Angles of peak sound radiation from axisymmetric duct inlets

The radiation of isolated spinning modes from five axisymmetric duct inlets having very different contours is examined experimentally in the absence of flow. It is found that when a mode is well cuton (cuton ratio ξ ≫ 1), the radiation pattern is much the same as if the duct were terminated at the inlet throat. However, as ξ decreases, the radiation pattern becomes more sensitive to the shape of the inlet. A physical model based on the concept of an effective plane of radiation explains the observed behavior and results in a simple expression for the angle of maximum radiation. This expression involves only two variables, ξ and the normalized inlet radius at the station where the contour has a slope of unity.