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  • 1
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    Acoustic waves superimposed on incompressible flows (1990)
    In:  CASI
    Details
    Publication Date: 2013-08-31
    Description: The use of incompressible approximations in deriving solutions to the Lighthill wave equation was investigated for problems where an analytical solution could be found. A particular model problem involves the determination of the sound field of a spherical oscillating bubble in an ideal fluid. It is found that use of incompressible boundary conditions leads to good approximations in the important region of high acoustic wave number.
    Keywords: ACOUSTICS
    Type: NASA/American Society for Engineering Education (ASEE) Summer Faculty Fellowship Program, 1990; NASA(American Societ
    Format: application/pdf
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    NASA TECHNICAL REPORTS
  • 2
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    Quiet aircraft design and operational characteristics (1991)
    In:  CASI
    Details
    Publication Date: 2016-06-07
    Description: The application of aircraft noise technology to the design and operation of aircraft is discussed. Areas of discussion include the setting of target airplane noise levels, operational considerations and their effect on noise, and the sequencing and timing of the design and development process. Primary emphasis is placed on commercial transport aircraft of the type operated by major airlines. Additionally, noise control engineering of other types of aircraft is briefly discussed.
    Keywords: ACOUSTICS
    Type: NASA. Langley Research Center, Aeroacoustics of Flight Vehicles: Theory and Practice. Volume 2: Noise Control; p 383-413
    Format: application/pdf
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  • 3
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    Evaluation of several non-reflecting computational boundary conditions for duct acoustics (1994)
    In:  CASI
    Details
    Publication Date: 2019-06-28
    Description: Several non-reflecting computational boundary conditions that meet certain criteria and have potential applications to duct acoustics are evaluated for their effectiveness. The same interior solution scheme, grid, and order of approximation are used to evaluate each condition. Sparse matrix solution techniques are applied to solve the matrix equation resulting from the discretization. Modal series solutions for the sound attenuation in an infinite duct are used to evaluate the accuracy of each non-reflecting boundary conditions. The evaluations are performed for sound propagation in a softwall duct, for several sources, sound frequencies, and duct lengths. It is shown that a recently developed nonlocal boundary condition leads to sound attenuation predictions considerably more accurate for short ducts. This leads to a substantial reduction in the number of grid points when compared to other non-reflecting conditions.
    Keywords: ACOUSTICS
    Type: NASA-TM-109118 , NAS 1.15:109118
    Format: application/pdf
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  • 4
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    A non-local computational boundary condition for duct acoustics (1994)
    In:  CASI
    Details
    Publication Date: 2019-06-28
    Description: A non-local boundary condition is formulated for acoustic waves in ducts without flow. The ducts are two dimensional with constant area, but with variable impedance wall lining. Extension of the formulation to three dimensional and variable area ducts is straightforward in principle, but requires significantly more computation. The boundary condition simulates a nonreflecting wave field in an infinite duct. It is implemented by a constant matrix operator which is applied at the boundary of the computational domain. An efficient computational solution scheme is developed which allows calculations for high frequencies and long duct lengths. This computational solution utilizes the boundary condition to limit the computational space while preserving the radiation boundary condition. The boundary condition is tested for several sources. It is demonstrated that the boundary condition can be applied close to the sound sources, rendering the computational domain small. Computational solutions with the new non-local boundary condition are shown to be consistent with the known solutions for nonreflecting wavefields in an infinite uniform duct.
    Keywords: ACOUSTICS
    Type: NASA-TM-109091 , NAS 1.15:109091
    Format: application/pdf
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  • 5
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    NASA rotor systems research aircraft: Fixed-wing configuration flight-test results (1986)
    In:  CASI
    Details
    Publication Date: 2019-06-28
    Description: The fixed-wing, airplane configuration flight-test results of the Rotor System Research Aircraft (RSRA), NASA 740, at Ames/Dryden Flight Research Center are documented. Fourteen taxi and flight tests were performed from December 1983 to October 1984. This was the first time the RSRA was flown with the main rotor removed; the tail rotor was installed. These tests confirmed that the RSRA is operable as a fixed-wing aircraft. Data were obtained for various takeoff and landing distances, control sensitivity, trim and dynamics stability characteristics, performance rotor-hub drag, and acoustics signature. Stability data were obtained with the rotor hub both installed and removed. The speed envelope was developed to 261 knots true airspeed (KTAS), 226 knots calibrated airspeed (KCAS) at 10,000 ft density altitude. The airplane was configured at 5 deg. wing incidence with 5 deg. wing flaps as a normal configuration. Level-flight data were acquired at 167 KCAS for wing incidence from 0 to 10 deg. Step inputs and doublet inputs of various magnitudes were utilized to acquire dynamic stability and control sensitivity data. Sine-wave inputs of constantly increasing frequency were used to generate parameter identification data. The maximum load factor attained was 2.34 g at 206 KCAS.
