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  • 1
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    Viscous stability of compressible axisymmetric jets (1983)
    In:  Other Sources
    Details
    Publication Date: 2019-06-28
    Description: (Previously cited in issue 16, p. 3060, Accession no. A80-38635)
    Keywords: ACOUSTICS
    Format: text
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  • 2
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    The radiation of sound by the instability waves of a compressible plane turbulent shear layer (1980)
    In:  Other Sources
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    Publication Date: 2019-06-27
    Description: The problem of acoustic radiation generated by instability waves of a compressible plane turbulent shear layer is solved. The solution provided is valid up to the acoustic far-field region. It represents a significant improvement over the solution obtained by classical hydrodynamic-stability theory which is essentially a local solution with the acoustic radiation suppressed. The basic instability-wave solution which is valid in the shear layer and the near-field region is constructed in terms of an asymptotic expansion using the method of multiple scales. This solution accounts for the effects of the slightly divergent mean flow. It is shown that the multiple-scales asymptotic expansion is not uniformly valid far from the shear layer. Continuation of this solution into the entire upper half-plane is described. The extended solution enables the near- and far-field pressure fluctuations associated with the instability wave to be determined. Numerical results show that the directivity pattern of acoustic radiation into the stationary medium peaks at 20 degrees to the axis of the shear layer in the downstream direction for supersonic flows. This agrees qualitatively with the observed noise-directivity patterns of supersonic jets.
    Keywords: ACOUSTICS
    Type: Journal of Fluid Mechanics; 98; May 29
    Format: text
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  • 3
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    Broadband jet noise amplification by a pure tone excitation (1980)
    In:  CASI
    Details
    Publication Date: 2019-06-27
    Description: A model is proposed for the change in turbulent structure of a round jet in the presence of an acoustic excitation. The excitation is initial amplitude at the jet exit. As these waves propagate downstream they extract energy from the mean flow and transfer it to the random turbulence. This results in an increase in the levels of the turbulence and a resulting increase in the radiated broadband noise. An examination is made of the effect of excitation level and frequency on the jet flow. The numerical procedure allows for radial as well as axial variations in the averaged properties of jet to be calculated. The results indicate that the presence of a finite amplitude instability wave increases the spreading of the jet.
    Keywords: ACOUSTICS
    Type: NASA-CR-163269
    Format: application/pdf
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  • 4
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    Turbulence characteristics of the noise producing region of an excited round jet. I - Time-average flow properties (1984)
    In:  Other Sources
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    Publication Date: 2019-06-28
    Description: Experiments were conducted to determine the mean turbulence characteristics of the noise-producing region of a 3 in. cold excited round jet. The jet was excited by plane acoustic waves with a high amplitude of excitation (2 percent of the jet dynamic head), and at a Strouhal number of 0.5. The flow Reynolds number was 280,000. The exit boundary layer was made turbulent by artificially tripping it with sandpaper strips. The data were obtained with single and X-hot-wires and processed in digital form. Mean and higher order statistics were also deduced. The results showed an increase in all three velocity components and stresses. However, most of the increase was noticed from the longitudinal component, while the changes in the radial velocity and the azimuthal component were much smaller. Substantial widening of the jet occurred, accompanied by a shortening of the potential core. Increased rates of production of energy close to the lip-line were a characteristic feature of the mixing region behavior.
    Keywords: ACOUSTICS
    Type: AIAA PAPER 84-2343
    Format: text
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  • 5
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    Turbulence characteristics of the noise producing region of an excited round jet. II - Large scale structure characteristics (1984)
    In:  Other Sources
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    Publication Date: 2019-06-28
    Description: A conditional sampling of digital hot wire records obtained at several radial positions of an axisymmetric jet indicates the existence of regions within the mixing layer which are more orderly than a fully chaotic turbulent flow. A Fourier decomposition of the flow has shown that most of the turbulent energy is contained in the fundamental frequency of the excitation and the first harmonic.
    Keywords: ACOUSTICS
    Type: AIAA PAPER 84-2342
    Format: text
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  • 6
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    Inflight simulation experiments on turbulent jet mixing noise (1976)
    In:  Other Sources
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    Publication Date: 2019-07-13
    Description: The effects of aircraft forward motion on pure turbulent mixing noise from unheated jets are examined experimentally in the inflight simulation mode. Both acoustic and flow characteristics are determined by testing model-scale nozzles in an anechoic free-jet facility and a wind tunnel, respectively. Scaling laws are derived from each set of experiments and are found to be complementary. The implications are discussed in detail. In particular, it is shown that the measured reduction in noise at 90 deg to the jet axis is a pure source alteration effect.
