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  • 2010-2014  (7)
  • 1
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    The Effects of Crosswind Flight on Rotor Harmonic Noise Radiation (2013)
    In:  CASI
    Details
    Publication Date: 2019-07-13
    Description: In order to develop recommendations for procedures for helicopter source noise characterization, the effects of crosswinds on main rotor harmonic noise radiation are assessed using a model of the Bell 430 helicopter. Crosswinds are found to have a significant effect on Blade-Vortex Interaction (BVI) noise radiation when the helicopter is trimmed with the fuselage oriented along the inertial flight path. However, the magnitude of BVI noise remains unchanged when the pilot orients the fuselage along the aerodynamic velocity vector, crabbing for zero aerodynamic sideslip. The effects of wind gradients on BVI noise are also investigated and found to be smaller in the crosswind direction than in the headwind direction. The effects of crosswinds on lower harmonic noise sources at higher flight speeds are also assessed. In all cases, the directivity of radiated noise is somewhat changed by the crosswind. The model predictions agree well with flight test data for the Bell 430 helicopter captured under various wind conditions. The results of this investigation would suggest that flight paths for future acoustic flight testing are best aligned across the prevailing wind direction to minimize the effects of winds on noise measurements when wind cannot otherwise be avoided.
    Keywords: Acoustics
    Type: NF1676L-15487 , 69th Annual American Helicopter Society Forum and Technology Display; May 21, 2013 - May 23, 2013; Phoenix, AZ; United States
    Format: application/pdf
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  • 2
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    Acoustics Reflections of Full-Scale Rotor Noise Measurements in NFAC 40- by 80-Foot Wind Tunnel (2012)
    In:  CASI
    Details
    Publication Date: 2019-07-13
    Description: The objective of current research is to identify the extent of acoustic time history distortions due to wind tunnel wall reflections. Acoustic measurements from the recent full-scale Boeing-SMART rotor test (Fig. 2) will be used to illustrate the quality of noise measurement in the NFAC 40- by 80-Foot Wind Tunnel test section. Results will be compared to PSU-WOPWOP predictions obtained with and without adjustments due to sound reflections off wind tunnel walls. Present research assumes a rectangular enclosure as shown in Fig. 3a. The Method of Mirror Images7 is used to account for reflection sources and their acoustic paths by introducing mirror images of the rotor (i.e. acoustic source), at each and every wall surface, to enforce a no-flow boundary condition at the position of the physical walls (Fig. 3b). While conventional approach evaluates the "combined" noise from both the source and image rotor at a single microphone position, an alternative approach is used to simplify implementation of PSU-WOPWOP for this reflection analysis. Here, an "equivalent" microphone position is defined with respect to the source rotor for each mirror image that effectively renders the reflection analysis to be a one rotor, multiple microphones problem. This alternative approach has the advantage of allowing each individual "equivalent" microphone, representing the reflection pulse from the associated wall surface, to be adjusted by the panel absorption coefficient illustrated in Fig. 1a. Note that the presence of parallel wall surfaces requires an infinite number of mirror images (Fig. 3c) to satisfy the no-flow boundary conditions. In the present analysis, up to four mirror images (per wall surface) are accounted to achieve convergence in the predicted time histories
    Keywords: Acoustics
    Type: ARC-E-DAA-TN741 , AHS Specialists'' Conference; Jan 20, 2012 - Jan 21, 2012; San Francisco, CA; United States
    Format: application/pdf
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  • 3
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    Localized, Non-Harmonic Active Flap Motions for Low Frequency In-Plane Rotor Noise Reduction (2012)
    In:  CASI
    Details
    Publication Date: 2019-07-13
    Description: A first-of-its-kind demonstration of the use of localized, non-harmonic active flap motions, for suppressing low frequency, in-plane rotor noise, is reported in this paper. Operational feasibility is verified via testing of the full-scale AATD/Sikorsky/UTRC active flap demonstration rotor in the NFAC's 40- by 80-Foot anechoic wind tunnel. Effectiveness of using localized, non-harmonic active flap motions are compared to conventional four-per-rev harmonic flap motions, and also active flap motions derived from closed-loop acoustics implementations. All three approaches resulted in approximately the same noise reductions over an in-plane three-by-three microphone array installed forward and near in-plane of the rotor in the nearfield. It is also reported that using an active flap in this localized, non-harmonic manner, resulted in no more that 2% rotor performance penalty, but had the tendency to incur higher hub vibration levels.