    Keywords: AIRCRAFT DESIGN, TESTING AND PERFORMANCE
    Type: NASA-TM-86789 , A-85363 , NAS 1.15:86789
    Format: application/pdf
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  • 6
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    On the cross-stream spectral method for the Orr-Sommerfeld equation (1993)
    In:  CASI
    Details
    Publication Date: 2019-06-28
    Description: Cross-stream models are defined as solutions to the Orr-Sommerfeld equation which are propagating normal to the flow direction. These models are utilized as a basis for a Hilbert space to approximate the spectrum of the Orr-Sommerfeld equation with plane Poiseuille flow. The cross-stream basis leads to a standard eigenvalue problem for the frequencies of Poiseuille flow instability waves. The coefficient matrix in the eigenvalue problem is shown to be the sum of a real matrix and a negative-imaginary diagonal matrix which represents the frequencies of the cross-stream modes. The real coefficient matrix is shown to approach a Toeplitz matrix when the row and column indices are large. The Toeplitz matrix is diagonally dominant, and the diagonal elements vary inversely in magnitude with diagonal position. The Poiseuille flow eigenvalues are shown to lie within Gersgorin disks with radii bounded by the product of the average flow speed and the axial wavenumber. It is shown that the eigenvalues approach the Gersgorin disk centers when the mode index is large, so that the method may be used to compute spectra with an essentially unlimited number of elements. When the mode index is large, the real part of the eigenvalue is the product of the axial wavenumber and the average flow speed, and the imaginary part of the eigen value is identical to the corresponding cross-stream mode frequency. The cross-stream method is numerically well-conditioned in comparison to Chebyshev based methods, providing equivalent accuracy for small mode indices and superior accuracy for large indices.
    Keywords: ACOUSTICS
    Type: NASA-TM-107737 , NAS 1.15:107737
    Format: application/pdf
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  • 7
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    Solution of the three-dimensional Helmholtz equation with nonlocal boundary conditions (1995)
    In:  CASI
    Details
    Publication Date: 2019-06-28
    Description: The Helmholtz equation is solved within a three-dimensional rectangular duct with a nonlocal radiation boundary condition at the duct exit plane. This condition accurately models the acoustic admittance at an arbitrarily-located computational boundary plane. A linear system of equations is constructed with second-order central differences for the Helmholtz operator and second-order backward differences for both local admittance conditions and the gradient term in the nonlocal radiation boundary condition. The resulting matrix equation is large, sparse, and non-Hermitian. The size and structure of the matrix makes direct solution techniques impractical; as a result, a nonstationary iterative technique is used for its solution. The theory behind the nonstationary technique is reviewed, and numerical results are presented for radiation from both a point source and a planar acoustic source. The solutions with the nonlocal boundary conditions are invariant to the location of the computational boundary, and the same nonlocal conditions are valid for all solutions. The nonlocal conditions thus provide a means of minimizing the size of three-dimensional computational domains.
    Keywords: ACOUSTICS
    Type: NASA-TM-110174 , NAS 1.15:110174
    Format: application/pdf
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  • 8
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    NASA rotor system research aircraft flight-test data report: Helicopter and compound configuration (1984)
    In:  CASI
    Details
    Publication Date: 2019-06-28
    Description: The flight test activities of the Rotor System Research Aircraft (RSRA), NASA 740, from June 30, 1981 to August 5, 1982 are reported. Tests were conducted in both the helicopter and compound configurations. Compound tests reconfirmed the Sikorsky flight envelope except that main rotor blade bending loads reached endurance at a speed about 10 knots lower than previously. Wing incidence changes were made from 0 to 10 deg.
    Keywords: AIRCRAFT DESIGN, TESTING AND PERFORMANCE
    Type: NASA-TM-85843 , NAS 1.15:85843
    Format: application/pdf
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  • 9
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    Comparison of analytical and flight test identified aerodynamic derivatives for a tandem-rotor transport helicopter (1980)
    In:  CASI
    Details
    Publication Date: 2019-06-27
    Description: Flight tests for verifying an analytical aerodynamic derivative model of a CH-47 helicopter were conducted for low cruise speeds and transition to hover portions of curved, decelerating landing approach trajectories. All testing was performed on a closed loop basis with the stability augmentation system of the helicopter operating, and response data were obtained using both manual and computer generated input maneuvers. The results indicate some differences between the measured response time histories and those predicted by both analytical and flight test identified derivatives. With some exceptions the discrepancies are not severe, and the overall agreement between the measured and computed time histories is reasonably good. No adverse effects attributable to closed loop testing were noted, and the use of computer generated inputs proved to be superior to manual ones.
    Keywords: AIRCRAFT DESIGN, TESTING AND PERFORMANCE
    Type: NASA-TP-1581 , L-13228
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  • 10
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    VALT parameter identification flight test (1978)
    In:  Other Sources
    Details
    Publication Date: 2019-07-27
    Description: The paper describes a method of establishing the accuracy of previously developed analytical models of research vehicles for a program for developing avionics technology for VTOL aircraft. The research vehicle is a Boeing-Vertol CH-47 tandem rotor transport helicopter equipped with a fly-by-wire control system. The specialized flight test was designed to take into account the presence of winds at flight conditions from hover through transition to cruise. The test provided data to obtain estimates of derivatives by parameter identification.
    Keywords: AIRCRAFT DESIGN, TESTING AND PERFORMANCE
    Type: European Rotorcraft and Powered Lift Aircraft Forum; Sept. 13-15, 1978; Stresa; Italy
    Format: text
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