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  • 1
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    Propulsive lift noise (1991)
    In:  CASI
    Details
    Publication Date: 2006-06-13
    Description: Propulsive life noise is the increase in noise that occurs when airframe surfaces are placed in the propulsive system's exhaust to increase their lift force. Increased local flow velocities and turbulence levels, due to the propulsive system exhaust gases passing along the airframe lifting surfaces, cause an increase in maximum lift coefficient. The airplane's flight speed required for takeoff and landing can then be significantly reduced, allowing operation from shorter runways than those of conventional commercial airports. Unfortunately, interaction of high velocity turbulent exhaust flow with the airframe's solid surfaces generates additional noise radiation. Aeroacoustic processes that cause propulsive lift noise also are present in airframe noise and propulsive system installation noise. Research studies of propulsive lift noise led to development of improved methods of predicting noise radiation from surfaces in turbulent flows. Noise reduction and prediction methods of aircraft noise are discussed.
    Keywords: ACOUSTICS
    Type: NASA. Langley Research Center, Aeroacoustics of Flight Vehicles: Theory and Practice. Volume 1: Noise Sources; p 449-481
    Format: text
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  • 2
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    Airframe Noise Reduction Studies and Clean-Airframe Noise Investigation (1980)
    In:  CASI
    Details
    Publication Date: 2019-06-27
    Description: Acoustic wind tunnel tests were conducted of a wing model with modified leading edge slat and trailing edge flap. The modifications were intended to reduce the surface pressure response to convected turbulence and thereby reduce the airframe noise without changing the lift at constant incidence. Tests were conducted at 70.7 and 100 m/sec airspeeds, with Reynolds numbers 1.5 x 10 to the 6th power and 2.1 x 10 to the 6th power. Considerable reduction of noise radiation from the side edges of a 40 deflection single slotted flap was achieved by modification to the side edge regions or the leading edge region of the flap panel. Total far field noise was reduced 2 to 3 dB over several octaves of frequency. When these panels were installed as the aft panel of a 40 deg deflection double slotted flap, 2 dB noise reduction was achieved.
    Keywords: ACOUSTICS
    Type: NASA-CR-159311 , R80-914626-12
    Format: application/pdf
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  • 3
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    A method for calculating externally blown flap noise (1978)
    In:  CASI
    Details
    Publication Date: 2019-06-27
    Description: Several basic noise components were described. These components are: (1) compact lift dipoles associated with the wing and flaps; (2) trailing edge noise associated with the last trailing edge; and (3) quadrupole noise associated with the undeflected exhaust jet and the free jet located downstream of the trailing edge. These noise components were combined to allow prediction of directivity and spectra for under the wing (UTW) slotted flaps with conventional or mixer nozzles, UTW slotless flaps, upper surface blowing (USB) slotless flaps, and engine in front of the wing slotted flaps. A digital computer program listing was given for this calculation method. Directivities and spectra calculated by this method were compared with free field data for UTW and USB configurations. The UTRC method best predicted the details of the measured noise emission, but the ANOP method best estimated the noise levels directly below these configurations.
    Keywords: ACOUSTICS
    Type: NASA-CR-2954 , R77-911739-17
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  • 4
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    A method for calculating strut and splitter plate noise in exit ducts: Theory and verification (1978)
    In:  CASI
    Details
    Publication Date: 2019-06-27
    Description: Portions of a four-year analytical and experimental investigation relative to noise radiation from engine internal components in turbulent flow are summarized. Spectra measured for such airfoils over a range of chord, thickness ratio, flow velocity, and turbulence level were compared with predictions made by an available rigorous thin-airfoil analytical method. This analysis included the effects of flow compressibility and source noncompactness. Generally good agreement was obtained. This noise calculation method for isolated airfoils in turbulent flow was combined with a method for calculating transmission of sound through a subsonic exit duct and with an empirical far-field directivity shape. These three elements were checked separately and were individually shown to give close agreement with data. This combination provides a method for predicting engine internally generated aft-radiated noise from radial struts and stators, and annular splitter rings. Calculated sound power spectra, directivity, and acoustic pressure spectra were compared with the best available data. These data were for noise caused by a fan exit duct annular splitter ring, larger-chord stator blades, and turbine exit struts.
    Keywords: ACOUSTICS
    Type: NASA-CR-2955 , R77-911739-18
    Format: application/pdf
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  • 5
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    Prediction of rotating-blade vortex noise from noise of nonrotating blades (1976)
    In:  CASI
    Details
    Publication Date: 2019-06-27
    Description: Measurements were conducted in an acoustic wind tunnel to determine vortex noise of nonrotating circular cylinders and NACA 0012 airfoils. Both constant-width and spanwise tapered models were tested at a low turbulence level. The constant-diameter cylinder and constant-chord airfoil also were tested in the turbulent wake generated by an upstream cylinder or airfoil. Vortex noise radiation from nonrotating circular cylinders at Reynolds numbers matching those of the rotating-blade tests were found to be strongly dependent on surface conditions and Reynolds number. Vortex noise of rotating circular cylinder blades, operating with and without the shed wake blown downstream, could be predicted using data for nonrotating circular cylinders as functions of Reynolds number. Vortex noise of nonrotating airfoils was found to be trailing-edge noise at a time frequence equal to that predicted for maximum-amplitude Tollmein-Schlichting instability waves at the trailing edge.
