modelled within the LNSE solver. The goal of this research is to assess the two-port acoustic scattering matrix of an array of tubes, as a generic model of heat-exchanger, with a hot cross flow.

Numerical analysis on the high-speed impulsive noise propagation characteristic of helicopter rotor in the presence of strong shear flow

Based on the Linearized Euler Equations (LEE) and the Computational Fluid Dynamics (CFD) technique, the propagation characteristics of helicopter rotor High-Speed Impulsive (HSI) noise in non-uniform flowfield are simulated and analyzed. A Hybrid Computational Aeroacoustics (HCAA) method for solving the sound field of rotor is first developed. The acoustic source region is simulated by solving Navier-Stokes equations, and the acoustic near-field is simulated by solving LEE based on the Runge-Kutta Discontinuous Galerkin (RKDG) method. The numerical analysis method is then validated through comparisons with experimental data, and the aeroacoustics characteristics of the UH-1H rotor are calculated. Afterwards, the propagation characteristics of HSI noise in the wind tunnel are calculated, and the “trap” effect of the shear layer on HSI noise is determined. Finally, the peak negative pressure, time-averaged acoustic energy, and instantaneous perturbed pressure field are analyzed in detail, and some conclusions are drawn. In jet flow, the HSI noise energy is reinforced, and the time of the peak negative pressure is significantly advanced. In still air, the HSI noise energy is attenuated, and the time of the peak negative pressure is slightly delayed. These investigations offer valuable data for developing advanced theoretical correction method to consider the effect of shear layer on rotating acoustic source.

Computational statistically optimized near-field acoustical holography (C-SONAH) for virtual noise source identification

In this paper, a new methodology, computational statistically optimized near-field acoustic holography (C-SONAH), that can be applied to virtually identify aeroacoustic sources, is described. Sound pressure is first obtained by a combined computational fluid dynamics and computational aeroacoustics (CAA) method, then SONAH is used to locate the acoustic sources and predict the sound field. C-SONAH provides computational advantages over the direct CAA methods in simulating sound emitted in systems with rotating elements, because CAA analyzes the sources on the moving elements which makes the sound field calculation computationally expensive. However, with the help of a SONAH procedure, those rotating sources are converted into a series of equivalent stationary planar or cylindrical waves, which reduces not only the number of sources but also the time required to compute the sound field from each source. This methodology is demonstrated by characterizing the aerodynamic noise emitted from a bladeless fan. In this validation case, the predicted acoustic maps are compared with the results obtained from acoustic array measurements, revealing that the C-SONAH method can predict the noise sources generated by the airflow and rotating components inside the fan. Thus, it can be used as an effective tool to gain insights into the aeroacoustic noise generation mechanism and to guide the design optimization of similar products.

Numerical study on the noise propagation characteristics of rotor in non-uniform downwash flowfield Based on Linearized Euler Equations

To study the influence of non-uniform flowfield on the propagation characteristics of helicopter rotor noise, a Hybrid Computational Aeroacoustics (HCAA) method is developed. The acoustic source region is simulated by Computational Fluid Dynamics (CFD) technique with the Unsteady Reynolds Averaged Navier-Stokes equations (URANS) as the governing equations. Acoustic near-field is simulated by Computational Aeroacoustics (CAA) technique with the Linearized Euler Equations (LEE) as the governing equations, and the numerical discretization of the LEE is accomplished by Runge-Kutta Discontinuous Galerkin (RKDG) method. A novel acoustic source extraction method based on pressure and pressure gradient is proposed to accomplish the one-way CFD-CAA weak coupling. The HCAA method is validated through comparisons with noise experimental data of the UH-1H model rotor and the BO-105 model rotor. Based on the proposed HCAA method, the convection and refraction effects of rotor noise under different collective pitch angles are analyzed. The results show that the distortion effect of the rotor noise is most affected by the non-uniformly distributed downwash velocity field, resulting in an increment of acoustic energy below the rotor plane. The effect of non-uniformly distributed downwash velocity on noise propagation increases with the increase of the collective pitch angle. For the UH-1H model rotor, the maximum change of the sound pressure level is 0.8 dB (about 10% change of the effective sound pressure).

