On the Generation of Pipeline Acoustic Resonance

Abstract

<p>A flow configuration capable of exciting and interacting with the acoustic plane waves modes of a piping system is examined both experimentally and theoretically. The acoustic source is generated by placing two standard geometry orifice plates in the flow. Strong acoustic pressures exceeding 125 dB inside the pipe are generated with the orifice plate separation distance small (<2%) in comparison to the wavelength of the lowest frequency excited. The acoustic source is shown to excite those modes possessing an acoustic pressure node (acoustic velocity anti-node) at or near the source location. The Strouhal number based on mean orifice velocity and orifice plate separation ranges from 0.5 to 1.0. and is sensitive to cavity diameter. Flow visualization photographs examining the fluid mechanics of the phenomenon are provided. The photographs reveal the presence of an oscillating shear layer near the upstream orifice plate, and subsequent roll up into a large scale vortex and propagation to the downstream orifice plate. The effect of mean turbulence levels at the upstream separation plane on the fluid dynamics and coupled acoustic production is studied.</p> <p>An acoustic model of the piping system is developed using the 4 pole method. The acoustic model, as well as the modelling procedures, are examined and tested in detail. A theoretical model of the coupled fluid/acoustic oscillator is developed by combining published characteristics of separated inviscid sheared flows with the developed acoustic model. The theoretical model predictions compare favorably, both qualitatively, and quantitatively with the experimental results.</p>