Showing papers on "Computational aeroacoustics published in 2015"

Compressible-Flow DNS with Application to Airfoil Noise

Abstract: Airfoil self-noise is the noise produced by the interaction between an airfoil with its own boundary layers and wake. Self-noise is of concern as it is an important contributor to the overall noise in many applications, e.g. wind turbines, cooling fan blades, or air frames, to name a few. The continued growth of available computing power has made direct numerical simulations (DNS) of compressible flows with application to airfoil noise possible. Challenges associated with such simulations and numerical details of a DNS code that is able to exploit modern high-performance computing systems are presented. Results obtained from DNS of flow over NACA-0012 airfoils at moderate Reynolds number are used to evaluate the accuracy of approximations commonly made in deriving trailing edge noise theories. The data are also used to identify additional noise sources present in airfoil configurations. Finally, DNS are employed to study the noise reducing effect of trailing-edge noise serrations, indicating that the noise reduction is mainly due to a changed scattering mechanisms and not a change in the incidence turbulence field.

A hybrid approach to the computational aeroacoustics of human voice production

Abstract: The aeroacoustic mechanisms in human voice production are complex coupled processes that are still not fully understood. In this article, a hybrid numerical approach to analyzing sound generation in human voice production is presented. First, the fluid flow problem is solved using a parallel finite-volume computational fluid dynamics (CFD) solver on a fine computational mesh covering the larynx. The CFD simulations are run for four geometrical configurations: both with and without false vocal folds, and with fixed convergent or convergent–divergent motion of the medial vocal fold surface. Then the aeroacoustic sources and propagation of sound waves are calculated using Lighthill’s analogy or acoustic perturbation equations on a coarse mesh covering the larynx, vocal tract, and radiation region near the mouth. Aeroacoustic sound sources are investigated in the time and frequency domains to determine their precise origin and correlation with the flow field. The problem of acoustic wave propagation from the larynx and vocal tract into the free field is solved using the finite-element method. Two different vocal-tract shapes are considered and modeled according to MRI vocal-tract data of the vowels /i/ and /u/. The spectra of the radiated sound evaluated from acoustic simulations show good agreement with formant frequencies known from human subjects.

Evaluation of Airframe Noise Reduction Concepts via Simulations Using a Lattice Boltzmann Approach

Abstract: Unsteady computations are presented for a high-fidelity, 18% scale, semi-span Gulfstream aircraft model in landing configuration, i.e. flap deflected at 39 degree and main landing gear deployed. The simulations employ the lattice Boltzmann solver PowerFLOW® to simultaneously capture the flow physics and acoustics in the near field. Sound propagation to the far field is obtained using a Ffowcs Williams and Hawkings acoustic analogy approach. In addition to the baseline geometry, which was presented previously, various noise reduction concepts for the flap and main landing gear are simulated. In particular, care is taken to fully resolve the complex geometrical details associated with these concepts in order to capture the resulting intricate local flow field thus enabling accurate prediction of their acoustic behavior. To determine aeroacoustic performance, the farfield noise predicted with the concepts applied is compared to high-fidelity simulations of the untreated baseline configurations. To assess the accuracy of the computed results, the aerodynamic and aeroacoustic impact of the noise reduction concepts is evaluated numerically and compared to experimental results for the same model. The trends and effectiveness of the simulated noise reduction concepts compare well with measured values and demonstrate that the computational approach is capable of capturing the primary effects of the acoustic treatment on a full aircraft model.

Single velocity-component modeling of leading edge turbulence interaction noise.

Abstract: A computational aeroacoustics approach is used to predict leading edge turbulence interaction noise for real airfoils. One-component (transverse), two-component (transverse and streamwise), and three-component (transverse, streamwise, and spanwise) synthesized turbulence disturbances are modeled instead of harmonic transverse gusts, to which previous computational studies of leading edge noise have often been confined. The effects of the inclusion of streamwise and spanwise disturbances on the noise are assessed. It is shown that accurate noise predictions can be made by modeling only transverse disturbances which reduces the computational expense of simulations. The accuracy of using only transverse disturbances is assessed for symmetric and cambered airfoils, and also for airfoils at non-zero angle of attack.

