The Transient Character of the Near-Field Acoustic Radiation From a Cylindrical Diaphragm Excited by Water Hammer Transients

An investigation of the transient character of the near-field acoustic radiation from a water hammer excited, cylindrical diaphragm was conducted. The investigation was principally experimental and focused on relating the radiated waveforms to pressure transients within the pipe. The basic experimental system consisted of a length of rigid pipe extending from a flow source into a large sonar tank. The rigid pipe was interrupted by a cylindrical section of elastomeric tubing which served as a diaphragm and, thus, as an acoustic coupler between the water inside the pipe and water surrounding the pipe. A quick-closing valve downstream of the diaphragm produced water hammer transients which were coupled to the surrounding water through the diaphragm and were monitored in the near-field by hydrophones. The physical system geometry was varied to include data for a range of pipe sizes from 0.5 in. to 4.0 in. inside dia and from 10 ft to 34 ft in length. The experiments revealed that two distinct major waves could always be identified in each radiated wave pattern. One was found to originate from the primary water hammer compressive wave downstream of the diaphragm. The other major wave was attributed to a precursor wave which originated upstream of the diaphragm. The propagation paths of the two waves are identified and it is shown that their relation to one another in time space is a predictable function of geometry. It is estimated, by extrapolating from the experimental data, that an acoustic pressure in excess of 88 psi at one yard is possible from a system of realistic mechanical design.