Measurement of the effects of fluid loading on the dispersion of flexural waves in a plate

Abstract

Dispersion curves for axially symmetric flexural waves in a circular aluminum plate were determined using both direct phase measurement and measurement of normal mode frequencies. Measurements were conducted with the 8 mm thick plate in air and with one surface loaded by water in an anechoic tank over a frequency range of 20–80 kHz. At frequencies larger than the critical frequency (at which the free plate flexural wave speed is equal to the sound speed in water) the phase speeds with the plate in air and in water are nearly equal and agree well with the Mindlin-Timoshenko theory. Below critical frequency, water loading causes significantly lower values, about fifteen percent here. The speed in the water-loaded plate appears to change abruptly at or just above the critical frequency, in reasonable agreement with the theory of Kurtze and Bolt [Acustica 9, 238–242 (1959)]. Significance of these results will be discussed. [Supported by the Naval Undersea Warfare Engineering Station, Keyport, WA.]