Acoustic power measurement for single and annular stream duct-nozzle systems utilizing a modal decomposition scheme

A refined acoustic impulse technique developed recently at Lockheed-Georgia Company [1,2] was used to evaluate the adequacy of single point in-duct pressure measurements for determining the acoustic power for incident, reflected, and transmitted fields for single and annular stream duct-nozzle systems at various flow conditions. The spatial distributions of incident and reflected pressure fields were measured at several radial and azimuthal locations inside the duct. A modal decomposition scheme, developed for this purpose, was used to derive the acoustic power of each duct mode from the complex pressure measurements. The total sums of these individual modal acoustic powers were compared with the area-weighted acoustic power distributions that were evaluated from measured pressure data. It was found that a single-point measurement near the duct wall is adequate for estimating the transmitted power for both single and annular stream duct-nozzle systems. The in-duct incident, reflected, and transmitted powers computed from the sum of the plane and higher order modes agrees well with corresponding powers derived from the area-weighted pressures.