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  • 1
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    AN EFFICIENT, TWO-DIMENSIONAL IMPLEMENTATION OF THE FFOWCS WILLIAMS AND HAWKINGS EQUATION (2000)
    Elsevier
    Details
    Publication Date: 2000-01-01
    Print ISSN: 0022-460X
    Electronic ISSN: 1095-8568
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics , Physics
    Published by Elsevier
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  • 2
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    An Overview of Computational Aeroacoustic Modeling at NASA Langley (2001)
    In:  CASI
    Details
    Publication Date: 2016-06-07
    Description: The use of computational techniques in the area of acoustics is known as computational aeroacoustics and has shown great promise in recent years. Although an ultimate goal is to use computational simulations as a virtual wind tunnel, the problem is so complex that blind applications of traditional algorithms are typically unable to produce acceptable results. The phenomena of interest are inherently unsteady and cover a wide range of frequencies and amplitudes. Nonetheless, with appropriate simplifications and special care to resolve specific phenomena, currently available methods can be used to solve important acoustic problems. These simulations can be used to complement experiments, and often give much more detailed information than can be obtained in a wind tunnel. The use of acoustic analogy methods to inexpensively determine far-field acoustics from near-field unsteadiness has greatly reduced the computational requirements. A few examples of current applications of computational aeroacoustics at NASA Langley are given. There remains a large class of problems that require more accurate and efficient methods. Research to develop more advanced methods that are able to handle the geometric complexity of realistic problems using block-structured and unstructured grids are highlighted.
    Keywords: Acoustics
    Type: The Tenth Thermal and Fluids Analysis Workshop; NASA/CP-2001-211141
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  • 3
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    In Search of the Physics: NASA's Approach to Airframe Noise (1999)
    In:  CASI
    Details
    Publication Date: 2018-06-02
    Description: An extensive numerical and experimental study of airframe noise mechanisms associated with a subsonic high-lift system has been performed at NASA Langley Research Center (LaRC). Investigations involving both steady and unsteady computations and experiments on small-scale models with part-span flaps and full-span flaps are presented. Both surface (steady and unsteady pressure measurements, hot films, oil flows, pressure sensitive paint) and off-surface (5 holeprobe, particle-imaged velocimetry, laser velocimetry, laser light sheet measurements) were taken in the LaRC Quiet Flow Facility (QFF) and several hard-wall tunnels. Experiments in the Low Turbulence Pressure Tunnel (LTPT) included Reynolds number variations up to flight conditions. Successful microphone array measurements were also taken providing both acoustic source maps on the model, and quantitative spectra. Critical directivity measurements were obtained in the QFF. NASA Langley unstructured and structured Reynolds-Averaged Navier-Stokes codes modeled the steady aspects of the flows. Excellent comparisons with surface and off-surface experimental data were obtained. Subsequently, these meanflow calculations were utilized in both linear stability and direct numerical simulations of the flow fields to calculate unsteady surface pressures and farfield acoustic spectra. Accurate calculations were critical in obtaining not only noise source characteristics, but shear layer correction data as well. Techniques utilized in these investigations as well as brief overviews of the results are given.
    Keywords: Aircraft Design, Testing and Performance
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  • 4
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    An Application of the Quadrature-Free Discontinuous Galerkin Method (2000)
    In:  CASI
    Details
    Publication Date: 2017-10-02
    Description: The process of generating a block-structured mesh with the smoothness required for high-accuracy schemes is still a time-consuming process often measured in weeks or months. Unstructured grids about complex geometries are more easily generated, and for this reason, methods using unstructured grids have gained favor for aerodynamic analyses. The discontinuous Galerkin (DG) method is a compact finite-element projection method that provides a practical framework for the development of a high-order method using unstructured grids. Higher-order accuracy is obtained by representing the solution as a high-degree polynomial whose time evolution is governed by a local Galerkin projection. The traditional implementation of the discontinuous Galerkin uses quadrature for the evaluation of the integral projections and is prohibitively expensive. Atkins and Shu introduced the quadrature-free formulation in which the integrals are evaluated a-priori and exactly for a similarity element. The approach has been demonstrated to possess the accuracy required for acoustics even in cases where the grid is not smooth. Other issues such as boundary conditions and the treatment of non-linear fluxes have also been studied in earlier work This paper describes the application of the quadrature-free discontinuous Galerkin method to a two-dimensional shear layer problem. First, a brief description of the method is given. Next, the problem is described and the solution is presented. Finally, the resources required to perform the calculations are given.
    Keywords: Theoretical Mathematics
    Type: Third Computational Aeroacoustics (CAA) Workshop on Benchmark Problems; 331-337; NASA/CP-2000-209790
    Format: text
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  • 5
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    A Hybrid Approach To Tandem Cylinder Noise (2004)
    In:  CASI
    Details
    Publication Date: 2018-06-06
    Description: Aeolian tone generation from tandem cylinders is predicted using a hybrid approach. A standard computational fluid dynamics (CFD) code is used to compute the unsteady flow around the cylinders, and the acoustics are calculated using the acoustic analogy. The CFD code is nominally second order in space and time and includes several turbulence models, but the SST k - omega model is used for most of the calculations. Significant variation is observed between laminar and turbulent cases, and with changes in the turbulence model. A two-dimensional implementation of the Ffowcs Williams-Hawkings (FW-H) equation is used to predict the far-field noise.