Numerical Predictions of Turbulence/Cascade-Interaction Noise Using Computational Aeroacoustics with a Stochastic Model

Abstract: Turbulent-flow interactions with the outlet guide vanes are known to mainly contribute to broadband-noise emission of aeroengines at approach conditions. This paper presents a three-dimensional computational aeroacoustics hybrid method aiming at simulating the aeroacoustic response of an annular cascade impacted by a prescribed homogeneous isotropic turbulent flow. It is based on a time-domain Euler solver coupled to a synthetic turbulence model implemented in the code by means of a suited inflow boundary condition. The fluctuating pressure over the airfoil surface provided by computational aeroacoustics is used as an input to a Ffowcs Williams and Hawkings integral method to calculate the radiated sound field. Euler computations are first validated against an academic computational aeroacoustics benchmark in the case of an harmonic gust interacting with an annular flat-plate cascade. Then, simulations are applied to turbulence–cascade interactions for annular configurations, in uniform and swirling mean ...

Efficient numerical simulation of aeroacoustics for low Mach number flows interacting with structures

Abstract: An integrated hybrid approach for the numerical simulation of aeroacoustics at low Mach numbers is presented. The method is based on a viscous/acoustic splitting. The turbulent incompressible background flow is computed with large eddy simulation, based on the incompressible Navier-Stokes equations, whereas the acoustics are computed from linearized Euler equations with a high-resolution scheme. The focus is on the numerical efficiency of the approach. To accelerate the computations, hierarchical grids and a frozen fluid approach for the acoustics are employed and investigated. For validation and the investigation of the numerical efficiency and accuracy the sound emission of a plate in the turbulent wake of a circular cylinder is employed as a test case.

Large-Scale Studies on Slat Noise Reduction

Abstract: Slat noise research activity within the EC co-financed project OPENAIR involved both experimental and numerical studies at the new large (1:3.3-scaled) swept high-lift wing model F15LS. Experiments were performed in the DNW-LLF (Large Low-speed Facility) to verify the noise reduction benefit of selected slat noise reduction concepts under more realistic test conditions than in precursor projects. Moreover, the gained test data served to extend current slat noise validation datasets towards larger Reynolds numbers, i.e. up to Re = 5.1 Mio. in the current experiment. Slat noise reduction concepts under review were 1) slat gap/overlap setting variations, and 2) an adaptive slat with the potential to reduce the gap width for noise reduction. Both concepts were proven highly e�fficient: Sealing of the gap leads to a maximum 5-dB noise reduction at wing level, equivalent to a full elimination of the slat noise source. Optimized slat settings or an adaptive slat with partially closed gap are suited to reduce slat noise by about 2-3 dB at wing level while producing negligible aerodynamic impact at the operative test angles of attack within the linear polar region. When transposing these results to overall aircraft flight conditions, optimized slat settings might bring about a 0.5-EPNdB reduction of approach certification noise levels, provided all other relevant noise sources than the slat remain untreated. CAA (Computational Aeroacoustics) prediction results derived with DLR's PIANO and DISCO codes coupled with RANS-based stochastic source models revealed a generally good reproduction of the measured trends.

Implementation of a Sharp Immersed Boundary Method in a 3-D Multi-block Large Eddy Simulation Tool for Jet Aeroacoustics

Abstract: Experimentation with different nozzle geometries has gained importance in recent years for reduction of jet noise. These geometrical complexities pose a significant challenge to the computational studies of these nozzles, especially when structured grids are employed with high-order numerical methods. In the present work, a ghost-point-based immersed boundary method is implemented in a highly-scalable, multi-block, high-order, structuredgrid large eddy simulation tool to tackle this problem. The implementation is not limited to using uniformly-spaced and/or Cartesian grids as background grids, and therefore allows an efficient grid point distribution, which is imperative for high-Reynolds number jet noise simulations. Three test cases selected to assess specific aspects of the implementation are presented. The impact of the immersed boundary method on order of accuracy of the base solver is quantified. The method is demonstrated to perform well in predicting scattering of acoustic waves from a solid cylinder, and in producing realistic low-Reynolds number viscous flow over a solid sphere.

Hybrid Discontinuous Galerkin and Finite Volume Method for Launch Environment Acoustics Prediction

Abstract: Launch vehicles experience extreme acoustic loads during liftoff driven by the interaction of rocket plumes and plume-generated acoustic waves with ground structures. Currently employed predictive capabilities to model the complex turbulent plume physics are too dissipative to accurately resolve the propagation of acoustic waves throughout the launch environment. Higher fidelity liftoff acoustic analysis tools to design mitigation measures are critically needed to optimize launch pads for the Space Launch System and commercial launch vehicles. To this end, a new coupled two-field simulation capability has been developed to enable accurate prediction of liftoff acoustic physics. Established unstructured computational fluid dynamics algorithms are used for simulation of acoustic generation physics and a high-order-accurate discontinuous Galerkin nonlinear Euler solver is employed to accurately propagate acoustic waves across large distances. An innovative hybrid computational fluid dynamics/computational ae...

