Longitudinal waves in liquid-filled tubes—II: Experiments

Abstract

An experimental investigation of the propagation of pulses in fluid-filled tubes produced by longitudinal impact was undertaken to correlate both wave speeds and pulse shapes in the tube and the fluid with corresponding theoretical predictions. Three tubes, composed of aluminum and a thick-walled and thin-walled acrylic (PMMA), and three fluids, distilled water, glycerin and an electrical insulating fluid (Fluorinert) were employed. Pulses were produced by the central longitudinal impact of a pneumatically-fired steel sphere on a closed end of the tube: both axial and transverse strains in the tubes and fluid pressures were measured at various positions. The tests involved both empty tubes and stationary liquids as well as laminar flow both in and opposed to the direction of impact. Excellent correspondence was found between measured tube and fluid wave speeds and the predictions of Skalak's simplified theory of water hammer; other theories did not provide as good a match. No significant difference was found in either wave speeds or pulse shapes whether the fluid was stationary or streaming, as would be expected on the basis of at least three orders of magnitude difference between particle and propagation velocity. Correspondence between predicted and measured initial portions of the axial strain ranged from very good to satisfactory, depending upon the combination of tube and fluid employed, with a maximum deviation of 15%, based upon experimentally-determined mechanical properties of the system components. The major portion of the deviation is attributed to inaccuracies in the determination of the viscoelastic behavior of the acrylic tubes.