A wake-scattering experiment in thermally stratified water

An underwater object moving through a stable thermocline leaves a trail of fluctuations in pressure, velocity, and temperature; the attendant nonuniformities in the index of refraction scatter acoustic waves. An acoustic scattering experiment was performed in a flooded quarry, which has a stable thermocline with a typical gradient of 1.0 to 1.5 °C/ft and a Brunt–Väissälä period of about 70 s. The experiment used a self-propelled, slender model about 9 in. in diameter and 48 in. long. The speed was 3 ft/s, the Froude number was 300, and the Reynolds number was 1.5×105 based on the body diameter. A 75-kHz directional sonar, driven with 0.1-ms pulses at a rate of 10/s, insonified the wake; the scattered return was received by a directional hydrophone. The transducer-wake geometries included vertical, horizontal, and 45° backscatter. A strong backscattered signal was received for several Brunt–Väissälä periods; in one instance, for ten periods. The amplitude of the scattered return compares favorably with theory.