Prediction of Porous Trailing Edge Noise Reduction via Acoustic Perturbation Equations and Volume Averaging

Abstract: Edge noise is generated if turbulence interacts with solid edges. Reduction of trailing edge noise of airfoils can be achieved by replacing the solid material at the trailing edge by inlays of porous permeable material. The acoustic benefit of approximately 6 dB of such treatment is known from experiments. Enroute to numerically optimized porous properties, this paper presents a first principle based Computational Aeroacoustics (CAA) method for predicting the acoustic effect of a porous NACA0012 trailing edge. In a hybrid two-step CFD/CAA procedure the turbulence statistics from a solution of the Volume Averaged Navier-Stokes (VANS) equations is used as a basis for the prediction of turbulent-boundary-layer trailing-edge noise (TBL-TEN). For the acoustic part of the calculation, the Acoustic Perturbation Equations (APE) are solved in the flow field. Inside the porous regions, a different set of governing equations, referred to as Linear Perturbation Equations (LPE) will be solved. The LPE represent a modified form of the Linearized Euler Equations (LEE) with the APE vorticity source term shifted to the right-hand side. The new set of equations is derived by volume averaging the Navier-Stokes equations and decomposing the flow variables into a time-averaged mean part and a fluctuating part and isolating the vorticity source term to the right-hand side of the momentum equation. The LPE are verified by an analytical solution. The simulation results of a NACA0012 airfoil geometry with and without porous trailing edge treatment are compared to wind tunnel measurements. The noise reduction effect of such a trailing edge treatment is successfully demonstrated.

Overset DNS with Application to Sound Source Prediction

Abstract: In this contribution, we present an application of a computational aeroacoustics code as a hybrid Zonal DNS tool. The extension of the Non-Linear Perturbation Equations (NLPE) with viscous terms is presented as well as information related to the numerical method. The applicability of the simulation tool is illustrated with two testcases, i.e., a 2D circular cylinder in a uniform flow at moderate Reynolds numbers and a 3D decaying flow initialised with Taylor-Green vortices. Both testcases provide results which match well with data reported in literature. The cylinder testcase verifies that the viscous terms are indeed correctly implemented (at least in 2D) and the Taylor-Green vortex case illustrates that the numerical scheme introduced minimal numerical dissipation.

Numerical Algorithm for Computing Acoustic and Vortical Spatial Instability Waves

Abstract: Local linear stability is often invoked in computational aeroacoustics to predict Mach wave radiation or to prescribe inflow conditions in order to drive turbulent transition in large-eddy simulations. In this work, the governing equations are reformulated for the nonoscillatory part of eigenfunctions. Boundary conditions can thus be explicitly enforced and, moreover, the numerical cost is drastically reduced regardless of the method chosen to solve this problem. An efficient method based on a matrix formulation is proposed in this study. One single, small collocation domain is used, even for computing the stability of supersonic flows. Vortical and acoustic instability waves of supersonic plane jets are briefly revisited to demonstrate the efficiency of this new approach.

Numerical Study on the Flow over a Simplified Vehicle Door Gap – an Old Benchmark Problem Is Revisited

Abstract: A simplified automobile door gap model, defined at the Third Computational Aeroacoustics (CAA) Workshop on Benchmark Problems (Category 6) was investigated. After a thorough mesh study compressible and incompressible simulations were carried out and various turbulence models were tried. The influence of three dimensional effects and boundary layer thickness effects were examined too. In case of compressible simulations stability problems were encountered with the non-reflective boundary condition of CFX (beta version at the time of the simulations). It was found that for deep cavities incompressible simulations are not applicable. In spite of the diculties a good agreement between measurements and simulations was found when the flow speed was 50m/s. In case of 26.8m/s flow speed it was found that the presence of the upper channel wall, not taken into account by the previous authors simulating this problem plays an important role – a hitherto unexplained peak in the measured spectrum appeared this way in the simulation.

Predictions of Fan Broadband Noise Using Lifting Surface Method

Abstract: This paper adopts the three-dimensional lifting surface method as a new cascade-response function to predict fan broadband interaction noise. The stator is modeled as an annular cascade, and the vanes are assumed to be zero-thickness plates with no stagger angle. The unsteady loading spectrum on the stator vanes is considered under a fully three-dimensional condition rather than calculated by the strip theories that are widely used in the existing analytical broadband fan noise models. The sound power spectrum is calculated in an annular duct using numerical integration weighted by statistical turbulence spectrum. The present theory is verified by calculating the category 4 benchmark problem of the Third Computational Aeroacoustics Workshop for the response function and by comparing the predictions of a benchmark test for the broadband formulations of the sound power spectra. Then, the predictions of the inlet and exhaust sound power spectra for the NASA source diagnostic test are presented, which both sh...

