Structural-acoustic measurements of pipes with ninety-degree elbows, under water loading

The structural effects of an elbow, along with the fluid-loading effects of internal and external water on a thin and thick-walled pipe were experimentally examined. The important vibroacoustic behavior of test structures consisting of thin-walled (Schedule 10) and thick-walled (Schedule 40) steel pipe with ninety-degree elbows was measured in ARL/Penn State's reverberant water tank. To characterize coupling between the vibrating structures and the surrounding acoustic fluid, several transfer functions between structural vibration and acoustic sound radiation were measured using force hammer impulse excitation. The sound power transfer functions show that the pipes behave generally like dipole sound sources when excited with structural transverse drives, and amplify sound at the cut-on frequencies of shell wall lobar modes. The radiation efficiencies of the thick-walled pipe are higher than those of the thinwalled pipe above the cut-on frequencies of lobar modes, and the radiation efficiencies of both elbowed pipes are lower than those given by simple statistical estimates for beams and shells.