Computational Aeroacoustics Research Articles

Optimized simulation of multiscale problems on high‐performance computers

AbstractClassic hybrid computational fluid dynamics ‐ computational aeroacoustics simulations rely on disk I/O to exchange large volumes of data between the flow solver and the aeroacoustics solver, which considerably limits the scalability of the overall scheme. A direct‐hybrid method is presented that eliminates this restriction by executing both solvers simultaneously on a joint hierarchical Cartesian mesh. Simulation results for a pair of co‐rotating vortices show the parallel efficiency of the approach. (© 2017 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Computational study for noise reduction and characteristic of unsteady flow field/flow-induced noise generated in a small radial fan

This study is an attempt to characterize unsteady flow field and predict flow noise produced by a radial fan installed in a system that has a tiny gap between the fan casing and upper wall (similar conditions found in actual products). The large-eddy simulation turbulence model was used to simulate unsteady flow conditions, and an impeller was rotated 13 times in total to sufficiently develop unsteady flow and obtain data for Computational aeroacoustics (CAA). Unsteady flow field due to the radial fan structure and narrow flow channel in a system was verified, and a location wherein flow properties rapidly changed over time due to the complicated flow field was identified. The noise spectrum obtained through CAA was compared with that measured through experiment. Both tonal noise components consisting of blade passing frequency with its harmonic frequencies and broadband noise were matched. The major source of noise of the radial fan was on the inner surface of the casing. However, the sources of noise in the upper and lower sides of the casing were not equivalent to each other due to the interaction between the flow coming through an inlet located in the upper part of the casing and the flow discharged through an outlet installed at its side, as well as the interaction between impeller rotation and cut-off. Sources of noise located at the upper casing are caused by the flow around impeller tips, whereas sources on the lower casing are related to the interaction of the flow between the impeller and casing. A low noise model with a modified impeller tip was proposed, and its noise reducing effects were evaluated. The modified model reduced overall sound pressure level by 0.8 dB compared with the base model.

Handling of spatially truncated source fields based on pressure time derivatives in hybrid computational aeroacoustics

In hybrid computational aeroacoustics, the acoustic computation domain is oftentimes larger than the domain of the flow simulation. Consequently, discontinuities of the source field occur on the acoustic mesh, which can diminish the quality of the results. A common way to handle truncated source fields is spatial windowing. The present work focuses on aeroacoustic approaches that use time derivatives of the incompressible base flow pressure as acoustic source terms and their distinctive character with respect to source domain truncation. Especially for those approaches, a novel alternative to spatial windowing is proposed, which uses a simplified pressure field continuation beyond the source domain. Numerical results display that spatial windowing and simplified pressure field continuation can both treat truncated source fields effectively. The windowing approach is computationally cheaper but exhibits issues for small flow computation domains. The simplified pressure field continuation can overcome these and other issues yielding very good results while adding little computational cost.

Hermite regularization of the lattice Boltzmann method for open source computational aeroacoustics

The lattice Boltzmann method (LBM) is emerging as a powerful engineering tool for aeroacoustic computations. However, the LBM has been shown to present accuracy and stability issues in the medium-low Mach number range, which is of interest for aeroacoustic applications. Several solutions have been proposed but are often too computationally expensive, do not retain the simplicity and the advantages typical of the LBM, or are not described well enough to be usable by the community due to proprietary software policies. An original regularized collision operator is proposed, based on the expansion of Hermite polynomials, that greatly improves the accuracy and stability of the LBM without significantly altering its algorithm. The regularized LBM can be easily coupled with both non-reflective boundary conditions and a multi-level grid strategy, essential ingredients for aeroacoustic simulations. Excellent agreement was found between this approach and both experimental and numerical data on two different benchmarks: the laminar, unsteady flow past a 2D cylinder and the 3D turbulent jet. Finally, most of the aeroacoustic computations with LBM have been done with commercial software, while here the entire theoretical framework is implemented using an open source library (palabos).

