ASA 124th Meeting New Orleans 1992 October

5aPAa8. Acoustic phase velocity measurements in a bubbly liquid using a fiber optic laser Doppler velocimeter.

Stanley A. Cheyne

Carl T. Stebbings

Dept. of Phys. and Astron., Hampden-Sydney College, Hampden-Sydney, VA 23943

Ronald A. Roy

Univ. of Washington, Seattle, WA 98105

Acoustic phase velocity measurements of a bubbly liquid in a cylindrical tube have been made with a fiber optic laser Doppler velocimeter (FOLDV). Past experimental configurations of sound propagation studies in tubes filled with air--water mixtures consisted of hydrophones or piezoelectric pill transducers (PZPT) embedded periodically in the tube wall. Hydrophones placed in the sample impose a lower limit on the tube diameter. (The tube diameter must be large compared to hydrophone.) Embedded PZPTs are typically spaced periodically imposing a limit on the resolution of the measured profile of the standing wave due to the finite size of the transducers. The advantages of the FOLDV are the nonintrusive nature of the detection scheme, enhanced resolution, and continuous scanning capabilities over a wider range of frequencies limited by the frequency response of the receiving photodiode. The FOLDV consisted of a fiber optic beam splitter, a miniature lens, a photodiode, and a very thin reflective membrane which is flush against the interior tube wall. The membrane is suspended by a small torus-shaped magnet on the exterior of the tube wall allowing continuous maneuverability in the vertical position of the membrane. A more comprehensive description of the experimental configuration and experimental results will be presented. [Work supported by the ONR and ONT.]