Numerical Study on Flow-Induced Noise of Deflector Jet Servo Valve Based on LES/Lighthill Hybrid Method

The flow-induced noise occurring in the prestage of the deflector jet servo valve (DJV) significantly affects the performance of DJV, which is a critical issue for electrohydraulic servo valves. To capture the root causes of flow-induced noise, DJV is numerically studied using computational aeroacoustics (CAA) methods in this paper. The acoustic field is investigated with Lighthill’s acoustic analogy based on the pulsation data from a large eddy simulation (LES). The flow-induced noise generation mechanism in DJV is revealed by analyzing the sound pressure level and vorticity distribution at different moments. Moreover, a kind of oscillation cavity model based on the Helmholtz oscillator is built, which reveals that the flow-induced noise is caused by self-excited oscillation and vortex. In addition, the result shows that continuous broadband dipole noise with medium- and high-frequency components of 1100 Hz and 2501 Hz dominates the DJV flow field. In order to avoid unexpected resonance, the natural frequency of the prestage should not coincide with the frequency of the flow oscillation frequency. Thus, this work can benefit the design and optimization of the prestage structure.

The third golden age of aeroacoustics

The present review covers the latest evolution of computational aeroacoustics, the field that deals with the noise generated by fluid flows and its propagation in the medium. It highlights the latest findings in both free flows (jet noise) and wall-bounded flows (airfoil, airframe, and turbomachinery noise) in more and more complex environments. Among the computational aero-acoustics methods, high-order schemes of the Navier–Stokes equations on unstructured grids and the lattice Boltzmann method on Cartesian grids have emerged as excellent candidates to tackle noise problems in realistic complex geometries. The latter is also shown to be particularly efficient for both noise generation and propagation, allowing to directly estimate the noise in the far field. Two examples of application of such methods to complex jet noise and to installed airfoil noise are first presented. The first one involves compressible subsonic and supersonic flows in dual-stream nozzles and the second one subsonic flow around an airfoil embedded in the potential core of the open-jet anechoic wind tunnel as in the actual trailing-edge noise experiment. For airframe noise, large eddy simulations of scaled nose landing gear noise and three-element high-lift devices can be tackled to decipher noise sources. For turbomachinery noise, simulations of installed low-speed fans have already unveiled a wealth of details on their noise sources, whereas high-speed turbofans remain a challenge giving the high Reynolds numbers and small tip gaps involved.

Further assessment of a time domain impedance boundary condition for the numerical simulation of noise-absorbing materials

In regard to the mitigation of environmental noise across major industry sectors, the present study focuses on the numerical prediction of passive noise reduction devices. Here, it is further explored how the noise attenuation induced by locally reacting noise absorbing materials (also called acoustic liners) can be simulated using a time domain highly accurate Computational AeroAcoustics (CAA) method. To this end, it is assessed how a classical Time Domain Impedance Boundary Condition (TDIBC) can effectively model acoustic liners of practical interest, including when the latter are exposed to realistic conditions (grazing flow and noise excitation). The investigation consists in numerically reproducing two experimental campaigns initially performed at NASA Langley Research Center. Two different materials are considered (honeycomb superimposed with perforate or wiremesh resistive face-sheet), each being characterized by a specific noise attenuation behaviour ( e.g. dependency on the flow conditions and/or noise excitation). Each material is tested under various flow conditions ( e.g. grazing flow of Mach up to 0.5) and/or noise source excitation ( e.g. multiple tones of level up to 140 dB each). The results demonstrate the ability of the underlying CAA/TDIBC approach to simulate realistic acoustic liners in non-trivial configurations, with enough physical accuracy ( e.g. correct capture of the noise attenuation characteristics) and numerical robustness ( e.g. absence of instabilities). The study also reveals that, independent from the CAA/TDIBC approach itself, some specific pre-processing tasks (e.g. impedance eduction and subsequent TDIBC calibration) may play a bigger role than expected, in practice.