Validation of aero-vibro acoustic simulation technique using experimental data of simplified fairing model

To predict harmful acoustic loading observed at lift-off of the launch vehicle, aero-vibro acoustic simulation technique is developed. High-fidelity large-eddy simulation with computational aeroacoustics based on the full Euler equations in time domain are employed to predict generation of the acoustic waves and their propagation to the fairing. Coupled vibro-acoustic analysis based on finite element method are applied to compute the transmitted acoustic wave into the fairing both in the time and frequency domains. Acoustic measurement of a simplified fairing model with a subscale liquid rocket engine is conducted, and validation study of the present technique is performed. Reasonable agreement is obtained for peaks of the acoustic spectrum taken inside the fairing model. Such peaks are found to be related to the internal acoustic modes and ring modes of the faring structure. However, further study is required to obtain quantitative agreement.

Computational Aeroacoustics Simulation of an Air Micro-Injector with Dimples Shape on the Surface of Divergent Nozzle

As for jet noise reduction of an air micro-injector with divergent nozzle, dimples shape in the inner surface of divergent nozzle is proposed, in the paper, the jet noise reduction effect of dimples shape is numerically confirmed at delivery pressure 1.33 atm. and 2.0 atm. (fully expanded jet Mach number 0.65 and 1.05), using Ffows Williams and Hawkings (FW-H) method base on time-accurate solutions of Large-Eddy Simulation (LES). In addition, the influence of dimples nozzle on the jet flow field is numerically investigated.

Accuracy consideration by DRP schemes for DNS and LES of compressible flow computations

Several dispersion relation-preserving (DRP) spatially central discretizations are considered as the base scheme in the framework of the Yee & Sjögreen low dissipative nonlinear filter approach. In addition, the nonlinear filter of Yee & Sjögreen with shock-capturing and long time integration capabilities is used to replace the standard DRP linear filter for both smooth flows and flows containing discontinuities. DRP schemes for computational aeroacoustics (CAA) focus on dispersion error consideration for long time linear wave propagation rather than the formal order of accuracy of the scheme. The resulting DRP schemes usually have wider grid stencils and increased CPU operations count compared with standard central schemes of the same formal order of accuracy. For discontinuous initial data and long time wave propagation of smooth acoustic waves, various space and time DRP linear filter are needed. For acoustic waves interacting with shocks and turbulence induced noise, DRP schemes with linear filters alone usually are not capable of simulating such flows. The investigation presented in this paper is focused on the possible gain in efficiency and accuracy by spatial DRP schemes over standard central schemes having the same grid stencil width for general direct numerical simulations (DNS) and large eddy simulations (LES) of compressible flows. Representative test cases for both smooth flows and problems containing discontinuities for 3D DNS of compressible gas dynamics are included.

Computational Aeroacoustics for Rotating Systems with Application to an Axial Fan

A hybrid aeroacoustic formulation, which is well suited for the computation of rotating systems, is presented. It is based on a decomposition of flow (incompressible part) and acoustic (compressible part) quantities, is applicable to high-Reynolds-number and low-Mach-number flows, accounts convective effects on the wave propagation, and features only a scalar unknown (i.e., the acoustic velocity potential), thus reducing the computational effort significantly. To show the accuracy and applicability of our approach, the proposed method is applied to an axial fan. Thereby, the experimental setup is discussed in detail, and measurements are performed toward the flow and acoustic field. Furthermore, the computation of the flowfield is presented along with the evaluation of the acoustic source terms, its efficient and accurate interpolation to the acoustic grid, and the computation of the generated acoustic sound. The comparison of local and global physical quantities, both for the flow as well as for acoustics, shows good agreement and demonstrates the applicability of the present aeroacoustic approach for rotating systems.

Numerical Simulation of Supersonic Twin-Jet Noise with High-Order Finite Difference Scheme