    Keywords: ACOUSTICS
    Type: AIAA PAPER 76-554 , Aero-Acoustics Conference; Jul 20, 1976 - Jul 23, 1976; Palo Alto, CA
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  • 7
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    Tone-excited jet: Theory and experiments (1982)
    In:  CASI
    Details
    Publication Date: 2019-06-28
    Description: A detailed study to understand the phenomenon of broadband jet-noise amplification produced by upstream discrete-tone sound excitation has been carried out. This has been achieved by simultaneous acquisition of the acoustic, mean velocity, turbulence intensities, and instability-wave pressure data. A 5.08 cm diameter jet has been tested for this purpose under static and also flight-simulation conditions. An open-jet wind tunnel has been used to simulate the flight effects. Limited data on heated jets have also been obtained. To improve the physical understanding of the flow modifications brought about by the upstream discrete-tone excitation, ensemble-averaged schlieren photographs of the jets have also been taken. Parallel to the experimental study, a mathematical model of the processes that lead to broadband-noise amplification by upstream tones has been developed. Excitation of large-scale turbulence by upstream tones is first calculated. A model to predict the changes in small-scale turbulence is then developed. By numerically integrating the resultant set of equations, the enhanced small-scale turbulence distribution in a jet under various excitation conditions is obtained. The resulting changes in small-scale turbulence have been attributed to broadband amplification of jet noise. Excellent agreement has been found between the theory and the experiments. It has also shown that the relative velocity effects are the same for the excited and the unexcited jets.
    Keywords: ACOUSTICS
    Type: NASA-CR-3538 , NAS 1.26:3538 , LG82ER0031
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  • 8
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    A model for broadband jet noise amplification (1980)
    In:  Other Sources
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    Publication Date: 2019-07-13
    Description: A model is proposed for the change in turbulent structure of a round jet in the presence of an acoustic excitation. The excitation is assumed to trigger instability waves of a known initial amplitude at the jet exit. As these waves propagate downstream they extract energy from the mean flow and transfer it to the random turbulence. This results in an increase in the levels of the turbulence and a resulting increase in the radiated broadband noise. No calculations are presented for the noise radiation; however, an examination is made of the effect of excitation level and frequency on the jet flow. The numerical procedure allows for radial as well as axial variations in the averaged properties of jet to be calculated. The results indicate that the presence of a finite amplitude instability wave increases the spreading of the jet. It does not vary the characteristic radial shapes of both the axial mean velocity and the turbulent kinetic energy. An energy budget for the random turbulence shows that it is fed energy from the excited wave predominantly on each side of the jet lip line. This results in a broader radial shape for the turbulent kinetic energy.
    Keywords: ACOUSTICS
    Type: AIAA PAPER 80-1004 , Aeroacoustics Conference; Jun 04, 1980 - Jun 06, 1980; Hartford, CT
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  • 9
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    Measurements and predictions of turbulence in sound excited jets (1981)
    In:  Other Sources
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    Publication Date: 2019-07-13
    Description: The modifications to the structure of turbulence in a round jet excited by acoustic tones upstream of the jet exit are measured and compared with predictions. The prediction scheme assumes that the acoustic waves excite instability waves or large turbulent structures in the jet. The presence of these waves at finite amplitude causes an increase in the levels of the random turbulence. Models are presented for the interaction between the instability waves and the turbulence. The numerical scheme permits the enhanced turbulence and large turbulent structures to modify the shape of the mean velocity profile. The changes in the turbulence spectra with excitation are measured. The jet has a Reynolds number of 3.75 x 10 to the 5th and the initial boundary layer is turbulent.
    Keywords: ACOUSTICS
    Type: AIAA PAPER 81-2006 , Aeroacoustics Conference; Oct 05, 1981 - Oct 07, 1981; Palo Alto, CA
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  • 10
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    Tone excited jets. V - A theoretical model and comparison with experiment (1985)
    In:  Other Sources
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    Publication Date: 2019-07-12
    Description: A mathematical model of tone-excited jets is developed which consists of two major components: a mathematical description of the process by which the intrinsic instability waves of the jet are excited by the upstream tones; and the modeling of the nonlinear interaction between the mean flow of the jet, the excited large-scale instability waves or turbulence structure, and the fine scale turbulence. It is assumed that each of these jet flow components can be characterized by a few parameters, which are then related by conservation equations that are supplemented by closure models. This quasi-linear model's results are compared with experimental measurements, and good agreement is obtained over a wide range of excitation frequencies and excitation levels.
    Keywords: ACOUSTICS
    Type: Journal of Sound and Vibration (ISSN 0022-460X); 102; 119-151
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