    Keywords: ACOUSTICS
    Type: NASA-CR-2611
    Format: application/pdf
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  • 6
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    The importance of quadrupole sources in prediction of transonic tip speed propeller noise (1978)
    In:  CASI
    Details
    Publication Date: 2019-07-13
    Description: A theoretical analysis is presented for the harmonic noise of high speed, open rotors. Far field acoustic radiation equations based on the Ffowcs-Williams/Hawkings theory are derived for a static rotor with thin blades and zero lift. Near the plane of rotation, the dominant sources are the volume displacement and the rho U(2) quadrupole, where u is the disturbance velocity component in the direction blade motion. These sources are compared in both the time domain and the frequency domain using two dimensional airfoil theories valid in the subsonic, transonic, and supersonic speed ranges. For nonlifting parabolic arc blades, the two sources are equally important at speeds between the section critical Mach number and a Mach number of one. However, for moderately subsonic or fully supersonic flow over thin blade sections, the quadrupole term is negligible. It is concluded for thin blades that significant quadrupole noise radiation is strictly a transonic phenomenon and that it can be suppressed with blade sweep. Noise calculations are presented for two rotors, one simulating a helicopter main rotor and the other a model propeller. For the latter, agreement with test data was substantially improved by including the quadrupole source term.
    Keywords: ACOUSTICS
    Type: NASA. Langley Res. Center Helicopter Acoustics; p 339-371|Spring Meeting of the Inst. of Acoustics; Apr 07, 1978; Cambridge
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  • 7
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    Comparison of predictions and under-the-wing EBF noise data (1976)
    In:  Other Sources
    Details
    Publication Date: 2019-07-13
    Description: Detailed three-dimensional free field noise data were obtained by NASA for an under-the-wing externally blown model three-flap wing, and for a similar slotless wing. Spatial (polar and azimuthal) and spectral characteristics of these data are presented. These data are compared with predictions from some published EBF noise calculation methods. Methods include the totally empirical ANOPP and GELAC procedures, and a semi-empirical noise component method. The latter method adds the separately computed dipole noise due to fluctuating lift and drag, trailing edge noise, and quadrupole noise due to the deflected jet. Each of these components is calculated for the local geometry and flow conditions.
    Keywords: ACOUSTICS
    Type: AIAA PAPER 76-501 , Aero-Acoustics Conference; Jul 20, 1976 - Jul 23, 1976; Palo Alto, CA
    Format: text
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  • 8
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    Investigation of scrubbing and impingement noise (1975)
    In:  CASI
    Details
    Publication Date: 2019-06-27
    Description: Tests were conducted in an acoustic wind tunnel to determine surface pressure spectra and far field noise caused by turbulence impinging on an airfoil and turbulence convected past a sharp trailing edge. Measured effects of flow velocity and turbulence intensity were compared with predictions from several theories. Also, tests were conducted in an anechoic chamber to determine surface pressure spectra and far field noise caused by a deflected airfoil scrubbed by a subsonic jet. This installation simulated both an under-the-wing and an upper-surface-blowing externally blown flap, depending on the deflection angle. Surface and far field spectra, and cross correlation coherence and delay time, were utilized to infer the major noise-producing mechanisms.
    Keywords: ACOUSTICS
    Type: NASA-CR-134762
    Format: application/pdf
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  • 9
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    Airframe noise component interaction studies (1979)
    In:  CASI
    Details
    Publication Date: 2019-06-27
    Description: Acoustic wind tunnel tests were conducted of a two-dimensional wing section with removable high-lift leading and trailing edge devices and a removeable two-wheel landing gear with open cavity. An array of far field conventional microphones and an acoustic mirror directional microphone were utilized to determine far field spectrum levels and noise source distribution. Data were obtained for the wing with components deployed separately and in various combinations. The basic wing model had 0.305 m (1.00 ft) chord, which is roughly 1/10 scale for a one-hundred passenger transport airplane. Most of the data were obtained at 70.7 and 100 m/sec (232 and 328 ft/sec) airspeeds, which bracket the range of practical approach speeds for such aircraft. Data were obtained at frequence to 40 kHz so that, when scaled to s typical full-airframe, the frequency region which strongly influences preceived noise level would be included.
    Keywords: ACOUSTICS
    Type: NASA-CR-3110 , R78-912996-12
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  • 10
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    Model tests of airframe noise reduction concepts (1980)
    In:  Other Sources
    Details
    Publication Date: 2019-07-13
    Description: Trailing-edge flap and leading-edge slat models were modified to reduce their acoustic response to convected turbulence. These components were tested on an 0.3-m chord wing at 70.7 and 100 m/sec velocities in an acoustic wind tunnel. Noise radiation from the side edges of the 40 deg deflected flap was reduced by use of porous and perforated surface treatment along the leading- and side-edge region. Up to 2 to 3 dB reduction of total flap noise was achieved over a 3-octave range of frequency. Leading-edge slat noise was reduced about 2 dB by use of a perforated trailing-edge region on the slat. These reductions were retained when the modified slat and flaps were tested together.
    Keywords: ACOUSTICS
    Type: AIAA PAPER 80-0979 , Aeroacoustics Conference; Jun 04, 1980 - Jun 06, 1980; Hartford, CT
    Format: text
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