    Keywords: Aircraft Design, Testing and Performance
    Type: AD-A558227 , ARC-E-DAA-TN4949 , American Helicopter Society 68th Annual Forum; May 01, 2012 - May 03, 2012; Fort Worth, TX; United States
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  • 4
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    Aeromechanics and Aeroacoustics Predictions of the Boeing-SMART Rotor Using Coupled-CFD/CSD Analyses (2010)
    In:  CASI
    Details
    Publication Date: 2019-07-13
    Description: This paper will highlight helicopter aeromechanics and aeroacoustics prediction capabilities developed by Georgia Institute of Technology, the Pennsylvania State University, and Northern Arizona University under the Helicopter Quieting Program (HQP) sponsored by the Tactical Technology Office of the Defense Advanced Research Projects Agency (DARPA). First initiated in 2004, the goal of the HQP was to develop high fidelity, state-of-the-art computational tools for designing advanced helicopter rotors with reduced acoustic perceptibility and enhanced performance. A critical step towards achieving this objective is the development of rotorcraft prediction codes capable of assessing a wide range of helicopter configurations and operations for future rotorcraft designs. This includes novel next-generation rotor systems that incorporate innovative passive and/or active elements to meet future challenging military performance and survivability goals.
    Keywords: Acoustics
    Type: ARC-E-DAA-TN910 , AHS 66th Annual Forum and Technology Display; May 11, 2010 - May 13, 2010; Phoenix, AZ; United States
    Format: application/pdf
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  • 5
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    Direct CFD Predictions of Low Frequency Sounds Generated by a Helicopter Main Rotor (2010)
    In:  CASI
    Details
    Publication Date: 2019-07-13
    Description: The use of CFD to directly predict helicopter main rotor noise is shown to be quite promising as an alternative mean for low frequency source noise evaluation. Results using existing state-of-the-art grid structures and finite-difference schemes demonstrated that small perturbation pressures, associated with acoustics radiation, can be extracted with some degree of fidelity. Accuracy of the predictions are demonstrated via comparing to predictions from conventional acoustic analogy-based models, and with measurements obtained from wind tunnel and flight tests for the MD-902 helicopter at several operating conditions. Findings show that the direct CFD approach is quite successfully in yielding low frequency results due to thickness and steady loading noise mechanisms. Mid-to-high frequency contents, due to blade-vortex interactions, are not predicted due to CFD modeling and grid constraints.
    Keywords: Acoustics
    Type: ARC-E-DAA-TN1528 , 2010 American Helicopter Society 66th Annual Forum; May 11, 2010 - May 13, 2010; Phoenix, AZ; United States
    Format: application/pdf
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  • 6
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    Direct CFD Predictions of Low Frequency Sounds Generated by Helicopter Main Rotors (2010)
    In:  CASI
    Details
    Publication Date: 2019-07-13
    Description: This proposed paper will highlight the application of a CSD/CFD methodology currently inuse by the US Army Aerfolightdynamics Directorate (AFDD) to assess the feasibility and fidelity of directly predicting low frequency sounds of helicopter rotors.
    Keywords: Fluid Mechanics and Thermodynamics
    Type: ARC-E-DAA-TN912 , AHS 66th Annual Forum and Technology Display; May 11, 2010 - May 13, 2010; Phoenix, AZ; United States
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  • 7
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    A Study of Acoustic Reflections in Full-Scale Rotor Low Frequency Noise Measurements Acquired in Wind Tunnels (2010)
    In:  CASI
    Details
    Publication Date: 2019-07-13
    Description: Difficulties in obtaining full-scale rotor low frequency noise measurements in wind tunnels are addressed via residual sound reflections due to non-ideal anechoic wall treatments. Examples illustrated with the Boeing-SMART rotor test in the National Full-Scale Aerodynamics Complex (NFAC) 40- by 80-Foot Wind Tunnel facility demonstrated that these reflections introduced distortions in the measured acoustic time histories that are not representative of free-field rotor noise radiation. A simplified reflection analysis, based on the method of images, is used to examine the sound measurement quality in such "less-than-anechoic" environment. Predictions of reflection-adjusted acoustic time histories are qualitatively shown to account for some of the spurious fluctuations observed in wind tunnel noise measurements
    Keywords: Acoustics
    Type: ARC-E-DAA-TN1156 , American Helicopter Society Specialists'' Conference on Aeromechanics; Jan 20, 2010 - Jan 22, 2010; San Francisco, CA; United States
    Format: application/pdf
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