Abstract
Recently, approximate equations [E.J. Rice, AIAA Paper No. 777–1281 (1977)] for the farfield acoustic radiation pattern in the forward quadrant from a flanged circular duct were presented for both single and multimodal excitation. The single mode radiation equation showed that modal identification could be suppressed and that the resulting approximate equation could be expressed as a function of mode cutoff ratio only. In this paper, several exact radiation patterns for modes with similar cutoff ratios will be compared to the approximate equation to check its range of validity. The multimodal approximate equation consisted only of a simple function of directivity angle and an acoustic power biasing function, and was checked in the above reference for the equal acoustic power per mode case. Several cases of exact calculations other than equal acoustic power per mode will be presented here to test the validity of the multimodal approximate equation. A discussion of significant modal participation in a multimodal radiation pattern will be included.
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