Implementation of Direct Acoustic Simulation using ANSYS Fluent

Direct Acoustic Simulation (DAS) is a powerful Computational Aero Acoustics method that obtains hydrodynamic and acoustic solutions simultaneously by solving compressible Navier-Stokes equation together with state equation of ideal gas. Thus, DAS has advantages for cases with flow acoustic coupling and high Mach numbers (). With an increasing demand of massive-scale calculations, a robust numerical solver for DAS is required. ANSYS Fluent is a suitable CFD platform with proven robustness. However, there is no direct implementation of DAS in the current version of ANSYS Fluent. The present study, therefore, aims to investigate an approach for implementing DAS using ANSYS Fluent. Given the acoustic part of fluctuations is much smaller than the hydrodynamic part in amplitude, a DAS solver requires high accuracy and low dissipation. Based on these needs, proper solution methods, spatial discrete methods and boundary conditions are firstly determined through simple calculations of two dimensional propagating plane waves. Afterwards aeroacoustics of a two-dimensional cavity flow at 0.6 is calculated to verify the capability for solving separating flow with the aforementioned set-up. Finally, aeroacoustics of a cylindrical bluff body at a turbulent regime and 0.2 is calculated in three-dimensions to verify the capability for solving turbulent flow using Monotonically Integrated Large Eddy Simulation.

Comparison of hydrodynamic and acoustic pressure on automotive front side window

The unsteady flow in the front side window region of the vehicle can generate hydrodynamic and acoustic pressure on the front side window, which can influence the interior sound field. The hydrodynamic pressure on the front side window was achieved by the incompressible wall-modeled large-scale eddy simulation (WMLES) or improved delayed detached eddy simulation (IDDES), and the hybrid computational aeroacoustics (CAA) method based on acoustic perturbation equations (APE) was employed to achieve the acoustic pressure on the front side window. The numerical results of both hydrodynamic and acoustic pressure ware validated by the wind tunnel experiment, especially the corrected force analysis technique (CFAT) is employed to validate the acoustic pressure. The comparison of hydrodynamic and acoustic pressure on the front side window was performed by the Dynamic Mode Decomposition (DMD). Results show that the hydrodynamic pressure regionally distributes on the front side window and most energy concentrates on area interacted with the side mirror wake, while the acoustic pressure distributes uniformly on the front side window acting as a diffusion field and the energy disperses in frequency region.

Aeroacoustic noise calculations of non-compact bodies with permeable boundaries

A hybrid computational aeroacoustic method with permeable boundary is developed to evaluate non-compact noise induced by low Mach number flow over arbitrarily shaped bodies. Based on Lighthill’s equation and the boundary element method, the unified integral equations are established in which the integral boundary surrounding the objects can be selected arbitrarily. Validation studies are developed for noise induced by two-dimensional NACA0012 airfoil and three-dimensional circular cylinder. For NACA0012 airfoil, the directivity patterns of calculated noise with different permeable boundaries agree well with Howe’s analytical solution for trailing edge model. The acoustic noise generated by circular cylinder has a good agreement with Revell’s experimental data and FW-H equation. It demonstrates that the noise predicted by different permeable boundary is as accurate as that calculated by the body surface.

Rapid Prediction Method of Fan Broadband Noise Based on A Three-dimensional Sound Source Model

The fan/compressor broadband noise is an important noise source of civil large bypass ratio turbofan engine.. The empirical method requires massive amounts of test data for support. Adopting the computational aeroacoustic (CAA) method to predict the fan/compressor broadband noise will cost high amounts of computing resources or time. Thus, this study combines flow field calculation with analytical model to study the prediction method of fan/compressor broadband noise, this method can be used for quick and efficient preliminary evaluation and parameter studies. On the basis of the simulation and modeling of the stator – rotor interference phenomenon of fans, rotor wake flow field and the corresponding parameters are obtained. On this basis, the three-dimensional sound source model is used to calculate the blade surface sound source and its propagation under different working conditions. The distribution of sound energy in the duct is obtained. Then, the broadband noise prediction of fan is further acquired. The broadband noise of a typical single-stage TA36 fan is predicted using this method. Study results suggest that (1) turbulence model and turbulence kinetic energy of inlet boundary condition will have an important effect on the prediction results of broadband noise. (2) When the inlet turbulence kinetic energy is small, different turbulence models slightly influence the prediction results of broadband noise; when the inlet turbulence kinetic energy is large, the sound pressure level of sound field obtained by SST model is approximately 5dB larger than that obtained by model. This method can be used in the early stage of fan/compressor design, and the characteristics of blade broadband noise can be rapidly obtained.

