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The University of Southampton
Engineering

Source location and mode detection techniques

Mode detection using microphones on a TCS.

Experimental methods are being developed to characterize the sound transmission and radiation from a turbofan inlet.

A study has been performed in which microphones situated over the turbulence control screen on a turbofan inlet is used to deduce the broadband modal composition of the sound field by inverting the measured sound field.

Photograph of a low speed ducted fan and a hemispherical grid in which 81 microphones were located to invert the sound field to deduce the mode amplitudes.

A typical result is shown here, in which the inverted mode amplitude is plotted against cut-on ratio (a = 1, for cut-off modes). Also shown is the theoretical result for an ‘equal energy per mode' sound field. 

Measured sound pressure level variation at ka = 13.8 of the broadband noise over a hemispherical surface enclosing the inlet of a low speed fan rig.

 

 

 

 

Inverted mode amplitudes versus cut-on ratio a. Dashed curve is the theoretical result for an ‘equal energy per mode’ sound field.

 

 

 

 

 

 

In-duct techniques for the estimation of the broadband noise far field directivity

In circumstances where it is not possible to measure directly the far field directivity, such as when a suitable test bed or anechoic facility is not available, a technique has been developed whereby the far field directivity can be estimated from measurements of the in-duct sound field. The technique assumes a unique relationship between the mode-ray angle of a mode propagating in the duct (detectable with an in-duct beamformer) and the angle of the radiation of the main lobe in the far field.

Axial microphone array for beamforming measurements in order to deduce the far field directivity.



A comparison of the far field directivity deduced from an axial microphone array (solid curve) and the directivity measured using far field microphones (X) versus angle is shown above. Here good agreement can be observed.

Journal publications on fan broadband noise:

P. Joseph and C.L. Morfey, Multi-mode radiation from an unflanged, semi-infinite circular duct.  Journal of the Acoustical Society of America 105(5), 1999, 2590-600.

C.L. Morfey and P.F. Joseph, Shear layer refraction corrections for off-axis sources in a jet flow.  Journal of Sound and Vibration 239(4), 2000, 817-46.

P. Joseph, C.L. Morfey and C. Lowis, Multi-mode sound transmission in ducts with flow.  Journal of Sound and Vibration 264, 2003, 523-44.

C. Cheong, P. Joseph, and S. Lee. High frequency formulation for the acoustic power spectrum due to cascade-turbulence interaction J. Acoust. Soc. Am. 119, 206, 108.

Q. Zhou, P. Joseph. A frequency domain method for rotor self-noise prediction. AIAA Journal 44(6), 2006, 1197-206.

C. R. Lowis, P. Joseph, Determining the strength of rotating broadband sources in ducts by inverse methods. Journal of Sound and Vibration 295, 2006, 614-32 (Awarded the Bob Chivers prize for best student paper in physical acoustics (£300)).

Q. Zhou, P. Joseph. A frequency domain numerical method for airfoil broadband self-noise prediction. Journal of Sound and Vibration 299, 2006, 504-19.

F. Castres, P. Joseph. Mode detection in turbofan inlets from near field sensor arrays. 796 - 807. J. Acoust. Soc. Am. 121(2), 2007.

F. Castres, P. Joseph. Experimental investigation of an inversion technique for the determination of broadband duct mode amplitudes by the use of near-field sensor arrays. 848-59. J. Acoust. Soc. Am. 122(2), 2007.

R.D. Sandberg, N. D. Sandham and P.F. Joseph. Direct numerical simulations of trailing edge noise generated by boundary-layer instabilities. Journal of Sound and Vibration, 304, 2007, 677-90.

C.R. Lowis and P. Joseph. An in-duct technique for the measurement of far field directivity J. Acoust. Soc. Am. (accepted subject to minor changes).

C. Cheong, P.F. Joseph, Y. Park and S. Lee. Computation of aeolian tone from a circular cylinder using source models. Applied Acoustics, 2006 (in press, available online).

T. P. Chong, P. F. Joseph and P. O. A. L. Davies. A parametric study of passive flow control for a short and high area ratio 90 degree curved diffuser. Submitted to the ASME Journal of Fluids, June 2007.

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