Computational Aeroacoustics Research Articles

The numerical oscillation suppression method for the large time step scheme

The large time step scheme, which is commonly used in hydrodynamic numerical simulation, adopts the multielement reconstruction method in order to improve both the computational efficiency and accuracy; however, this results in numerical oscillation and calculation divergence. Therefore, it is necessary to develop methods to suppress the occurrence of numerical oscillation near the discontinuities. The numerical oscillation that is induced can be suppressed by using the random choice method (RCM) with the van der Corput random number sequence. In order to optimize the suppression performance, error analysis has been carried out in this paper for the calculation results from the RCM with the popular van der Corput random sequence combinations for selecting the optimal sequence combination. In order to further eliminate any short-wave numerical oscillations near the discontinuities, a method that combines the RCM with a nonlinear filter, which often used in aeroacoustic calculations, has been proposed in this paper to suppress short-wave oscillations. Specifically, the RCM and the nonlinear filter method were sequentially implemented in order to process the calculation results at each time step. The results showed that this method had a good suppression effect on the numerical oscillations near the discontinuities, which was demonstrated by the numerical experiments.

Cabin noise prediction using wave‐resolving aircraft models

AbstractWithin the Coordinated Research Center (CRC) 880, future civil aircraft with short take‐off and landing capabilities are investigated. A main motivation are efficient point‐to‐point connections within Europe. Environmental friendliness is a key factor in the development of such aircraft systems in order to ensure the acceptance of new technologies. Therefore, the sound immission at the ground and in the passenger cabin is a major focus in the project. In this paper, wave‐resolving models of complex aircraft fuselage structures and solution strategies of these large systems in frequency domain are presented.On the basis of preliminary aircraft design data available in the project, a mechanical model of all fluid and structural parts is developed. The model comprises the back partition starting from the wing‐box and assumes a linear behavior in deflection, strain and material. The thickness distribution on the outer surface, the stiffeners, the floor, the bulk heads and the inner panels are considered concerning the structural domain. The insulation is modeled by an equivalent fluid approach. The cabin fluid, the structural domain, and the insulation are fully coupled. All domains are discretized automatically applying coincident nodes at the coupling interfaces. Frequency‐dependent pressure fluctuations distributed on the outer surface as a result of Computational Aeroacoustic (CAA) simulations of jet noise are applied to the model. A realistic consideration of such characteristic noise footprints is only possible due to the wave‐resolving approach. The direct result of the aircraft model is the cabin's pressure field in frequency domain.The finite element discretization leads to a system of equations with 1‐3 mio degrees of freedom to be solved in frequency domain. As one solver run is easily realized, many solutions in the frame of uncertainty analyses or sensitivity studies require a highly efficient solution process. Different strategies applying direct solvers, iterative solvers or its combination are discussed within this contribution as well.

A technique of flux reconstruction at the interfaces of nonconforming grids for aeroacoustic simulations

SummaryA flux reconstruction technique is presented to perform aeroacoustic computations using implicit high‐order spatial schemes on multiblock structured grids with nonconforming interfaces. The use of such grids, with mesh spacing discontinuities across the block interfaces, eases local mesh refinements, simplifies the mesh generation process, and thus facilitates the computation of turbulent flows. In this work, the spatial discretization consists of sixth‐order finite‐volume implicit schemes with low‐dispersion and low‐dissipation properties. The flux reconstruction is based on the combination of noncentered schemes with local interpolations to define ghost cells and compute flux values at the grid interfaces. The flow variables in the ghost cells are calculated from the flow field in the grid cells using a meshless interpolation with radial basis functions. In this study, the flux reconstruction is applied to both plane and curved nonconforming interfaces. The performance of the method is first evaluated by performing two‐dimensional simulations of the propagation of an acoustic pulse and of the convection of a vortex on Cartesian and wavy grids. No significant spurious noise is produced at the grid interfaces. The applicability of the flux reconstruction to a three‐dimensional computation is then demonstrated by simulating a jet at a Mach number of 0.9 and a diameter‐based Reynolds number of 4×105 on a Cartesian grid. The nonconforming grid interface located downstream of the jet potential core does not appreciably affect the flow development and the jet sound field, while reducing the number of mesh points by a factor of approximately two.