In this study, the noise generated from a cold supersonic circular twin jet is simulated with a high-order finite difference solver. The three-dimensional compressible Favre-filtered Navier–Stokes equations in Cartesian form are solved by computational aeroacoustics methods. To handle the complex geometry of two closely spaced circular nozzles, the grid block interface flux reconstruction method for high-order finite difference scheme is used. The supersonic twin jet with fully expanded Mach number 1.358 is simulated in this study. A single jet operating at the same condition is also computed for comparison. It is found that the two coupling jets are both oscillating in flapping mode but out of phase with each other. Therefore the near-field pressure is symmetric about two planes, the midplane separating the two nozzles and the plane involving the two jets’ axes. The computed noise spectra are compared with the experimental data of Walker [“Twin Jet Screech Suppression Concepts Tested for 4.7% Axisymmetric and Two-Dimensional Nozzle Configurations,” AIAA Paper 1990–2150, 1990]. The dominant and first harmonic modes in the twin jet are amplified dramatically comparing with the single jet. An increment of 12 dB for the dominant component is observed, whereas 17 dB for the first harmonic. The predicted amplitudes of the screech tone and its harmonic agree well with the experimental data. Comparing with the single jet, a frequency shift of the screech tone in the twin jet is observed. The pressure field of the jet plumes is analyzed with the dynamic mode decomposition (DMD) method. The eigenvalues and most energetic eigen-modes are presented. The coupling mechanism of the twin jet is analyzed based on the DMD results. The upstream shift of the DMD modes in the twin jet indicates stronger interactions between the instability waves and the shock cells, which generate stronger tone noise. The stronger interactions in turn alter the shock-cell length and the convective velocity of the instability waves and result in the frequency shift of the screech tone in the twin jet.

Simulation of viscous flows by highly accurate aeroacoustic schemes on regular grids

Highly accurate finite-difference schemes that are widely used in computational aeroacoustics and referred to as dispersion-relation-preserving (DRP) schemes are adapted for the simulation of viscous flows. The main result consists in the construction and verification of numerical boundary conditions on a solid body, artificial boundaries, and on their interface. Calculations are performed for several test problems, including dissipation of the Lamb–Oseen vortex and decay of the Taylor–Green vortex in two-and three-dimensional cases and in infinite and semi-infinite (open-outlet) plane channels.

Aerodynamic noise prediction of a MW-class HAWT using shear wind profile

In this paper, the results of aerodynamic and aeroacoustic calculation of a MW-class horizontal axes wind turbine (HAWT) are presented. Improved Delayed Detached Eddy Simulation (IDDES), as a hybrid LES-RANS turbulence model, is applied to simulate instantaneous flow field around the HAWT. The noise attenuation in the far field is estimated with the Ffowcs Williams and Hawking's acoustic analogy. Improved Blade Element Momentum (IBEM) theory and Brooks, Pope and Marcolini (BPM) semi-empirical methods also are implemented for wind turbine output power and aeroacoustic noise calculation. The semi-empirical and CFD methods are validated against experimental data of AOC 50/15 HAWT. The results show good agreement with experimental data for both IBEM-BPM and CFD techniques. The contribution of the thickness, loading, and quadrupole sources in far field noise propagation are investigated. Then, those methods are implemented for aerodynamic and aeroacoustic prediction of the NREL's 1.5 MW WP_Baseline HAWT. The Results obtained by CFD and IBEM-BPM techniques are compatible with each other for the 1.5 MW HAWT.

Quadrupole Corrections for the Permeable-Surface Ffowcs Williams–Hawkings Equation

This paper focuses on the spurious noise of the Ffowcs Williams and Hawkings equation with a permeable integral surface, which is now commonly used in far-field noise prediction for computational aeroacoustics methodologies. Spurious noise arises in the surface integral terms as an adverse effect of truncating a computational domain resolved for the quadrupole sources so that hydrodynamic fluctuations pass through the integral surface with mean flow. First, the quadrupole contribution is reformulated in the form of surface integrals for additive correction by introducing a frozen turbulence assumption for uniform mean flow. The math formulation is verified for simple convecting vortices. The derived formulation is then extended to nonuniform convection for locally subsonic flow. By adding the correction to the conventional Ffowcs Williams and Hawkings surface integrals, spurious noise is suppressed in two-dimensional wake flow. In a three-dimensional turbulent jet-noise simulation, the proposed correction, extended to nonuniform convection, is demonstrated to reduce the dependence of the prediction on the streamwise end-cap location of the integral surface. The present method requires no tunable parameters, with only a minimal additional cost compared to the conventional permeable-surface integrals in the far-field noise computation.

Acoustic multipole sources for the regularized lattice Boltzmann method: Comparison with multiple-relaxation-time models in the inviscid limit.