Variability in the Propagation Phase of CFD-Based Noise Prediction: Summary of Results From Category 8 of the BANC-III Workshop

Abstract: The usage of Computational Fluid Dynamics (CFD) in noise prediction typically has been a two part process: accurately predicting the flow conditions in the near-field and then propagating the noise from the near-field to the observer. Due to the increase in computing power and the cost benefit when weighed against wind tunnel testing, the usage of CFD to estimate the local flow field of complex geometrical structures has become more routine. Recently, the Benchmark problems in Airframe Noise Computation (BANC) workshops have provided a community focus on accurately simulating the local flow field near the body with various CFD approaches. However, to date, little effort has been given into assessing the impact of the propagation phase of noise prediction. This paper includes results from the BANC-III workshop which explores variability in the propagation phase of CFD-based noise prediction. This includes two test cases: an analytical solution of a quadrupole source near a sphere and a computational solution around a nose landing gear. Agreement between three codes was very good for the analytic test case, but CFD-based noise predictions indicate that the propagation phase can introduce 3dB or more of variability in noise predictions.

Influence of Spanwise Boundary Conditions on Slat Noise Simulations

Abstract: The slat noise from the 30P/30N high-lift system is being investigated through computational fluid dynamics simulations with the OVERFLOW code in conjunction with a Ffowcs Williams-Hawkings acoustics solver. In the present study, two different spanwise grids are being used to investigate the effect of the spanwise extent and periodicity on the near-field unsteady structures and radiated noise. The baseline grid with periodic boundary conditions has a short span equal to 1/9th of the stowed chord, whereas the other, longer span grid adds stretched grids on both sides of the core, baseline grid to allow inviscid surface boundary conditions at both ends. The results indicate that the near-field mean statistics obtained using the two grids are similar to each other, as are the directivity and spectral shapes of the radiated noise. However, periodicity forces all acoustic waves with less than one wavelength across the span to be two-dimensional, without any variation in the span. The spanwise coherence of the acoustic waves is what is needed to make estimates of the noise that would be radiated from realistic span lengths. Simulations with periodic conditions need spans of at least six slat chords to allow spanwise variation in the low-frequencies associated with the peak of broadband slat noise. Even then, the full influence of the periodicity is unclear, so employing grids with a fine, central region and highly stretched meshes that go to slip walls may be a more efficient means of capturing the spanwise decorrelation of low-frequency acoustic phenomena.

Implementation of High-Order Discontinuous Galerkin Method for Solution of Practical Tasks in External Aerodynamics and Aeroacoustics

Abstract: For solution of 3D stationary RANS equations, closed by EARSM turbulence model, high order Discontinuous Galerkin method (degree of basic polynomials K = 2, 3 with 1st order implicit smoother is proposed. Modifications, which were introduced in the method to achieve stability and fast convergence, are described. The method is enhanced by the use of improved Gauss quadrature rules and by the use of improved Gauss quadrature rules and of h − p multigrid multigrid acceleration and is implemented into NUMECA FINETM/Hexa code in version for massive parallel calculations. For solution of 3D nonstationary Isentropic Linearized Euler Equations within the perturbation approach in aeroacoustics, explicit high order Discontinuous Galerkin method is implemented. Calculations of various tests, including U3, U2 and A14, demonstrate the efficiency of developed methods.

Numerical Investigation of the Refraction Effects by Jet Flows in Anechoic Wind Tunnels, with Application to NASA/LaRC Quiet Flow Facility

Abstract: In regard to the problem of aircraft noise mitigation, the present study focuses on the refraction effects to be possibly induced by anechoic facility jet flows on the measured acoustic signatures, during typical airframe noise experiments. To this end, numerical simulations are conducted, enabling the characterization of the refraction effects induced by a typical open-jet, anechoic wind tunnel, namely the NASA Langley’s Quiet Flow Facility (QFF). Computations are conducted using two complementary approaches, that is, Computational AeroAcoustics (CAA) relying on the Perturbed Euler Equations (PEE) and Geometrical Acoustics (GA) based on a Ray Tracing (RT) technique. These complementary CAA/PEE and GA/RT simulations allow exploring a wide range of canonical situations, that is, comprising jet flows of various natures (planar vs. round, parallel vs. spreading, etc.) and acoustic sources of diverse characteristics (frequency, location, etc.). The analysis of numerical results highlights the respective effects by the flow features, delivering insights about what are the key parameters (jet characteristics, source-to-jet aspect ratio, etc.) to play a major role in the refraction phenomena by the facility jet flow.