Skew-symmetric splitting of high-order central schemes with nonlinear filters for computational aeroacoustics turbulence with shocks

A class of high-order nonlinear filter schemes by Yee et al. (J Comput Phys 150:199–238, 1999), Sjogreen and Yee (J Comput Phys 225:910–934, 2007), and Kotov et al. (Commun Comput Phys 19:273–300, 2016; J Comput Phys 307:189–202, 2016) is examined for long-time integrations of computational aeroacoustics (CAA) turbulence applications. This class of schemes was designed for an improved nonlinear stability and accuracy for long-time integration of compressible direct numerical simulation and large eddy simulation computations for both shock-free turbulence and turbulence with shocks. They are based on the skew-symmetric splitting version of the high-order central base scheme in conjunction with adaptive low-dissipation control via a nonlinear filter step to help with stability and accuracy capturing at shock-free regions as well as in the vicinity of discontinuities. The central dispersion-relation-preserving schemes as well as classical central schemes of arbitrary orders fit into the framework of skew-symmetric splitting of the inviscid flux derivatives. Numerical experiments on CAA turbulence test cases are validated.

On the differences in noise predictions based on solid and permeable surface Ffowcs Williams–Hawkings integral solutions

The Ffowcs Williams–Hawkings equation is widely used in computational aeroacoustics to post-process unsteady simulations and provide the sound at distances beyond the accurate range of the grid. It...

Numerical prediction of passenger cabin noise due to jet noise by an ultra–high–bypass ratio engine

Within the framework of the Collaborative Research Center 880, future civil transportation aircraft are investigated. One major aim is a drastic reduction of the noise emission to the ground and an appropriate noise immission into the passenger cabin. The latter forms the focus of this contribution with the aim to ensure an equal or lower noise level in the cabin for new aircraft configurations. Numerical methods are applied to establish a multidisciplinary modeling chain resulting in a prediction of cabin noise due to jet noise by two different engine configurations. An ultra–high–bypass ratio engine is compared to a conventional engine on the basis of a preliminary aircraft design used for both configurations. On the basis of flow calculations in cruise flight as operation point, the hybrid Computational Aeroacoustics solver PIANO combined with the Fast Random Particle Mesh method is applied to compute the pressure fluctuations due to jet noise on the outer skin of the fuselage. These loads are applied to a finite element model considering the structure and the fluid of the aircraft cabin. The acoustic model resolving the structure-borne and airborne sound waves is derived from the design data. The results show a lower sound pressure level induced by the ultra–high–bypass ratio engine in the entire frequency range on the outer skin. Within the cabin, the modern engine is still much quieter, though this fact is not generally valid for the entire frequency range as transmission effects of the double wall occur.

Assessment of aeroacoustic resolution properties of DG schemes and comparison with DRP schemes

We discuss the aeroacoustic resolution properties of the discontinuous Galerkin (DG) schemes in detail and compare their performance with the state of the art finite difference (FD) schemes, including the classical dispersion-relation-preserving (DRP) schemes. Analysis shows that, even though the DG schemes are slightly dissipative, their overall dispersion and dissipation properties are comparable with the corresponding DRP schemes on the same stencil. For the convenience of a direct comparison with FD schemes, we write the DG schemes in the form of block finite difference schemes. Ample numerical tests, including the tests on nozzle flow problems, are performed and we observe that the DG schemes and DRP schemes with the same stencil produce comparable numerical results. Since the DG schemes are flexible on non-uniform meshes and general unstructured meshes, they should be good candidates for computational aeroacoustics, especially those on complex geometry.

Direct anisotropic filter method of generating synthetic turbulence applied to turbulence-airfoil interaction noise prediction

Synthetic turbulence methods are widely used in computational aeroacoustics to study broadband noise physics. In this work, a direct anisotropic filter (DAF) method is proposed to produce desired velocity spectra of three-dimensional anisotropic turbulence, based on a direct form of anisotropic filter that can be implemented directly to obtain non-Gaussian spectra. It contains fewer model constraints than the existing methods, getting rid of the inconvenience induced by complex hypergeometric functions and Gaussian superposition with Mach number dependent model parameters. In addition, compared with the numerical results given by existing methods, e.g., the advanced digital filter method, this technique offers improvement in accuracy and cost. In this study, the DAF method is combined with the linearised Euler equations in computational aeroacoustics to study the turbulence-airfoil interaction noise, with the results showing good agreement with benchmarking analytical solutions and experimental measurement. The numerical study also investigates the effect of anisotropy on leading edge noise for thick airfoils, revealing that the anisotropic turbulence stretched in the axial direction will suppress the thickness-induced noise reduction. This study also highlights the contribution of streamwise energy spectra to noise generation when considering the effects of anisotropy.

