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  • ACOUSTICS  (2)
  • 42.60D  (1)
  • 1990-1994  (3)
  • 1965-1969
  • 1950-1954
  • 1935-1939
Collection
Keywords
Publisher
Years
  • 1990-1994  (3)
  • 1965-1969
  • 1950-1954
  • 1935-1939
Year
  • 1
    Electronic Resource
    Electronic Resource
    Four-branch oscillation from a dual-polarization cavity CO2 laser (1993)
    Springer
    Applied physics 57 (1993), S. 227-230 
    Details
    ISSN: 1432-0649
    Keywords: 42.55E ; 42.60D
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract We report here for the first time multiline oscillation of a cw-CO2 laser on all the four branches 10P, 10R, 9P, and 9R where all laser wavelengths share a common gain medium. The 9R, 9P and 10R branch lines lased at the expense of the high-gain 10P branch lines for which the cavity offered very high transmission losses. The 10P(20) line was made to lase by constructing another cavity with orthogonal polarization, thereby easing the strong competition between this line and the relatively low-gain 9R, 9P and 10R branch lines within the gain medium which is common to both cavities.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00334539
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    Paper (German National Licenses)
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  • 2
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    Computation of helicopter rotor acoustics in forward flight (1994)
    In:  CASI
    Details
    Publication Date: 2019-07-13
    Description: This paper presents a new method for computing acoustic signals from helicopter rotors in forward flight. The aerodynamic and acoustic solutions in the near field are computed with a finite-difference solver for the Euler equations. A nonrotating cylindrical Kirchhoff surface is then placed around the entire rotor system. This Kirchhoff surface moves subsonically with the rotor in forward flight. The finite-difference solution is interpolated onto this cylindrical surface at each time step and a Kirchhoff integration is used to carry the acoustic signal to the far field. Computed values for high-speed impulsive noise show excellent agreement with model-rotor and flight-test experimental data. Results from the new method offer high accuracy with reasonable computer resource requirements.
    Keywords: ACOUSTICS
    Type: NASA-CR-196132 , NAS 1.26:196132 , RIACS-TR-94-06 , Army Science Conference; Jun 20, 1994 - Jun 24, 1994; Orlando, FL; United States
    Format: application/pdf
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    NASA TECHNICAL REPORTS
  • 3
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    Unstructured adaptive mesh computations of rotorcraft high-speed impulsive noise (1993)
    In:  CASI
    Details
    Publication Date: 2019-07-13
    Description: A new method is developed for modeling helicopter high-speed impulsive (HSI) noise. The aerodynamics and acoustics near the rotor blade tip are computed by solving the Euler equations on an unstructured grid. A stationary Kirchhoff surface integral is then used to propagate these acoustic signals to the far field. The near-field Euler solver uses a solution-adaptive grid scheme to improve the resolution of the acoustic signal. Grid points are locally added and/or deleted from the mesh at each adaptive step. An important part of this procedure is the choice of an appropriate error indicator. The error indicator is computed from the flow field solution and determines the regions for mesh coarsening and refinement. Computed results for HSI noise compare favorably with experimental data for three different hovering rotor cases.
    Keywords: ACOUSTICS
    Type: NASA-CR-195090 , NAS 1.26:195090 , RIACS-TR-93-10 , AIAA PAPER 93-4359 , AIAA Aeroacoustics Conference; Oct 25, 1993 - Oct 27, 1993; Long Beach, CA; United States
    Format: application/pdf
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    NASA TECHNICAL REPORTS
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