Second Computational Aeroacoustics (CAA) Workshop on ...

accuracy. The numberof shapefunctionsJ is chosensufficiently large to captureat Ieast 2-3 t_mes the
number of oscillations in _ expected along the duct; this number can be inferred from the incident field
given in eq. (6).
RESULTS
Numerical predictions from the theory outlined in thispaper are illustrated in figs. 2 and 3.
Figure 2 corresponds to the case of primary interest in the current proceedings: the forward Mach
number is 0.5, the duct (D) diameter and length are both I m, and the dimensionless clrcular frequency
is tzD/c=16r_. On the figure is shown the sound pressure level in dB re lO-51.tPa radiated from a
monopole for which pc tAI= IN. The polar plot gives the SPL of the incident field alone and that of the
total field in the presence of the duct on a spherical radius 2.5m from the origin of _. The angles 0 °
and 180 ° correspond to the exit and inlet ends of the duct, respectively. As would be expected at this
relatively high frequency, the duct causes very little scattering in the axial directions. There is, however,
a lateral shielding effect that cuts the SPL about 20dB around the 90 ° direction as is seen in all
problems of this type [1, 2, 7].
Figure 3 illustrates the directivity found for the same case except with the frequency reduced to
wD/c=4.409n (corresponding to 750 Hz). While the axial scattering is somewhat stronger in this case,
the duct obviously affords minimal lateral shielding at the lower frequency.
ACKNOWLEDGMENTS
This research was supported by NASA Langley Research Center under Cooperative Agreement
NCCI-14. Essential assistance with computer code development was provided by Mr. Melvin
Kosanchick III, Ms. Barbara Lakota and Mr. Jason Buhler.
.
.
.
REFERENCES
Myers, M. K. and Lan, J. H., "Sound Radiation from Ducted Rotating Sources in Uniform Motion."
AIAA Paper No. 93-4429, October 1993.
Myers, M. K., "Boundary Integral Formulations for Ducted Fan Radiation Calculations."
Proceedings of First Joint CEAS/AIAA Aerocoustics Conference, Munich, Germany, Vol. I, June
1995, pp. 565-573.
Eversman, W., "Ducted Fan Acoustic Radiation Including the Effects of Nonuniform Mean Flow and
Acoustic Treatment." AIAA Paper No. 93-4424, October 1993.
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