Noise Sources of Lean Premixed Flames

The thermoacoustic sound generation mechanisms of lean premixed laminar and turbulent flames are investigated by a two-step approach. First, the conservation equations of a reacting compressible fluid are solved. This solution is used to determine the acoustic source terms of the acoustic perturbation equations (APE). Second, the contributions of the different source terms to the amplitude and phase of the acoustic pressure signal are analyzed by solving the APE in a computational aeroacoustics (CAA) simulation. The results show that it is not sufficient to only consider the unsteady heat release rate fluctuations which occur in the substantial pressure-density relation. The acceleration of density gradients occurring at the flame front is a significant contributor to the overall sound emission. For the investigated laminar and turbulent flames the amplitude and phase of the acoustic pressure signal can only be predicted accurately if both source terms are included in the acoustic analysis.

Vibro-acoustics response of a simplified glass window excited by the turbulent wake of a quarter-spherocylinder body.

Inside vehicle cabins, an important part of interior noise is generated from cabin window vibration. The vibration is stimulated by surface pressure fluctuations that are produced by exterior flows and flow-induced noise. To numerically investigate the cabin noise, both exterior flows and noise must be resolved in simulations. This requirement motivates us to utilize advanced computational fluid dynamics (CFD) and computational aeroacoustics (CAA) based on a quarter-spherocylinder body, which is a general model for vehicle mirrors. The blunt body is mounted upstream of a rectangular window of a cuboid cavity. The turbulent flow is simulated using compressible large eddy simulation, compressible detached eddy simulation, and incompressible detached eddy simulation (I-DES). The exterior noise is either predicted by coupling the I-DES with an acoustic wave modeling method, or directly solved using compressible CFD methods. Given surface pressure fluctuations on the window from the CFD and CAA methods, the window vibration and interior noise are simulated with a finite element method. The effects of compressibility, turbulence modeling methods, and grid topology (polyhedral and trimmed elements) are discussed. The computational efficiency of the numerical methods is addressed. The contributions of hydrodynamic and acoustic pressure fluctuations to the interior noise are clarified.

Computation of Aeroacoustics and Fluid Flow Problems Using a Novel Dispersion Relation Preserving Scheme

A new spectrally optimized physical dispersion relation preserving scheme has been introduced to solve computational acoustics and aeroacoustics problems, accurately. The derived fourth-order accurate scheme has significant spectral resolution and physical dispersion relation preserving (DRP) nature. The scheme displays neutral stability at high CFL numbers. The developed scheme has an ability to add numerical diffusion as and when required to attenuate spurious waves present in the computed solutions. These features make the proposed scheme suitable for solving computational aeroacoustic problems. The scheme has been validated by comparing solutions of model computational acoustic problems with the available analytical solutions. Scheme has also been tested to solve the incompressible flow field around a circular cylinder executing rotary oscillations. Ability of the scheme to perform direct simulation of the computational aeroacoustic problems has been shown by computing acoustic field triggered by a laminar flow past a stationary circular cylinder. Excellent match has been observed between the present computed results and the available results in the literature which justifies applicability of the present DRP scheme to solve complex flow and aeroacoustic problems.

Numerical Simulation and Experiment on Impulse Noise in a Small Caliber Rifle with Muzzle Brake

Blast waves generated from the muzzles of weapons may exert negative effects, such as shock waves and impulse noise. If the weapon is fired with a muzzle brake, these effects are recognized to be more severe. This paper discusses the influence of the muzzle brake on certain aeroacoustic noise characteristics based on numerical simulations and a corresponding experiment. The impulse noise, which is induced by complex jet flows discharging from small caliber rifles with muzzle brakes, is focused in this study. Computational fluid dynamics (CFD) and computational aeroacoustics (CAA) are combined to calculate the muzzle flow field and jet noise for cases with and without a muzzle brake, and then the data sets are carefully compared. The simulations indicate that the muzzle brake alters the muzzle flow field and directional distribution of the jet noise compared to a rifle sans muzzle brake. Deviations less than 7.6% between experimental data and simulation results validate the simulation model. The results presented in this paper may provide a workable reference for the prediction of muzzle noise and the optimization of muzzle brake designs.

Numerical Simulation Study on Aeroacoustic Characteristics within Deformable Cavities

Cavity flow phenomena are encountered in many kinds of aviation vehicles. The flow‐induced noise can easily cause structure resonance and fatigue damages. Therefore, the study on the mechanism and effective control methods of cavity noise are very important to engineering applications. A new active control method was proposed based on the deformable cavity in order to mitigate the cavity noise. Large eddy simulation (LES) and computational aeroacoustics (CAA) are combined to simulate a typical open cavity noise. The results show the first mode sound‐pressure level (SPL) of tonal noise decreases gradually while the first mode frequency sharply jumps within a small range of the slant angle of the trailing and bottom wall. In addition, with the increase in the slant angle, the decrease of the first mode SPL of tonal noise at Mach 0.6 is more significant than that at Mach 0.85, but the increase of the first mode frequency at Mach 0.85 is more dramatical than that at Mach 0.6. The proposed method can not only reduce the first mode SPL obviously but also increase the first mode frequency dramatically, which makes it different from the natural frequency of the cavity structure and sequentially helps the cavity avoid fatigue damages from resonance.