Optimized Explicit Dissipation Operators for Computational Aeroacoustics

Abstract: The field of Computational Aeroacoustics (CAA) is focused on the accurate simulation of unsteady flow and noise (see Refs.1−4 for reviews of the progress made and challenges for computational aeroacoustics). To achieve this goal, it is necessary to accurately simulate the linear and nonlinear propagation of disturbances as they move, which requires the use of high-accuracy numerical schemes in space and time. These high-accuracy schemes are often optimized for improved wave propagation accuracy with few grid points per wavelength of a simple harmonic wave. The optimization methods for wave propagation are designed to minimize the magnitude of the error in the predicted wave speed, allowing the waves to propagate either faster or slower than the exact value. At some point as the number of grid points per wavelength decreases, the numerical scheme will not adequately predict the unsteady dynamics of the flow. This will result in excessive errors in wave speed, viscous damping, and nonlinear wave interactions. The presence of these unresolved disturbances will reduce the accuracy of the overall solution. Thus, it is desirable to selectively remove unresolved disturbances from the flow solution without altering the resolved flow disturbances. This is the goal of artificial dissipation. Artificial dissipation is a nonphysical term added to the governing equations, designed to damp unresolved disturbances from the solution. Just as numerical schemes are

High-Order Hybrid Cell-Centered Method for Computational Aeroacoustics

Abstract: High-order cell-vertex finite difference schemes applied to multi-block structured grids are used widely in computational aeroacoustics for their low-dispersion and low-dissipation properties. Structured grids for complex geometries may contain discontinuous grid metrics at multi-block interfaces. In this work it is demonstrated that the grid-induced errors from such interfaces can be reduced by applying finite difference schemes in the cell-centered space. Further reduction of these grid-induced errors can be achieved by applying an additional finite volume method, which serves as an interface condition. In this paper, the development of a hybrid cell-centered finite difference and finite volume method is demonstrated. An interpolation scheme is derived from a high-order finite difference scheme to apply the finite volume method at interfaces. The order of accuracy of this hybrid method is demonstrated and the method is used to simulate the flow around a single cylinder, tandem cylinders, and a complex isolated wheel. Comparisons with experimental measurements and numerical predictions show that the hybrid method can provide accurate results at block interfaces and can be applied to high-order simulations of complex geometries.

Open-Rotor Noise Assessment with CFD/CAA Chaining

Abstract: A numerical strategy based upon a coupling of Computational Fluid Dynamics (CFD) and Computational AeroAcoustics (CAA) solvers is set up for open-rotor noise predictions. The method simulates the near field noise, and is possibly followed by an integral method calculation which leads to the far field noise. The goal of such a method is to take into account the mean flow and acoustical installation effects that a chaining restricted to the CFD and the integral method cannot predict at low computational costs. The chaining is assessed on the Airbus Clean Sky generic open rotor configuration, first on the front rotor only, as an isolated single propeller noise evaluation, second on the isolated CROR. In both configurations, the CFD/CAA results relying on freestream uniform flow are first compared to the CFD/Integral method ones for validation. Then, the chaining is performed considering the mean background flow, allowing to observe the effects of the latter on acoustics. In the case of the CROR, these results are compared to experimental measurements. Results are believed to be sensitive to the data injection into CAA, with more or less effects according to the injection technique.

Airfoil Self-Noise - Investigation with Particle Image Velocimetry

Abstract: Noise generated aerodynamically by the airflow over a lifting surface is often of concern for applications as diverse as air and ground transportation, heating, ventilation, air-conditioning systems, and wind turbines. The thesis describes the application of advanced optical flow measurements techniques for the visualization and description of the sources of sound on airfoils. These measurement techniques include high-speed stereoscopic and tomographic Particle Image Velocimetry (PIV) together with advanced methods for post-processing to obtain a representation of the aeroacoustic source field. Following this innovative and evolving approach in experimental aeroacoustics, a novel methodology for broadband trailing-edge noise diagnostics by tomographic PIV is proposed. Moreover, on the basis of simultaneous high-speed PIV and acoustic measurements, new and fundamental insights into the mechanism of tonal noise generation due to the interaction of amplified laminar boundary layer instability waves with the trailing edge are presented. Both examples demonstrate the potential impact of advanced PIV methods on present and future research in experimental aeroacoustics.