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The University of Southampton
Engineering

Sound radiation from jet pipes (in collaboration with Jeremy Astley (ISVR))

An analytical model was developed for sound radiation from turbofan bypass ducts [1,2]. This problem is modelled by a semi-infinite unflanged annular duct carrying a jet that issues into a uniform mean flow (see figure below). The turbofan afterbody is represented by an infinite cylindrical centre body extending downstream from the duct exit. Noise propagates along the annular bypass duct, refracts through the external bypass stream and radiates to the far field. The instability wave of the vortex sheet and the refraction of sound waves by the jet shear layer are accounted for.

Geometry of the jet pipe.

Good agreement was found between analytical solutions and experimental data from the ANCF test rig at NASA Glenn Research Center [3], see figure. The capability to obtain near field solutions is particularly useful in providing reference solutions with which to assess extensively the accuracy of numerical solutions. The problem of sound radiation from a jet pipe includes several difficulties often found in applications of computational aero-acoustics to aero-engines, such as the scattering by duct terminations, radiation to the free field, wave refraction at vortex layers and the presence of instability waves. 

Comparison of analytical solution (solid line) with experimental data (dashed line) for the far-field directivity.

 

 

Example of near-field pressure field (note the presence of the instability wave).

 

 

 

 

 

1] Gabard, G. and Astley, R.J. Theoretical model for sound radiation from annular jet pipes: far- and near-field solutions. Journal of Fluid Mechanics, 549, 2006, 315-41.

[2] Gabard, G. Near- to far-field characteristics of acoustic radiation through plug flow jets. Journal of the Acoustical Society of America, 124(5), 2008, 2755-66.

[3] Sutliff, D., Nallasamy, M., Heidelberg, L., Elliott, D. Baseline acoustic levels of the NASA active noise control fan rig. NASA Technical Memorandum 107214, 1996.

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