In this paper, a variant of the acoustic multipole source (AMS) method is proposed within the framework of the lattice Boltzmann method. A quadrupole term is directly included in the stress system (equilibrium momentum flux), and the dependency of the quadrupole source in the inviscid limit upon the fortuitous discretization error reported in the works of E. M. Viggen [Phys. Rev. E 87, 023306 (2013)PLEEE81539-375510.1103/PhysRevE.87.023306] is removed. The regularized lattice Boltzmann (RLB) method with this variant AMS method is presented for the 2D and 3D acoustic problems in the inviscid limit, and without loss of generality, the D3Q19 model is considered in this work. To assess the accuracy and the advantage of the RLB scheme with this AMS for acoustic point sources, the numerical investigations and comparisons with the multiple-relaxation-time (MRT) models and the analytical solutions are performed on the 2D and 3D acoustic multipole point sources in the inviscid limit, including monopoles, x dipoles, and xx quadrupoles. From the present results, the good precision of this AMS method is validated, and the RLB scheme exhibits some superconvergence properties for the monopole sources compared with the MRT models, and both the RLB and MRT models have the same accuracy for the simulations of acoustic dipole and quadrupole sources. To further validate the capability of the RLB scheme with AMS, another basic acoustic problem, the acoustic scattering from a solid cylinder presented at the Second Computational Aeroacoustics Workshop on Benchmark Problems, is numerically considered. The directivity pattern of the acoustic field is computed at r=7.5; the present results agree well with the exact solutions. Also, the effects of slip and no-slip wall treatments within the regularized boundary condition on this pure acoustic scattering problem are tested, and compared with the exact solution, the slip wall treatment can present a better result. All simulations demonstrate that the RLB scheme with the AMS method is capable of accurately simulating 2D and 3D acoustic generation, propagation, and scattering at zero viscosity.

An Efficiently Parallelized High-Order Aeroacoustics Solver Using a Characteristic-Based Multi-Block Interface Treatment and Optimized Compact Finite Differencing

This paper presents the development of a fourth-order finite difference computational aeroacoustics solver. The solver works with a structured multi-block grid domain strategy, and it has been parallelized efficiently by using an interface treatment based on the method of characteristics. More importantly, it extends the characteristic boundary condition developments of previous researchers by introducing a characteristic-based treatment at the multi-block interfaces. In addition, most characteristic methods do not satisfy Pfaff’s condition, which is a requirement for any mathematical relation to be valid. A mathematically-consistent and valid method is used in this work to derive the characteristic interface conditions. Furthermore, a robust and efficient approach for the matching of turbulence quantities at the multi-block interfaces is developed. Finally, the implementation of grid metric relations to minimise grid-induced errors has been adopted. The code was validated against a number of benchmark cases, which demonstrated its accuracy and robustness across a range of problem types.

Numerical characterization of landing gear aeroacoustics using advanced simulation and analysis techniques

With the objective of aircraft noise mitigation, we here address the numerical characterization of the aeroacoustics by a simplified nose landing gear (NLG), through the use of advanced simulation and signal processing techniques. To this end, the NLG noise physics is first simulated through an advanced hybrid approach, which relies on Computational Fluid Dynamics (CFD) and Computational AeroAcoustics (CAA) calculations. Compared to more traditional hybrid methods (e.g. those relying on the use of an Acoustic Analogy), and although it is used here with some approximations made (e.g. design of the CFD-CAA interface), the present approach does not rely on restrictive assumptions (e.g. equivalent noise source, homogeneous propagation medium), which allows to incorporate more realism into the prediction. In a second step, the outputs coming from such CFD-CAA hybrid calculations are processed through both traditional and advanced post-processing techniques, thus offering to further investigate the NLG's noise source mechanisms. Among other things, this work highlights how advanced computational methodologies are now mature enough to not only simulate realistic problems of airframe noise emission, but also to investigate their underlying physics.

On the computation of low-subsonic turbulent pipe flow noise with a hybrid LES/LPCE method

Aeroacoustic computation of a fully-developed turbulent pipe flow at Reτ = 175 and M = 0.1 is conducted by LES/LPCE hybrid method. The generation and propagation of acoustic waves are computed by solving the linearized perturbed compressible equations (LPCE), with acoustic source DP(x,t)/Dt attained by the incompressible large eddy simulation (LES). The computed acoustic power spectral density is closely compared with the wall shear-stress dipole source of a turbulent channel flow at Re τ = 175. A constant decaying rate of the acoustic power spectrum, f -8/5 is found to be related to the turbulent bursts of the correlated longitudinal structures such as hairpin vortex and their merged structures (or hairpin packets). The power spectra of the streamwise velocity fluctuations across the turbulent boundary layer indicate that the most intensive noise at ω + z R < 1.5).

