The Structure of Wobbling Sound Fields

2006

A method is derived for the fast, exact prediction of acoustic fields around rotating sources by using a series expansion which generalizes a previously published method for a circular piston. The technique gives exact predictions for the field outside the sphere containing the rotor in a computational time two orders of magnitude less than that required for direct numerical evaluation of the acoustic integrals. Its use is demonstrated by application to two sample problems characteristic of real aircraft propellers.

“…The structure of rotating sound fields has been presented in detail in previous work 1, [3][4][5] and so the examples shown here serve to illustrate application of the method.…”

Section: B Acoustic Fieldsmentioning

confidence: 96%

Series expansion for the sound field of rotating sources

2006

“…This is a transformation which has been used, in the axisymmetric case, in studies of transient radiation from pistons, 21,22 and, with azimuthal variation, in studies of rotor acoustics. [13][14][15][16] The first stage is to switch from the source-centered cylindrical coordinates ͑r , , z͒ of Fig. 1 to the observer-centered coordinates ͑r 2 , 2 , z͒ of Fig.…”

Section: Equivalent Line Sourcementioning

confidence: 99%

“…Previous studies of the structure of spinning fields [13][14][15][16] have shown that the field decays exponentially away from the source. This means that in the forward projection problem, errors in the estimated acoustic source will decay and the predicted acoustic field may well be quite accurate, even if the source is not well recovered.…”

Section: Introductionmentioning

confidence: 99%

Analysis of the radiated information in spinning sound fields

2010

The information content of a spinning sound field is analyzed using a combination of exact and asymptotic results, in order to set limits on how accurately source identification can be carried out. Using a transformation of the circular source to an exactly equivalent set of line source modes, given by Chebyshev polynomials, it is found that the line source modes of order greater than the source wavenumber generate exponentially small fields. Asymptotic analysis shows that the remaining, lower order, modes radiate efficiently only into a region around the source plane, with this region shrinking as the mode order is increased. The results explain the ill-conditioning of source identification methods; the successful use of low order models in active noise control; and the low radiation efficiency of subsonic jets.

“…The formulation yields a set of coupled equations for azimuthal mode amplitudes on a distorted circular source, not unlike the "wobbling modes" studied in propeller noise. 11,12 The resulting equations include a hypersingular integral which is evaluated using recently developed quadratures, easing the analysis of the integral kernel.…”

Section: Numerical Techniquementioning

confidence: 99%

Scattering by quasi-symmetric pipes

2006

A hypersingular boundary integral method for the prediction of radiation from a straight circular pipe with arbitrary end profile has been developed. The technique represents an extension of established procedures for axisymmetric pipes with the addition of recent advances in special function and quadrature theory to simplify the implementation. The resulting code is applied to two sample problems: first, the prediction of radiation of a plane wave mode from a pipe with its ends cut by an inclined plane, representing the "scarfed intake" proposed for reduction of aircraft engine noise. Second, the method is used to examine scattering of an incident azimuthal mode by a multi-lobed profile, characteristic of the "chevron" nozzles proposed for jet engine exhausts.