    Keywords: Acoustics
    Type: Fourth Computational Aeroacoustics (CAA) Workshop on Benchmark Problems; 235-240; NASA/CP-2004-212954
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  • 6
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    Frequency Domain Calculations Of Acoustic Propagation (2004)
    In:  CASI
    Details
    Publication Date: 2018-06-06
    Description: Two complex geometry problems are solved using the linearized Euler equations. The impedance mismatch method1 is used to impose the solid surfaces without the need to use a body-fitted grid. The problem is solved in the frequency domain to avoid long run times. Although the harmonic assumption eliminates all time dependence, a pseudo-time term is added to allow conventional iterative methods to be employed. A Jameson type, Runge-Kutta scheme is used to advance the solution in pseudo time. The spatial operator is based on a seven-point, sixth-order finite difference. Constant coefficient, sixth-derivative artificial dissipation is used throughout the domain. A buffer zone technique employing a complex frequency to damp all waves near the boundaries is used to minimize reflections. The results show that the method is capable of capturing the salient features of the scattering, but an excessive number of grid points are required to resolve the phenomena in the vicinity of the solid bodies because the wavelength of the acoustics is relatively short compared with the size of the bodies. Smoothly transitioning into the immersed boundary condition alleviates the difficulties, but a fine mesh is still required.
    Keywords: Acoustics
    Type: Fourth Computational Aeroacoustics (CAA) Workshop on Benchmark Problems; 291-296; NASA/CP-2004-212954
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  • 7
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    Comparison of Computational and Experimental Microphone Array Results for an 18% Scale Aircraft Model (2017)
    In:  CASI
    Details
    Publication Date: 2019-06-11
    Description: An 18% scale semispan model is used as a platform for examining the efficacy of microphone array processing using synthetic data from numerical simulations. Two hybrid Reynolds-Averaged-Navier-Stokes/Large-Eddy-Simulation (RANS/LES) codes coupled with Ffowcs WilliamsHawkings solvers are used to calculate 97 microphone signals at the locations of an array employed in the NASA Langley Research Center 14 22 tunnel. Conventional, DAMAS, and CLEAN-SC array processing is applied in an identical fashion to the experimental and computational results for three different configurations involving deploying and retracting the main landing gear and a part-span flap. Despite the short time records of the numerical signals, the beamform maps are able to isolate the noise sources, and the appearance of the DAMAS synthetic array maps is generally better than those from the experimental data. The experimental CLEAN-SC maps are similar in quality to those from the simulations indicating that CLEAN-SC may have less sensitivity to background noise. The spectrum obtained from DAMAS processing of synthetic array data is nearly identical to the spectrum of the center microphone of the array, indicating that for this problem array processing of synthetic data does not improve spectral comparisons with experiment. However, the beamform maps do provide an additional means of comparison that can reveal differences that cannot be ascertained from spectra alone.
    Keywords: Acoustics
    Type: NF1676L-26125 , International Journal of Aeroacoustics (ISSN 1475-472X); 16; 4-5; 358-381
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  • 8
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    Efficient Implementations of the Quadrature-Free Discontinuous Galerkin Method (1999)
    In:  CASI
    Details
    Publication Date: 2019-07-13
    Description: The efficiency of the quadrature-free form of the dis- continuous Galerkin method in two dimensions, and briefly in three dimensions, is examined. Most of the work for constant-coefficient, linear problems involves the volume and edge integrations, and the transformation of information from the volume to the edges. These operations can be viewed as matrix-vector multiplications. Many of the matrices are sparse as a result of symmetry, and blocking and specialized multiplication routines are used to account for the sparsity. By optimizing these operations, a 35% reduction in total CPU time is achieved. For nonlinear problems, the calculation of the flux becomes dominant because of the cost associated with polynomial products and inversion. This component of the work can be reduced by up to 75% when the products are approximated by truncating terms. Because the cost is high for nonlinear problems on general elements, it is suggested that simplified physics and the most efficient element types be used over most of the domain.
    Keywords: Numerical Analysis
    Type: AIAA Paper 99-3309 , 14th AIAA CFD Conference; Jun 28, 1999 - Jul 01, 1999; Norfolk, VA; United States
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  • 9
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    Computational Aeroacoustic Analysis of Slat Trailing-Edge Flow (2000)
    In:  CASI
    Details
    Publication Date: 2019-07-13
    Description: An acoustic analysis based on the Ffowcs Williams and Hawkings equation was performed for a high-lift system. As input, the acoustic analysis used un- steady flow data obtained from a highly resolved, time-dependent, Reynolds-averaged Navier-Stokes calculation. The analysis strongly suggests that vor- tex shedding from the trailing edge of the slat results in a high-amplitude, high-frequency acoustic signal, similar to that which was observed in a correspond- ing experimental study of the high-lift system.
    Keywords: Acoustics
    Type: AIAA Paper 99-1802 , AIAA/CEAS Aeroacoustics Conference; May 10, 1999 - May 12, 1999; Greater Seattle, WA; United States
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  • 10
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    Simulation of Acoustic Scattering from a Trailing Edge (1999)
    In:  CASI
    Details
    Publication Date: 2019-07-13
    Description: Three model problems were examined to assess the difficulties involved in using a hybrid scheme coupling flow computation with the the Ffowcs Williams and Hawkings equation to predict noise generated by vortices passing over a sharp edge. The results indicate that the Ffowcs Williams and Hawkings equation correctly propagates the acoustic signals when provided with accurate flow information on the integration surface. The most difficult of the model problems investigated inviscid flow over a two-dimensional thin NACA airfoil with a blunt-body vortex generator positioned at 98 percent chord. Vortices rolled up downstream of the blunt body. The shed vortices possessed similarities to large coherent eddies in boundary layers. They interacted and occasionally paired as they convected past the sharp trailing edge of the airfoil. The calculations showed acoustic waves emanating from the airfoil trailing edge. Acoustic directivity and Mach number scaling are shown.
    Keywords: Aircraft Propulsion and Power
    Type: AIAA Paper 99-0231 , Aerospace Sciences Meeting and Exhibit; Jan 11, 1999 - Jan 14, 1999; Reno, NV; United States
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