A hybrid prediction for wind buffeting noises of vehicle rear window based on LES-LAA method

Based on the theories of computational fluid dynamics (CFD) and aeroacoustics, a hybrid simulation technique, the so-called LES-LAA method is proposed in this paper, for predicting the wind buffeting noises generated by opening rear windows of a running vehicle. The LES-LAA is developed by combining the large eddy simulation (LES) and the Lighthill acoustic analogy (LAA) methods. Based on the established vehicle and wind tunnel models, the wind buffeting noises from rear windows are predicted by using the proposed LES-LAA method and the obtained results are compared with experimental data. The results show that the calculation error of sound pressure level (SPL) from the LES-LAA method is less than 2%, which suggests that the proposed method has good accuracy in predicting the wind noise of the rear window of a vehicle. The wind noise when both sides of the rear window are opened at the same time is much lower than the case when just one window is opened. In conclusion, the hybrid LES-LAA technique presented in this paper is effective and feasible for predicting the wind buffeting noise, which can be applied to other types of vehicle and is a promising approach for solving other aero-acoustical engineering problems.

Higher-order multilevel framework for ADER scheme in computational aeroacoustics

The versatile Arbitrary-DERivative (ADER) scheme is cast in a multilevel framework (ML-ADER) for fast solution of system of linear hyperbolic partial differential equations. The solution is cycled through spatial operators of varying accuracy while retaining highest-order accuracy by the use of a forcing function. Accuracy analysis of the multilevel framework including in the ML-ADER form is carried out in time-domain as well as frequency-domain. Results are obtained for benchmark problems in computational aeroacoustics at a much reduced computational cost.

Prediction of jet mixing noise with Lighthill's Acoustic Analogy and geometrical acoustics.

A computational aeroacoustics prediction tool based on the application of Lighthill's theory is presented to compute noise from subsonic turbulent jets. The sources of sound are modeled by expressing Lighthill's source term as two-point correlations of the velocity fluctuations and the sound refraction effects are taken into account by a ray tracing methodology. Both the source and refraction models use the flow information collected from a solution of the Reynolds Averaged Navier-Stokes equations with a standard k-epsilon turbulence model. By adopting the ray tracing method to compute the refraction effects a high-frequency approximation is implied, while no assumption about the mean flow is needed, enabling the application of the method to jet noise problems with inherently three-dimensional propagation effects. Predictions show good agreement with narrowband measurements for the overall sound pressure levels and spectrum shape in polar angles between 60° and 110° for isothermal and hot jets with acoustic Mach number ranging from 0.5 to 1.0. The method presented herein can be applied as a relatively low cost and robust engineering tool for industrial optimization purposes.

A flow control technique for noise reduction of a rod-airfoil configuration

Adaptive and flow control techniques have been investigated as possible noise reduction approaches in modern commercial aircraft and small unmanned air vehicles. In the present paper, a rotating cylinder is examined as a noise reduction device in a simplified airframe component. The rod-airfoil canonical benchmark is used as a test case and noise prediction is realized by a 3D hybrid computational aeroacoustics approach. The rod is rotated at frequencies ranging from 0.5 to 2 times the natural shedding frequency of the nonrotating case. Evaluation of the directivity of generated noise around the rod and airfoil demonstrated that cylinder rotation leads to reduction of noise emissions, which became more pronounced for the highest rotational frequencies. Rod rotation also proved to be beneficial from an aerodynamic perspective, generally increasing the lift forces and reducing the drag forces acting on the rod and airfoil. Evaluation of the shedding Strouhal number displayed an ascending trend with increasing rotational frequency, which in turn resulted in a shift of the dominant acoustic tonal components towards higher frequencies. Moreover, rod rotation led to gradual suppression and deflection of the vortex street away from the symmetry plane. Since periodic vortex shedding is the dominant noise mechanism, vortex shedding suppression and minimization of the airfoil interaction with the vortex street is the cause of reduced acoustic emissions. The present study thus shows the potential of the rotating cylinder as a noise reduction device in aeronautical applications, while underlining its capabilities of enhancing aerodynamic performance.