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    Towards Full Aircraft Airframe Noise Prediction: Detached Eddy Simulations

    Mehdi R. Khorrami* NASA Langley Research Center, Hampton, Virginia, 23681

    and

    Raymond E. Mineck† Yorktown, Virginia 23692

    Results from a computational study on the aeroacoustic characteristics of an 18%-scale, semi-span Gulf- stream aircraft model are presented in this paper. NASA’s FUN3D unstructured compressible Navier-Stokes solver was used to perform steady and unsteady simulations of the flow field associated with this high-fidelity aircraft model. Solutions were obtained for free-air at a Mach number of 0.2 with the flap deflected at 39º, with the main gear off and on (the two baseline configurations). Initially, the study focused on accurately predicting the prominent noise sources at both flap tips for the baseline configuration with deployed flap on- ly. Building upon the experience gained from this initial effort, subsequent work involved the full landing configuration with both flap and main landing gear deployed. For the unsteady computations, we capitalized on the Detached Eddy Simulation capability of FUN3D to capture the complex time-dependent flow features associated with the flap and main gear. To resolve the noise sources over a broad frequency range, the tai- lored grid was very dense near the flap inboard and outboard tips and the region surrounding the gear. Ex- tensive comparison of the computed steady and unsteady surface pressures with wind tunnel measurements showed good agreement for the global aerodynamic characteristics and the local flow field at the flap inboard tip. However, the computed pressure coefficients indicated that a zone of separated flow that forms in the vicinity of the outboard tip is larger in extent along the flap span and chord than measurements suggest. Computed farfield acoustic characteristics from a FW-H integral approach that used the simulated pressures on the model solid surface were in excellent agreement with corresponding measurements.

    Nomenclature AOA = angle-of-attack CL = lift coefficient Cp = pressure coefficient c = local chord DES = detached eddy simulation DDES = delayed detached eddy simulation FW-H = Ffowcs Williams-Hawkings GAC = Gulfstream Aerospace Corporation Hz = Hertz, cycles per second M = Mach number MDDES = modified delayed detached eddy simulation PSD = power spectral density in psi2/Hz psi = pounds per square inch RANS = Reynolds-Averaged Navier-Stokes Re = Reynolds number RMS = root mean square r = radial distance s = seconds X, Y, Z = right handed coordinate system

    * Aerospace Engineer, Computational AeroSciences Branch, Associate Fellow AIAA † Consultant

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    20th AIAA/CEAS Aeroacoustics Conference

    16-20 June 2014, Atlanta, GA

    AIAA 2014-2480

    AIAA Aviation

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    I. Introduction Aircraft noise is an environmental hazard that affects metropolitan areas adjacent to major airports. With the pro-

    jected growth in air travel, noise exposure levels are expected to increase substantially. NASA’s Environmentally Responsible Aviation (ERA) project is involved with the development, evaluation, and maturation of novel noise reduction technologies that aim to confine aircraft noise footprints on the ground within airport boundaries. This is to be accomplished without detrimental effects to the aerodynamic efficiency of the aircraft. To most people, aircraft noise is synonymous with jet engine noise. During approach to landing, however, noise generated by the airframe is comparable to, and in some instances even louder than, propulsion noise. Acoustic measurements from flight tests1-6 and model-scale experiments7-8 have identified the landing gear and lift enhancing devices, such as wing slats and flaps, as the dominant airframe noise sources. Efficient design and development of viable noise reduction technolo- gies demand the availability of high-fidelity simulation tools with accurate predictive capabilities. Ultimately, the goal of creating such tools is to promote a paradigm shift in the design procedure from the current time-consuming and costly “cut-and-try” approach to a physics-based virtual design environment whereby the aeroacoustic evalua- tion of a noise reduction concept and its subsequent optimization can take place in an integrated fashion.

    As part of the NASA-Gulfstream partnership on airframe noise research, a series of flight tests and model scale experiments is being performed with a Gulfstream aircraft as the baseline configuration. An 18% scale, semi-span replica of a Gulfstream aircraft has been designed and fabricated specifically to conduct airframe noise studies and evaluate noise reduction concepts for mitigating landing gear, flap, and gear–flap interaction noise. Aeroacoustic testing of the 18% scale semi-span model was performed as a series of carefully planned entries in the NASA Lang- ley Research Center (LaRC) 14- by 22-Foot Subsonic Tunnel (14x22). The initial entry, completed in November 2010, focused on acquiring global forces (lift and drag) and measurements of steady and unsteady surface pressures. Detailed accounts of that entry and the processed aerodynamic data are given in Khorrami et al.9 and Khorrami and Neuhart.10 The second 14x22 tunnel entry, completed in April 2013, was executed in two segments. The first seg- ment was dedicated to simultaneous acoustic11 and surface pressure measurements,12 while the second segment was devoted to off-surface flow measurements13 for the nominal aircraft landing configuration.

    Concurrent with the experimental test campaign, there is a comprehensive computational effort aimed at extend- ing the state-of-the-art airframe noise prediction capabilities from component-level to full aircraft system-level con- figurations. A unique aspect of the NASA-Gulfstream collaboration is the fully complementary, and sometimes leading, role that the computational simulations play in paving the way for better design, evaluation, and testing of noise reduction technologies. The computational approach is built upon the knowledge and experience gained from previous aeroacoustic simulations of an isolated nose landing gear for a Gulfstream aircraft and other relevant con- figurations.14-19 Following those earlier nose landing gear simulations, the current full-aircraft airframe noise predic- tion effort is being pursued via execution of two distinct computational methodologies. In this paper, results ob- tained with NASA’s FUN3D solver are presented. The code, a standard unstructured compressible Navier-Stokes solver, is used to perform detached eddy simulations (DES) of the 18% scale, semi-span Gulfstream aircraft model in landing configuration (with the main landing gear off and on). In a companion paper by Khorrami, Fares and Ca- salino,20 we utilize Exa Corporation’s PowerFLOW® Lattice-Boltzmann flow solver to conduct aeroacoustic simu- lations of the same model configurations.

    II. Simulated Model Geometry The simulated model geometry corresponds to an 18% scale, semi-span, high-fidelity reproduction of a Gulf-

    stream aircraft (see Fig. 1). The model geometry consists of a fuselage, wing, flap, flow-through nacelle, pylon, and main landing gear. A full description of this model, including its instrumentation relative to the surface distribution of steady pressure ports and unsteady transducers on various components, is provided in Refs. 9 and 10. As an in- termediate building-block configuration, the main landing gear was omitted in the initial comprehensive set of simu- lations used to determine solution convergence and other grid dependency issues. Figures 2 and 3, reproduced from Ref. 9, show the top view of the spanwise location and numbering of the static pressure rows on the model wing, winglet, and flap. Note in Fig. 2a that the wing pressure rows extend to include the area that overlaps the stowed flap top surface, shown in purple. This region represents the wing spoiler, which was treated as part of the wing for in- strumentation purposes.

    Figures 4 and 5, taken from Ref. 10, display the locations and numbering of the unsteady pressure sensors on the model flap used extensively to validate the present computations. Altogether, the flap has 40 dynamic sensors. These sensors (herein after referred to as probes) are distributed in three distinct areas: along the mid-span section (not shown) and two narrow chordwise strips adjacent to the inboard (see Fig. 4) and outboard (see Fig. 5) edges where significant flow unsteadiness is expected. Note that the outboard flap side-edge has a cavity that nearly spans the

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    entire chord length. In the actual aircraft, this cavity houses a bulb seal that prevents metal to metal contact during flap retraction. To document wh