LES Analysis for Incompressible Flow of Internal Flow in a Box Fan and Computational Aeroacoustics of Radiated Sound from It

LES Analysis for Incompressible Flow of Internal Flow in a Box Fan and Computational Aeroacoustics of Radiated Sound from It

Selection of a leading edge noise prediction method for PNoise

Wind energy expansion is worldwide followed by various limitations, i.e. land availability, the NIMBY (not in my backyard) attitude, interference on birds migration routes and so on. This undeniable expansion is pushing wind farms near populated areas throughout the years, where noise regulation is more stringent. That demands solutions for the wind turbine (WT) industry, in order to produce quieter WT units. Focusing in the subject of airfoil noise prediction, it can help the assessment and design of quieter wind turbine blades. Considering the airfoil noise as a composition of many sound sources, and in light of the fact that the main noise production mechanisms are the airfoil self-noise and the turbulent inflow (TI) noise, this work is concentrated on the latter. TI noise is classified as an interaction noise, produced by the turbulent inflow, incident on the airfoil leading edge (LE). Theoretical and semi-empirical methods for the TI noise prediction are already available, based on Amiet’s broadband noise theory. Analysis of many TI noise prediction methods is provided by this work in the literature review, as well as the turbulence energy spectrum modeling. This is then followed by comparison of the most reliable TI noise methodologies, qualitatively and quantitatively, with the error estimation, compared to the Ffowcs Williams-Hawkings solution for computational aeroacoustics. Basis for integration of airfoil inflow noise prediction into a wind turbine noise prediction code is the final goal of this work.

Tonal noise reduction by unevenly spaced blades in a forward-curved-blades centrifugal fan

Tonal noise is an essential characteristic of the noise generated from a forward-curved-blades centrifugal fan. Herein, methods to unevenly arrange blades, in particular, a random modulation method, were studied to weaken the undesired tonal noise and reshape the noise spectrum. First, the balance and aerodynamic performance requirements of the impeller were considered when designing the unevenly spaced blades. The modulated blade positions had to satisfy the dynamic balance condition, while the variation of spacing between every two adjacent blades should not exceed 30% of the original spacing in the even impeller. Next, the influence of non-uniform blades on the noise spectrum characteristics of the fan was theoretically predicted by calculating the interference functions at each discrete frequency. Finally, computational fluid dynamics and computational aeroacoustics analyses were performed to investigate the acoustic performance of each uneven blade spacing scheme. Both the pressure fluctuation on the tongue and the noise frequency spectrum from the acoustic receiver demonstrated that unevenly spaced blades could reduce the tonal noise by dispersing the acoustic energy at the blade passing frequency to other specific frequencies as a form of sidebands. Compared with the existing sinusoidal modulation method, the random modulation method has no limits on the blade number. Moreover, this method has a higher degree of freedom that could overcome all limitations of blade spacing. It is shown that a reasonable blade distribution designed by this method could reduce tonal noise without negatively affecting the aerodynamic performance and the overall noise level.

A small leakage detection approach for oil pipeline using an inner spherical ball

Direct acoustic inspection with an inner detector has recently been suggested as an efficient leakage detection method. Because of its high sensitivity, this method is very promising for pipeline health monitoring. However, few studies have used automatic recognition based on the acoustic signature of leakage signals for direct acoustic inspection. In this study, a hybrid computational fluid dynamics and computational aeroacoustics mathematical model of small leakage based on Lighthill’s analogy was constructed to simulate the acoustic signature in the vicinity of a leakage source. The obtained results show that the leakage sound is a broadband signal with dispersion characteristics. Furthermore, because of the appearance of a stationary wave, amplitude peaks emerge at specific frequencies in the sound pressure spectrum. According to the simulation parameters, a specific experimental environment was constructed that matched the simulation results. Inspired by the formant recognition method used in automatic speech recognition, a novel acoustic signature extraction approach was proposed based on Mel-frequency cepstral coefficients. Moreover, instead of a full band, a redesigned frequency band was applied in response to the influence of noise interference and the sound pressure spectrum. An efficient support vector machine classifier was employed to solve the binary classification problem (regardless of leakage), and the obtained results are very encouraging: an almost 10% performance improvement was achieved with a redesigned frequency band, and both the accuracy and specificity of the recognition system reached up to 97%.