Aerodynamic noise of high-speed train pantographs: Comparisons between field measurements and an updated component-based prediction model

Aerodynamic noise from pantographs becomes an important source of noise from trains at high speeds. Previous studies have mostly been based on numerical predictions using computational aeroacoustic methods, which require large computing resources, or measurements conducted in a wind tunnel which cannot take all the real conditions into account. A component-based model relying on empirical constants obtained from the literature has been shown to predict aerodynamic noise from pantographs that agrees well with wind tunnel measurements. This model is extended in this paper by making use of simulation results on individual cylinders to refine the model constants and the Reynolds number dependence. In addition, allowance for the effect of incoming turbulence and cylinder aspect ratio is also extended. The updated model shows improved agreement with wind tunnel measurements, particularly at low frequencies. This model is then used to predict pantograph noise in more realistic conditions during train pass-by. The incoming flow conditions in terms of the incident flow speed, the turbulence intensity and the turbulence length scale are estimated from the literature considering the development of the boundary layer along the train roof. The sensitivity of the model to these assumptions is assessed using Monte Carlo simulations. The predicted results are compared with field measurements obtained using microphone array techniques for pantograph on different operational trains. Good agreement is obtained between the predictions and the measurements in terms of the far-field noise spectra and the dependence of noise level on speed. Differences are noted between measured levels for different orientations of the pantograph which according to the model are mainly related to the distance of the pantograph from the front of the train.

Numerical simulation of aerodynamic noise and noise reduction of range hood

The hybrid CAA (Computational Aeroacoustics) method is used to numerical study on the aerodynamic noise of a certain range hood. The CFD numerical calculation method was used to calculate the unsteady flow field, and the velocity and pressure distribution of the range hood under unsteady conditions were obtained. The particle imaging velocimetry (PIV) technology experiment was carried out by set up an internal flow field test bench, to record the flow status of the flow field in the measurement area by a CCD camera. Also, applied with FLUENT and Actran acoustic software, and based on the results of unsteady flow field calculation of the range hood as well as the Lighthill sound analogy method, the internal flow field information of the range hood is converted into the sound field information, and the noise of the range hood can be numerically simulated. The results showed that the range hood noise mainly concentrated on low and middle frequencies and characterized by discrete noise. The noise of the range hood Having distinct feature of direction at the blade passing frequency, and the noise is mainly transmitted to the outside through the main air inlet and secondary air inlet of the centrifugal fan in the range hood. Installing noise reduction structures at the main and secondary air inlet can effectively reduce noise.

A semi-implicit immersed boundary method for simulating viscous flow-induced sound with moving boundaries

In this paper, a semi-implicit immersed boundary body force model is derived from the compressible Navier–Stokes equations, to directly predict the viscous flow-induced sound from moving objects on a fixed Cartesian grid. To overcome the conflict of grid quality with efficiency in simulating moving-boundary problems with high-order computational aeroacoustics methods, a prediction–correction technique is utilized. This accurately satisfies no-slip wall boundary conditions at every time step without any feedback treatment. A particular contribution of the work is the introduction of a numerical model equation to analyze the body force convergence. This is useful to pre-evaluate the generated Cartesian and body-surface grids. Several benchmark aeroacoustic problems are simulated to validate the present model. Results show that the unsteady force and far-field sound directivity agree well with the previous direct numerical simulation results. The work further suggests that the developed body force model/CAA methods are capable of predicting interaction noise, especially those associated with oscillating multiple objects.