New very high-order upwind multi-layer compact (MLC) schemes with spectral-like resolution for flow simulations

Numerical simulations of multi-scale flow problems such as hypersonic boundary layer transition, turbulent flows, computational aeroacoustics and various other flow problems with complex physics require high-order methods with high spectral resolutions. For instance, the receptivity mechanisms in the hypersonic boundary layer are the resonant interactions between forcing waves and boundary-layer waves, and the complex wave interactions are difficult to be accurately predicted by conventional low-order numerical methods. High-order methods, which are robust and accurate in resolving a wide range of time and length scales, are required. Currently, the high-order finite difference methods for simulations of hypersonic flows are usually upwind schemes or compact schemes with fifth-order accuracy or lower [1]. The objective of this paper is to develop and analyze a new very high-order numerical scheme with the spectral-like resolution for flow simulations on structured grids, with focus on smooth flow problems involving multiple scales. Specifically, a new upwind multi-layer compact (MLC) scheme with spectral-like resolution up to seventh order is derived in a finite difference framework. By using the ‘multi-layer’ idea, which introduces first derivatives into the MLC schemes and approximates the second derivatives, the resolution of the MLC schemes can be significantly improved within a compact grid stencil. The auxiliary equations are required and they are the only nontrivial equations, which contributes to good computational efficiency. In addition, the upwind MLC schemes are derived based on the idea of constructing upwind schemes on centered stencils with adjustable parameters to control the dissipation. Fourier analysis is performed to show that the MLC schemes have small dissipation and dispersion in a very wide range of wavenumbers in both one- and two-dimensional cases, and the anisotropic error is much smaller than conventional finite difference methods in the two-dimensional case. Furthermore, the stability analysis with matrix method shows that high-order boundary closure schemes are stable because of compactness of the stencils. The accuracies and rates of convergence of the new schemes are validated by numerical experiments of the linear advection equation, the nonlinear Euler equations, and the Navier–Stokes equations. The numerical results show that good computational efficiency, very high-order accuracies, and high spectral resolutions especially on coarse meshes can be attained with the MLC schemes. Overall, the MLC scheme has the properties of simple formulations, high-order accuracies, spectral-like resolutions, and compact stencils, and it is suitable for accurate simulation of smooth multi-scale flows with complex physics.

Aero–acoustics in constricted pipe flow at low mach number

In this study, the sound generation in a constricted pipe at low Mach number (M=0.003, Re=1170 at the inlet) flow is investigated using a hybrid modeling approach of computational aero-acoustics (CAA). Large Eddy Simulation (LES) model is used simulate the flow and accurately capture the flow instabilities associated with the constriction. The Acoustic Perturbation Equation (APE) method is used to model the acoustic field where the source terms are derived from the incompressible flow simulation results. The acoustic spectra from the APE simulation results showed good agreement (within 9%) with measured values on the pipe wall downstream of the constriction. The strongest APE acoustic sources were found to occur in the jet core breakdown region that contained the highest pressure fluctuations in the flow field. Proper Orthogonal Decomposition (POD) was used to study the Coherent flow structures in the jet core breakdown region in order to further understand the characteristics of acoustic sources. The strongest structure corresponds to the peak frequency of the measured sound pressure spectra. The study results improve our understanding of the sound generation mechanisms in biomedical applications involving the airways such as phonation and partial airway obstruction.

Computational aeroacoustics to identify sound sources in the generation of sibilant /s/.

A sibilant fricative /s/ is generated when the turbulent jet in the narrow channel between the tongue blade and the hard palate is deflected downwards through the space between the upper and lower incisors and then impinges the space between the lower incisors and the lower lip. The flow eddies in that region become a source of direct aerodynamic sound, which is also diffracted by the speech articulators and radiated outwards. The numerical simulation of these phenomena is complex. The spectrum of an /s/ typically peaks between 4 and 10kHz, which implies that very fine computational meshes are needed to capture the eddies producing such high frequencies. In this work, a large-scale computation of the aeroacoustics of /s/ has been performed for a realistic vocal tract geometry, resorting to two different acoustic analogies. A stabilized finite element method that acts as a large eddy simulation model has been adopted to solve the flow dynamics. Also, a numerical strategy has been implemented that allows the determination, in a single computational run, of the separate contribution of the sound diffracted by the upper incisors from the overall radiated sound. Results are presented for points located close to the lip opening showing the relative influence of the upper teeth depending on frequency.